1================
2Event Histograms
3================
4
5Documentation written by Tom Zanussi
6
71. Introduction
8===============
9
10  Histogram triggers are special event triggers that can be used to
11  aggregate trace event data into histograms.  For information on
12  trace events and event triggers, see Documentation/trace/events.rst.
13
14
152. Histogram Trigger Command
16============================
17
18  A histogram trigger command is an event trigger command that
19  aggregates event hits into a hash table keyed on one or more trace
20  event format fields (or stacktrace) and a set of running totals
21  derived from one or more trace event format fields and/or event
22  counts (hitcount).
23
24  The format of a hist trigger is as follows::
25
26        hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
27          [:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
28          [:clear][:name=histname1][:<handler>.<action>] [if <filter>]
29
30  When a matching event is hit, an entry is added to a hash table
31  using the key(s) and value(s) named.  Keys and values correspond to
32  fields in the event's format description.  Values must correspond to
33  numeric fields - on an event hit, the value(s) will be added to a
34  sum kept for that field.  The special string 'hitcount' can be used
35  in place of an explicit value field - this is simply a count of
36  event hits.  If 'values' isn't specified, an implicit 'hitcount'
37  value will be automatically created and used as the only value.
38  Keys can be any field, or the special string 'stacktrace', which
39  will use the event's kernel stacktrace as the key.  The keywords
40  'keys' or 'key' can be used to specify keys, and the keywords
41  'values', 'vals', or 'val' can be used to specify values.  Compound
42  keys consisting of up to two fields can be specified by the 'keys'
43  keyword.  Hashing a compound key produces a unique entry in the
44  table for each unique combination of component keys, and can be
45  useful for providing more fine-grained summaries of event data.
46  Additionally, sort keys consisting of up to two fields can be
47  specified by the 'sort' keyword.  If more than one field is
48  specified, the result will be a 'sort within a sort': the first key
49  is taken to be the primary sort key and the second the secondary
50  key.  If a hist trigger is given a name using the 'name' parameter,
51  its histogram data will be shared with other triggers of the same
52  name, and trigger hits will update this common data.  Only triggers
53  with 'compatible' fields can be combined in this way; triggers are
54  'compatible' if the fields named in the trigger share the same
55  number and type of fields and those fields also have the same names.
56  Note that any two events always share the compatible 'hitcount' and
57  'stacktrace' fields and can therefore be combined using those
58  fields, however pointless that may be.
59
60  'hist' triggers add a 'hist' file to each event's subdirectory.
61  Reading the 'hist' file for the event will dump the hash table in
62  its entirety to stdout.  If there are multiple hist triggers
63  attached to an event, there will be a table for each trigger in the
64  output.  The table displayed for a named trigger will be the same as
65  any other instance having the same name. Each printed hash table
66  entry is a simple list of the keys and values comprising the entry;
67  keys are printed first and are delineated by curly braces, and are
68  followed by the set of value fields for the entry.  By default,
69  numeric fields are displayed as base-10 integers.  This can be
70  modified by appending any of the following modifiers to the field
71  name:
72
73	=============  =================================================
74        .hex           display a number as a hex value
75	.sym           display an address as a symbol
76	.sym-offset    display an address as a symbol and offset
77	.syscall       display a syscall id as a system call name
78	.execname      display a common_pid as a program name
79	.log2          display log2 value rather than raw number
80	.buckets=size  display grouping of values rather than raw number
81	.usecs         display a common_timestamp in microseconds
82	=============  =================================================
83
84  Note that in general the semantics of a given field aren't
85  interpreted when applying a modifier to it, but there are some
86  restrictions to be aware of in this regard:
87
88    - only the 'hex' modifier can be used for values (because values
89      are essentially sums, and the other modifiers don't make sense
90      in that context).
91    - the 'execname' modifier can only be used on a 'common_pid'.  The
92      reason for this is that the execname is simply the 'comm' value
93      saved for the 'current' process when an event was triggered,
94      which is the same as the common_pid value saved by the event
95      tracing code.  Trying to apply that comm value to other pid
96      values wouldn't be correct, and typically events that care save
97      pid-specific comm fields in the event itself.
98
99  A typical usage scenario would be the following to enable a hist
100  trigger, read its current contents, and then turn it off::
101
102    # echo 'hist:keys=skbaddr.hex:vals=len' > \
103      /sys/kernel/debug/tracing/events/net/netif_rx/trigger
104
105    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
106
107    # echo '!hist:keys=skbaddr.hex:vals=len' > \
108      /sys/kernel/debug/tracing/events/net/netif_rx/trigger
109
110  The trigger file itself can be read to show the details of the
111  currently attached hist trigger.  This information is also displayed
112  at the top of the 'hist' file when read.
113
114  By default, the size of the hash table is 2048 entries.  The 'size'
115  parameter can be used to specify more or fewer than that.  The units
116  are in terms of hashtable entries - if a run uses more entries than
117  specified, the results will show the number of 'drops', the number
118  of hits that were ignored.  The size should be a power of 2 between
119  128 and 131072 (any non- power-of-2 number specified will be rounded
120  up).
121
122  The 'sort' parameter can be used to specify a value field to sort
123  on.  The default if unspecified is 'hitcount' and the default sort
124  order is 'ascending'.  To sort in the opposite direction, append
125  .descending' to the sort key.
126
127  The 'pause' parameter can be used to pause an existing hist trigger
128  or to start a hist trigger but not log any events until told to do
129  so.  'continue' or 'cont' can be used to start or restart a paused
130  hist trigger.
131
132  The 'clear' parameter will clear the contents of a running hist
133  trigger and leave its current paused/active state.
134
135  Note that the 'pause', 'cont', and 'clear' parameters should be
136  applied using 'append' shell operator ('>>') if applied to an
137  existing trigger, rather than via the '>' operator, which will cause
138  the trigger to be removed through truncation.
139
140- enable_hist/disable_hist
141
142  The enable_hist and disable_hist triggers can be used to have one
143  event conditionally start and stop another event's already-attached
144  hist trigger.  Any number of enable_hist and disable_hist triggers
145  can be attached to a given event, allowing that event to kick off
146  and stop aggregations on a host of other events.
147
148  The format is very similar to the enable/disable_event triggers::
149
150      enable_hist:<system>:<event>[:count]
151      disable_hist:<system>:<event>[:count]
152
153  Instead of enabling or disabling the tracing of the target event
154  into the trace buffer as the enable/disable_event triggers do, the
155  enable/disable_hist triggers enable or disable the aggregation of
156  the target event into a hash table.
157
158  A typical usage scenario for the enable_hist/disable_hist triggers
159  would be to first set up a paused hist trigger on some event,
160  followed by an enable_hist/disable_hist pair that turns the hist
161  aggregation on and off when conditions of interest are hit::
162
163   # echo 'hist:keys=skbaddr.hex:vals=len:pause' > \
164      /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
165
166    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
167      /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
168
169    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
170      /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
171
172  The above sets up an initially paused hist trigger which is unpaused
173  and starts aggregating events when a given program is executed, and
174  which stops aggregating when the process exits and the hist trigger
175  is paused again.
176
177  The examples below provide a more concrete illustration of the
178  concepts and typical usage patterns discussed above.
179
180'special' event fields
181------------------------
182
183  There are a number of 'special event fields' available for use as
184  keys or values in a hist trigger.  These look like and behave as if
185  they were actual event fields, but aren't really part of the event's
186  field definition or format file.  They are however available for any
187  event, and can be used anywhere an actual event field could be.
188  They are:
189
190    ====================== ==== =======================================
191    common_timestamp       u64  timestamp (from ring buffer) associated
192                                with the event, in nanoseconds.  May be
193			        modified by .usecs to have timestamps
194			        interpreted as microseconds.
195    common_cpu             int  the cpu on which the event occurred.
196    ====================== ==== =======================================
197
198Extended error information
199--------------------------
200
201  For some error conditions encountered when invoking a hist trigger
202  command, extended error information is available via the
203  tracing/error_log file.  See Error Conditions in
204  :file:`Documentation/trace/ftrace.rst` for details.
205
2066.2 'hist' trigger examples
207---------------------------
208
209  The first set of examples creates aggregations using the kmalloc
210  event.  The fields that can be used for the hist trigger are listed
211  in the kmalloc event's format file::
212
213    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
214    name: kmalloc
215    ID: 374
216    format:
217	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
218	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
219	field:unsigned char common_preempt_count;		offset:3;	size:1;	signed:0;
220	field:int common_pid;					offset:4;	size:4;	signed:1;
221
222	field:unsigned long call_site;				offset:8;	size:8;	signed:0;
223	field:const void * ptr;					offset:16;	size:8;	signed:0;
224	field:size_t bytes_req;					offset:24;	size:8;	signed:0;
225	field:size_t bytes_alloc;				offset:32;	size:8;	signed:0;
226	field:gfp_t gfp_flags;					offset:40;	size:4;	signed:0;
227
228  We'll start by creating a hist trigger that generates a simple table
229  that lists the total number of bytes requested for each function in
230  the kernel that made one or more calls to kmalloc::
231
232    # echo 'hist:key=call_site:val=bytes_req.buckets=32' > \
233            /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
234
235  This tells the tracing system to create a 'hist' trigger using the
236  call_site field of the kmalloc event as the key for the table, which
237  just means that each unique call_site address will have an entry
238  created for it in the table.  The 'val=bytes_req' parameter tells
239  the hist trigger that for each unique entry (call_site) in the
240  table, it should keep a running total of the number of bytes
241  requested by that call_site.
242
243  We'll let it run for awhile and then dump the contents of the 'hist'
244  file in the kmalloc event's subdirectory (for readability, a number
245  of entries have been omitted)::
246
247    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
248    # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
249
250    { call_site: 18446744072106379007 } hitcount:          1  bytes_req:        176
251    { call_site: 18446744071579557049 } hitcount:          1  bytes_req:       1024
252    { call_site: 18446744071580608289 } hitcount:          1  bytes_req:      16384
253    { call_site: 18446744071581827654 } hitcount:          1  bytes_req:         24
254    { call_site: 18446744071580700980 } hitcount:          1  bytes_req:          8
255    { call_site: 18446744071579359876 } hitcount:          1  bytes_req:        152
256    { call_site: 18446744071580795365 } hitcount:          3  bytes_req:        144
257    { call_site: 18446744071581303129 } hitcount:          3  bytes_req:        144
258    { call_site: 18446744071580713234 } hitcount:          4  bytes_req:       2560
259    { call_site: 18446744071580933750 } hitcount:          4  bytes_req:        736
260    .
261    .
262    .
263    { call_site: 18446744072106047046 } hitcount:         69  bytes_req:       5576
264    { call_site: 18446744071582116407 } hitcount:         73  bytes_req:       2336
265    { call_site: 18446744072106054684 } hitcount:        136  bytes_req:     140504
266    { call_site: 18446744072106224230 } hitcount:        136  bytes_req:      19584
267    { call_site: 18446744072106078074 } hitcount:        153  bytes_req:       2448
268    { call_site: 18446744072106062406 } hitcount:        153  bytes_req:      36720
269    { call_site: 18446744071582507929 } hitcount:        153  bytes_req:      37088
270    { call_site: 18446744072102520590 } hitcount:        273  bytes_req:      10920
271    { call_site: 18446744071582143559 } hitcount:        358  bytes_req:        716
272    { call_site: 18446744072106465852 } hitcount:        417  bytes_req:      56712
273    { call_site: 18446744072102523378 } hitcount:        485  bytes_req:      27160
274    { call_site: 18446744072099568646 } hitcount:       1676  bytes_req:      33520
275
276    Totals:
277        Hits: 4610
278        Entries: 45
279        Dropped: 0
280
281  The output displays a line for each entry, beginning with the key
282  specified in the trigger, followed by the value(s) also specified in
283  the trigger.  At the beginning of the output is a line that displays
284  the trigger info, which can also be displayed by reading the
285  'trigger' file::
286
287    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
288    hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
289
290  At the end of the output are a few lines that display the overall
291  totals for the run.  The 'Hits' field shows the total number of
292  times the event trigger was hit, the 'Entries' field shows the total
293  number of used entries in the hash table, and the 'Dropped' field
294  shows the number of hits that were dropped because the number of
295  used entries for the run exceeded the maximum number of entries
296  allowed for the table (normally 0, but if not a hint that you may
297  want to increase the size of the table using the 'size' parameter).
298
299  Notice in the above output that there's an extra field, 'hitcount',
300  which wasn't specified in the trigger.  Also notice that in the
301  trigger info output, there's a parameter, 'sort=hitcount', which
302  wasn't specified in the trigger either.  The reason for that is that
303  every trigger implicitly keeps a count of the total number of hits
304  attributed to a given entry, called the 'hitcount'.  That hitcount
305  information is explicitly displayed in the output, and in the
306  absence of a user-specified sort parameter, is used as the default
307  sort field.
308
309  The value 'hitcount' can be used in place of an explicit value in
310  the 'values' parameter if you don't really need to have any
311  particular field summed and are mainly interested in hit
312  frequencies.
313
314  To turn the hist trigger off, simply call up the trigger in the
315  command history and re-execute it with a '!' prepended::
316
317    # echo '!hist:key=call_site:val=bytes_req' > \
318           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
319
320  Finally, notice that the call_site as displayed in the output above
321  isn't really very useful.  It's an address, but normally addresses
322  are displayed in hex.  To have a numeric field displayed as a hex
323  value, simply append '.hex' to the field name in the trigger::
324
325    # echo 'hist:key=call_site.hex:val=bytes_req' > \
326           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
327
328    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
329    # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]
330
331    { call_site: ffffffffa026b291 } hitcount:          1  bytes_req:        433
332    { call_site: ffffffffa07186ff } hitcount:          1  bytes_req:        176
333    { call_site: ffffffff811ae721 } hitcount:          1  bytes_req:      16384
334    { call_site: ffffffff811c5134 } hitcount:          1  bytes_req:          8
335    { call_site: ffffffffa04a9ebb } hitcount:          1  bytes_req:        511
336    { call_site: ffffffff8122e0a6 } hitcount:          1  bytes_req:         12
337    { call_site: ffffffff8107da84 } hitcount:          1  bytes_req:        152
338    { call_site: ffffffff812d8246 } hitcount:          1  bytes_req:         24
339    { call_site: ffffffff811dc1e5 } hitcount:          3  bytes_req:        144
340    { call_site: ffffffffa02515e8 } hitcount:          3  bytes_req:        648
341    { call_site: ffffffff81258159 } hitcount:          3  bytes_req:        144
342    { call_site: ffffffff811c80f4 } hitcount:          4  bytes_req:        544
343    .
344    .
345    .
346    { call_site: ffffffffa06c7646 } hitcount:        106  bytes_req:       8024
347    { call_site: ffffffffa06cb246 } hitcount:        132  bytes_req:      31680
348    { call_site: ffffffffa06cef7a } hitcount:        132  bytes_req:       2112
349    { call_site: ffffffff8137e399 } hitcount:        132  bytes_req:      23232
350    { call_site: ffffffffa06c941c } hitcount:        185  bytes_req:     171360
351    { call_site: ffffffffa06f2a66 } hitcount:        185  bytes_req:      26640
352    { call_site: ffffffffa036a70e } hitcount:        265  bytes_req:      10600
353    { call_site: ffffffff81325447 } hitcount:        292  bytes_req:        584
354    { call_site: ffffffffa072da3c } hitcount:        446  bytes_req:      60656
355    { call_site: ffffffffa036b1f2 } hitcount:        526  bytes_req:      29456
356    { call_site: ffffffffa0099c06 } hitcount:       1780  bytes_req:      35600
357
358    Totals:
359        Hits: 4775
360        Entries: 46
361        Dropped: 0
362
363  Even that's only marginally more useful - while hex values do look
364  more like addresses, what users are typically more interested in
365  when looking at text addresses are the corresponding symbols
366  instead.  To have an address displayed as symbolic value instead,
367  simply append '.sym' or '.sym-offset' to the field name in the
368  trigger::
369
370    # echo 'hist:key=call_site.sym:val=bytes_req' > \
371           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
372
373    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
374    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]
375
376    { call_site: [ffffffff810adcb9] syslog_print_all                              } hitcount:          1  bytes_req:       1024
377    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
378    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
379    { call_site: [ffffffff8154acbe] usb_alloc_urb                                 } hitcount:          1  bytes_req:        192
380    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
381    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
382    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
383    { call_site: [ffffffff811febd5] fsnotify_alloc_group                          } hitcount:          2  bytes_req:        528
384    { call_site: [ffffffff81440f58] __tty_buffer_request_room                     } hitcount:          2  bytes_req:       2624
385    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          2  bytes_req:         96
386    { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211]      } hitcount:          2  bytes_req:        464
387    { call_site: [ffffffff81672406] tcp_get_metrics                               } hitcount:          2  bytes_req:        304
388    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
389    { call_site: [ffffffff81089b05] sched_create_group                            } hitcount:          2  bytes_req:       1424
390    .
391    .
392    .
393    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1185  bytes_req:     123240
394    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:       1185  bytes_req:     104280
395    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       1402  bytes_req:     190672
396    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       1518  bytes_req:     146208
397    { call_site: [ffffffffa029070e] drm_vma_node_allow [drm]                      } hitcount:       1746  bytes_req:      69840
398    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       2021  bytes_req:     792312
399    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       2592  bytes_req:     145152
400    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2629  bytes_req:     378576
401    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2629  bytes_req:    3783248
402    { call_site: [ffffffff81325607] apparmor_file_alloc_security                  } hitcount:       5192  bytes_req:      10384
403    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       5529  bytes_req:     110584
404    { call_site: [ffffffff8131ebf7] aa_alloc_task_context                         } hitcount:      21943  bytes_req:     702176
405    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:      55759  bytes_req:    5074265
406
407    Totals:
408        Hits: 109928
409        Entries: 71
410        Dropped: 0
411
412  Because the default sort key above is 'hitcount', the above shows a
413  the list of call_sites by increasing hitcount, so that at the bottom
414  we see the functions that made the most kmalloc calls during the
415  run.  If instead we we wanted to see the top kmalloc callers in
416  terms of the number of bytes requested rather than the number of
417  calls, and we wanted the top caller to appear at the top, we can use
418  the 'sort' parameter, along with the 'descending' modifier::
419
420    # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
421           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
422
423    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
424    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
425
426    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2186  bytes_req:    3397464
427    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1790  bytes_req:     712176
428    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       8132  bytes_req:     513135
429    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        106  bytes_req:     440128
430    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2186  bytes_req:     314784
431    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       2174  bytes_req:     208992
432    { call_site: [ffffffff811ae8e1] __kmalloc                                     } hitcount:          8  bytes_req:     131072
433    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:        859  bytes_req:     116824
434    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       1834  bytes_req:     102704
435    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:        972  bytes_req:     101088
436    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:        972  bytes_req:      85536
437    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       3333  bytes_req:      66664
438    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        209  bytes_req:      61632
439    .
440    .
441    .
442    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
443    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
444    { call_site: [ffffffff812d8406] copy_semundo                                  } hitcount:          2  bytes_req:         48
445    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          1  bytes_req:         48
446    { call_site: [ffffffffa027121a] drm_getmagic [drm]                            } hitcount:          1  bytes_req:         48
447    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
448    { call_site: [ffffffff811c52f4] bprm_change_interp                            } hitcount:          2  bytes_req:         16
449    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
450    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
451    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
452
453    Totals:
454        Hits: 32133
455        Entries: 81
456        Dropped: 0
457
458  To display the offset and size information in addition to the symbol
459  name, just use 'sym-offset' instead::
460
461    # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
462           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
463
464    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
465    # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
466
467    { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915]                  } hitcount:       4569  bytes_req:    3163720
468    { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915]                      } hitcount:       4569  bytes_req:     657936
469    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915]      } hitcount:       1519  bytes_req:     472936
470    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915]      } hitcount:       3050  bytes_req:     211832
471    { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50                                 } hitcount:         34  bytes_req:     148384
472    { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915]                  } hitcount:       1385  bytes_req:     144040
473    { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0                                   } hitcount:          8  bytes_req:     131072
474    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm]              } hitcount:       1385  bytes_req:     121880
475    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm]                  } hitcount:       1848  bytes_req:     103488
476    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915]            } hitcount:        461  bytes_req:      62696
477    { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm]                      } hitcount:       1541  bytes_req:      61640
478    { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0                                } hitcount:         57  bytes_req:      57456
479    .
480    .
481    .
482    { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0                       } hitcount:          2  bytes_req:        128
483    { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm]                      } hitcount:          3  bytes_req:         96
484    { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0                         } hitcount:          8  bytes_req:         96
485    { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650                            } hitcount:          3  bytes_req:         84
486    { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110                              } hitcount:          1  bytes_req:          8
487    { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid]                     } hitcount:          1  bytes_req:          7
488    { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid]                    } hitcount:          1  bytes_req:          7
489
490    Totals:
491        Hits: 26098
492        Entries: 64
493        Dropped: 0
494
495  We can also add multiple fields to the 'values' parameter.  For
496  example, we might want to see the total number of bytes allocated
497  alongside bytes requested, and display the result sorted by bytes
498  allocated in a descending order::
499
500    # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
501           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
502
503    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
504    # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]
505
506    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       7403  bytes_req:    4084360  bytes_alloc:    5958016
507    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        541  bytes_req:    2213968  bytes_alloc:    2228224
508    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       7404  bytes_req:    1066176  bytes_alloc:    1421568
509    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1565  bytes_req:     557368  bytes_alloc:    1037760
510    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       9557  bytes_req:     595778  bytes_alloc:     695744
511    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       5839  bytes_req:     430680  bytes_alloc:     470400
512    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       2388  bytes_req:     324768  bytes_alloc:     458496
513    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       3911  bytes_req:     219016  bytes_alloc:     250304
514    { call_site: [ffffffff815f8d7b] sk_prot_alloc                                 } hitcount:        235  bytes_req:     236880  bytes_alloc:     240640
515    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        557  bytes_req:     169024  bytes_alloc:     221760
516    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       9378  bytes_req:     187548  bytes_alloc:     206312
517    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1519  bytes_req:     157976  bytes_alloc:     194432
518    .
519    .
520    .
521    { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach                 } hitcount:          2  bytes_req:        144  bytes_alloc:        192
522    { call_site: [ffffffff81097ee8] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
523    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
524    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
525    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
526    { call_site: [ffffffff81213e80] load_elf_binary                               } hitcount:          3  bytes_req:         84  bytes_alloc:         96
527    { call_site: [ffffffff81079a2e] kthread_create_on_node                        } hitcount:          1  bytes_req:         56  bytes_alloc:         64
528    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7  bytes_alloc:          8
529    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8  bytes_alloc:          8
530    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7  bytes_alloc:          8
531
532    Totals:
533        Hits: 66598
534        Entries: 65
535        Dropped: 0
536
537  Finally, to finish off our kmalloc example, instead of simply having
538  the hist trigger display symbolic call_sites, we can have the hist
539  trigger additionally display the complete set of kernel stack traces
540  that led to each call_site.  To do that, we simply use the special
541  value 'stacktrace' for the key parameter::
542
543    # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
544           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
545
546  The above trigger will use the kernel stack trace in effect when an
547  event is triggered as the key for the hash table.  This allows the
548  enumeration of every kernel callpath that led up to a particular
549  event, along with a running total of any of the event fields for
550  that event.  Here we tally bytes requested and bytes allocated for
551  every callpath in the system that led up to a kmalloc (in this case
552  every callpath to a kmalloc for a kernel compile)::
553
554    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
555    # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]
556
557    { stacktrace:
558         __kmalloc_track_caller+0x10b/0x1a0
559         kmemdup+0x20/0x50
560         hidraw_report_event+0x8a/0x120 [hid]
561         hid_report_raw_event+0x3ea/0x440 [hid]
562         hid_input_report+0x112/0x190 [hid]
563         hid_irq_in+0xc2/0x260 [usbhid]
564         __usb_hcd_giveback_urb+0x72/0x120
565         usb_giveback_urb_bh+0x9e/0xe0
566         tasklet_hi_action+0xf8/0x100
567         __do_softirq+0x114/0x2c0
568         irq_exit+0xa5/0xb0
569         do_IRQ+0x5a/0xf0
570         ret_from_intr+0x0/0x30
571         cpuidle_enter+0x17/0x20
572         cpu_startup_entry+0x315/0x3e0
573         rest_init+0x7c/0x80
574    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
575    { stacktrace:
576         __kmalloc_track_caller+0x10b/0x1a0
577         kmemdup+0x20/0x50
578         hidraw_report_event+0x8a/0x120 [hid]
579         hid_report_raw_event+0x3ea/0x440 [hid]
580         hid_input_report+0x112/0x190 [hid]
581         hid_irq_in+0xc2/0x260 [usbhid]
582         __usb_hcd_giveback_urb+0x72/0x120
583         usb_giveback_urb_bh+0x9e/0xe0
584         tasklet_hi_action+0xf8/0x100
585         __do_softirq+0x114/0x2c0
586         irq_exit+0xa5/0xb0
587         do_IRQ+0x5a/0xf0
588         ret_from_intr+0x0/0x30
589    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
590    { stacktrace:
591         kmem_cache_alloc_trace+0xeb/0x150
592         aa_alloc_task_context+0x27/0x40
593         apparmor_cred_prepare+0x1f/0x50
594         security_prepare_creds+0x16/0x20
595         prepare_creds+0xdf/0x1a0
596         SyS_capset+0xb5/0x200
597         system_call_fastpath+0x12/0x6a
598    } hitcount:          1  bytes_req:         32  bytes_alloc:         32
599    .
600    .
601    .
602    { stacktrace:
603         __kmalloc+0x11b/0x1b0
604         i915_gem_execbuffer2+0x6c/0x2c0 [i915]
605         drm_ioctl+0x349/0x670 [drm]
606         do_vfs_ioctl+0x2f0/0x4f0
607         SyS_ioctl+0x81/0xa0
608         system_call_fastpath+0x12/0x6a
609    } hitcount:      17726  bytes_req:   13944120  bytes_alloc:   19593808
610    { stacktrace:
611         __kmalloc+0x11b/0x1b0
612         load_elf_phdrs+0x76/0xa0
613         load_elf_binary+0x102/0x1650
614         search_binary_handler+0x97/0x1d0
615         do_execveat_common.isra.34+0x551/0x6e0
616         SyS_execve+0x3a/0x50
617         return_from_execve+0x0/0x23
618    } hitcount:      33348  bytes_req:   17152128  bytes_alloc:   20226048
619    { stacktrace:
620         kmem_cache_alloc_trace+0xeb/0x150
621         apparmor_file_alloc_security+0x27/0x40
622         security_file_alloc+0x16/0x20
623         get_empty_filp+0x93/0x1c0
624         path_openat+0x31/0x5f0
625         do_filp_open+0x3a/0x90
626         do_sys_open+0x128/0x220
627         SyS_open+0x1e/0x20
628         system_call_fastpath+0x12/0x6a
629    } hitcount:    4766422  bytes_req:    9532844  bytes_alloc:   38131376
630    { stacktrace:
631         __kmalloc+0x11b/0x1b0
632         seq_buf_alloc+0x1b/0x50
633         seq_read+0x2cc/0x370
634         proc_reg_read+0x3d/0x80
635         __vfs_read+0x28/0xe0
636         vfs_read+0x86/0x140
637         SyS_read+0x46/0xb0
638         system_call_fastpath+0x12/0x6a
639    } hitcount:      19133  bytes_req:   78368768  bytes_alloc:   78368768
640
641    Totals:
642        Hits: 6085872
643        Entries: 253
644        Dropped: 0
645
646  If you key a hist trigger on common_pid, in order for example to
647  gather and display sorted totals for each process, you can use the
648  special .execname modifier to display the executable names for the
649  processes in the table rather than raw pids.  The example below
650  keeps a per-process sum of total bytes read::
651
652    # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
653           /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger
654
655    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
656    # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]
657
658    { common_pid: gnome-terminal  [      3196] } hitcount:        280  count:    1093512
659    { common_pid: Xorg            [      1309] } hitcount:        525  count:     256640
660    { common_pid: compiz          [      2889] } hitcount:         59  count:     254400
661    { common_pid: bash            [      8710] } hitcount:          3  count:      66369
662    { common_pid: dbus-daemon-lau [      8703] } hitcount:         49  count:      47739
663    { common_pid: irqbalance      [      1252] } hitcount:         27  count:      27648
664    { common_pid: 01ifupdown      [      8705] } hitcount:          3  count:      17216
665    { common_pid: dbus-daemon     [       772] } hitcount:         10  count:      12396
666    { common_pid: Socket Thread   [      8342] } hitcount:         11  count:      11264
667    { common_pid: nm-dhcp-client. [      8701] } hitcount:          6  count:       7424
668    { common_pid: gmain           [      1315] } hitcount:         18  count:       6336
669    .
670    .
671    .
672    { common_pid: postgres        [      1892] } hitcount:          2  count:         32
673    { common_pid: postgres        [      1891] } hitcount:          2  count:         32
674    { common_pid: gmain           [      8704] } hitcount:          2  count:         32
675    { common_pid: upstart-dbus-br [      2740] } hitcount:         21  count:         21
676    { common_pid: nm-dispatcher.a [      8696] } hitcount:          1  count:         16
677    { common_pid: indicator-datet [      2904] } hitcount:          1  count:         16
678    { common_pid: gdbus           [      2998] } hitcount:          1  count:         16
679    { common_pid: rtkit-daemon    [      2052] } hitcount:          1  count:          8
680    { common_pid: init            [         1] } hitcount:          2  count:          2
681
682    Totals:
683        Hits: 2116
684        Entries: 51
685        Dropped: 0
686
687  Similarly, if you key a hist trigger on syscall id, for example to
688  gather and display a list of systemwide syscall hits, you can use
689  the special .syscall modifier to display the syscall names rather
690  than raw ids.  The example below keeps a running total of syscall
691  counts for the system during the run::
692
693    # echo 'hist:key=id.syscall:val=hitcount' > \
694           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
695
696    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
697    # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]
698
699    { id: sys_fsync                     [ 74] } hitcount:          1
700    { id: sys_newuname                  [ 63] } hitcount:          1
701    { id: sys_prctl                     [157] } hitcount:          1
702    { id: sys_statfs                    [137] } hitcount:          1
703    { id: sys_symlink                   [ 88] } hitcount:          1
704    { id: sys_sendmmsg                  [307] } hitcount:          1
705    { id: sys_semctl                    [ 66] } hitcount:          1
706    { id: sys_readlink                  [ 89] } hitcount:          3
707    { id: sys_bind                      [ 49] } hitcount:          3
708    { id: sys_getsockname               [ 51] } hitcount:          3
709    { id: sys_unlink                    [ 87] } hitcount:          3
710    { id: sys_rename                    [ 82] } hitcount:          4
711    { id: unknown_syscall               [ 58] } hitcount:          4
712    { id: sys_connect                   [ 42] } hitcount:          4
713    { id: sys_getpid                    [ 39] } hitcount:          4
714    .
715    .
716    .
717    { id: sys_rt_sigprocmask            [ 14] } hitcount:        952
718    { id: sys_futex                     [202] } hitcount:       1534
719    { id: sys_write                     [  1] } hitcount:       2689
720    { id: sys_setitimer                 [ 38] } hitcount:       2797
721    { id: sys_read                      [  0] } hitcount:       3202
722    { id: sys_select                    [ 23] } hitcount:       3773
723    { id: sys_writev                    [ 20] } hitcount:       4531
724    { id: sys_poll                      [  7] } hitcount:       8314
725    { id: sys_recvmsg                   [ 47] } hitcount:      13738
726    { id: sys_ioctl                     [ 16] } hitcount:      21843
727
728    Totals:
729        Hits: 67612
730        Entries: 72
731        Dropped: 0
732
733  The syscall counts above provide a rough overall picture of system
734  call activity on the system; we can see for example that the most
735  popular system call on this system was the 'sys_ioctl' system call.
736
737  We can use 'compound' keys to refine that number and provide some
738  further insight as to which processes exactly contribute to the
739  overall ioctl count.
740
741  The command below keeps a hitcount for every unique combination of
742  system call id and pid - the end result is essentially a table
743  that keeps a per-pid sum of system call hits.  The results are
744  sorted using the system call id as the primary key, and the
745  hitcount sum as the secondary key::
746
747    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
748           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
749
750    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
751    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]
752
753    { id: sys_read                      [  0], common_pid: rtkit-daemon    [      1877] } hitcount:          1
754    { id: sys_read                      [  0], common_pid: gdbus           [      2976] } hitcount:          1
755    { id: sys_read                      [  0], common_pid: console-kit-dae [      3400] } hitcount:          1
756    { id: sys_read                      [  0], common_pid: postgres        [      1865] } hitcount:          1
757    { id: sys_read                      [  0], common_pid: deja-dup-monito [      3543] } hitcount:          2
758    { id: sys_read                      [  0], common_pid: NetworkManager  [       890] } hitcount:          2
759    { id: sys_read                      [  0], common_pid: evolution-calen [      3048] } hitcount:          2
760    { id: sys_read                      [  0], common_pid: postgres        [      1864] } hitcount:          2
761    { id: sys_read                      [  0], common_pid: nm-applet       [      3022] } hitcount:          2
762    { id: sys_read                      [  0], common_pid: whoopsie        [      1212] } hitcount:          2
763    .
764    .
765    .
766    { id: sys_ioctl                     [ 16], common_pid: bash            [      8479] } hitcount:          1
767    { id: sys_ioctl                     [ 16], common_pid: bash            [      3472] } hitcount:         12
768    { id: sys_ioctl                     [ 16], common_pid: gnome-terminal  [      3199] } hitcount:         16
769    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:       1808
770    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:       5580
771    .
772    .
773    .
774    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2690] } hitcount:          3
775    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2688] } hitcount:         16
776    { id: sys_inotify_add_watch         [254], common_pid: gmain           [       975] } hitcount:          2
777    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3204] } hitcount:          4
778    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2888] } hitcount:          4
779    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3003] } hitcount:          4
780    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2873] } hitcount:          4
781    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3196] } hitcount:          6
782    { id: sys_openat                    [257], common_pid: java            [      2623] } hitcount:          2
783    { id: sys_eventfd2                  [290], common_pid: ibus-ui-gtk3    [      2760] } hitcount:          4
784    { id: sys_eventfd2                  [290], common_pid: compiz          [      2994] } hitcount:          6
785
786    Totals:
787        Hits: 31536
788        Entries: 323
789        Dropped: 0
790
791  The above list does give us a breakdown of the ioctl syscall by
792  pid, but it also gives us quite a bit more than that, which we
793  don't really care about at the moment.  Since we know the syscall
794  id for sys_ioctl (16, displayed next to the sys_ioctl name), we
795  can use that to filter out all the other syscalls::
796
797    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
798           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
799
800    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
801    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]
802
803    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2769] } hitcount:          1
804    { id: sys_ioctl                     [ 16], common_pid: evolution-addre [      8571] } hitcount:          1
805    { id: sys_ioctl                     [ 16], common_pid: gmain           [      3003] } hitcount:          1
806    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2781] } hitcount:          1
807    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2829] } hitcount:          1
808    { id: sys_ioctl                     [ 16], common_pid: bash            [      8726] } hitcount:          1
809    { id: sys_ioctl                     [ 16], common_pid: bash            [      8508] } hitcount:          1
810    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2970] } hitcount:          1
811    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2768] } hitcount:          1
812    .
813    .
814    .
815    { id: sys_ioctl                     [ 16], common_pid: pool            [      8559] } hitcount:         45
816    { id: sys_ioctl                     [ 16], common_pid: pool            [      8555] } hitcount:         48
817    { id: sys_ioctl                     [ 16], common_pid: pool            [      8551] } hitcount:         48
818    { id: sys_ioctl                     [ 16], common_pid: avahi-daemon    [       896] } hitcount:         66
819    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:      26674
820    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:      73443
821
822    Totals:
823        Hits: 101162
824        Entries: 103
825        Dropped: 0
826
827  The above output shows that 'compiz' and 'Xorg' are far and away
828  the heaviest ioctl callers (which might lead to questions about
829  whether they really need to be making all those calls and to
830  possible avenues for further investigation.)
831
832  The compound key examples used a key and a sum value (hitcount) to
833  sort the output, but we can just as easily use two keys instead.
834  Here's an example where we use a compound key composed of the the
835  common_pid and size event fields.  Sorting with pid as the primary
836  key and 'size' as the secondary key allows us to display an
837  ordered summary of the recvfrom sizes, with counts, received by
838  each process::
839
840    # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
841           /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger
842
843    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
844    # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]
845
846    { common_pid: smbd            [       784], size:          4 } hitcount:          1
847    { common_pid: dnsmasq         [      1412], size:       4096 } hitcount:        672
848    { common_pid: postgres        [      1796], size:       1000 } hitcount:          6
849    { common_pid: postgres        [      1867], size:       1000 } hitcount:         10
850    { common_pid: bamfdaemon      [      2787], size:         28 } hitcount:          2
851    { common_pid: bamfdaemon      [      2787], size:      14360 } hitcount:          1
852    { common_pid: compiz          [      2994], size:          8 } hitcount:          1
853    { common_pid: compiz          [      2994], size:         20 } hitcount:         11
854    { common_pid: gnome-terminal  [      3199], size:          4 } hitcount:          2
855    { common_pid: firefox         [      8817], size:          4 } hitcount:          1
856    { common_pid: firefox         [      8817], size:          8 } hitcount:          5
857    { common_pid: firefox         [      8817], size:        588 } hitcount:          2
858    { common_pid: firefox         [      8817], size:        628 } hitcount:          1
859    { common_pid: firefox         [      8817], size:       6944 } hitcount:          1
860    { common_pid: firefox         [      8817], size:     408880 } hitcount:          2
861    { common_pid: firefox         [      8822], size:          8 } hitcount:          2
862    { common_pid: firefox         [      8822], size:        160 } hitcount:          2
863    { common_pid: firefox         [      8822], size:        320 } hitcount:          2
864    { common_pid: firefox         [      8822], size:        352 } hitcount:          1
865    .
866    .
867    .
868    { common_pid: pool            [      8923], size:       1960 } hitcount:         10
869    { common_pid: pool            [      8923], size:       2048 } hitcount:         10
870    { common_pid: pool            [      8924], size:       1960 } hitcount:         10
871    { common_pid: pool            [      8924], size:       2048 } hitcount:         10
872    { common_pid: pool            [      8928], size:       1964 } hitcount:          4
873    { common_pid: pool            [      8928], size:       1965 } hitcount:          2
874    { common_pid: pool            [      8928], size:       2048 } hitcount:          6
875    { common_pid: pool            [      8929], size:       1982 } hitcount:          1
876    { common_pid: pool            [      8929], size:       2048 } hitcount:          1
877
878    Totals:
879        Hits: 2016
880        Entries: 224
881        Dropped: 0
882
883  The above example also illustrates the fact that although a compound
884  key is treated as a single entity for hashing purposes, the sub-keys
885  it's composed of can be accessed independently.
886
887  The next example uses a string field as the hash key and
888  demonstrates how you can manually pause and continue a hist trigger.
889  In this example, we'll aggregate fork counts and don't expect a
890  large number of entries in the hash table, so we'll drop it to a
891  much smaller number, say 256::
892
893    # echo 'hist:key=child_comm:val=hitcount:size=256' > \
894           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
895
896    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
897    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
898
899    { child_comm: dconf worker                        } hitcount:          1
900    { child_comm: ibus-daemon                         } hitcount:          1
901    { child_comm: whoopsie                            } hitcount:          1
902    { child_comm: smbd                                } hitcount:          1
903    { child_comm: gdbus                               } hitcount:          1
904    { child_comm: kthreadd                            } hitcount:          1
905    { child_comm: dconf worker                        } hitcount:          1
906    { child_comm: evolution-alarm                     } hitcount:          2
907    { child_comm: Socket Thread                       } hitcount:          2
908    { child_comm: postgres                            } hitcount:          2
909    { child_comm: bash                                } hitcount:          3
910    { child_comm: compiz                              } hitcount:          3
911    { child_comm: evolution-sourc                     } hitcount:          4
912    { child_comm: dhclient                            } hitcount:          4
913    { child_comm: pool                                } hitcount:          5
914    { child_comm: nm-dispatcher.a                     } hitcount:          8
915    { child_comm: firefox                             } hitcount:          8
916    { child_comm: dbus-daemon                         } hitcount:          8
917    { child_comm: glib-pacrunner                      } hitcount:         10
918    { child_comm: evolution                           } hitcount:         23
919
920    Totals:
921        Hits: 89
922        Entries: 20
923        Dropped: 0
924
925  If we want to pause the hist trigger, we can simply append :pause to
926  the command that started the trigger.  Notice that the trigger info
927  displays as [paused]::
928
929    # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
930           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
931
932    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
933    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]
934
935    { child_comm: dconf worker                        } hitcount:          1
936    { child_comm: kthreadd                            } hitcount:          1
937    { child_comm: dconf worker                        } hitcount:          1
938    { child_comm: gdbus                               } hitcount:          1
939    { child_comm: ibus-daemon                         } hitcount:          1
940    { child_comm: Socket Thread                       } hitcount:          2
941    { child_comm: evolution-alarm                     } hitcount:          2
942    { child_comm: smbd                                } hitcount:          2
943    { child_comm: bash                                } hitcount:          3
944    { child_comm: whoopsie                            } hitcount:          3
945    { child_comm: compiz                              } hitcount:          3
946    { child_comm: evolution-sourc                     } hitcount:          4
947    { child_comm: pool                                } hitcount:          5
948    { child_comm: postgres                            } hitcount:          6
949    { child_comm: firefox                             } hitcount:          8
950    { child_comm: dhclient                            } hitcount:         10
951    { child_comm: emacs                               } hitcount:         12
952    { child_comm: dbus-daemon                         } hitcount:         20
953    { child_comm: nm-dispatcher.a                     } hitcount:         20
954    { child_comm: evolution                           } hitcount:         35
955    { child_comm: glib-pacrunner                      } hitcount:         59
956
957    Totals:
958        Hits: 199
959        Entries: 21
960        Dropped: 0
961
962  To manually continue having the trigger aggregate events, append
963  :cont instead.  Notice that the trigger info displays as [active]
964  again, and the data has changed::
965
966    # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
967           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
968
969    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
970    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
971
972    { child_comm: dconf worker                        } hitcount:          1
973    { child_comm: dconf worker                        } hitcount:          1
974    { child_comm: kthreadd                            } hitcount:          1
975    { child_comm: gdbus                               } hitcount:          1
976    { child_comm: ibus-daemon                         } hitcount:          1
977    { child_comm: Socket Thread                       } hitcount:          2
978    { child_comm: evolution-alarm                     } hitcount:          2
979    { child_comm: smbd                                } hitcount:          2
980    { child_comm: whoopsie                            } hitcount:          3
981    { child_comm: compiz                              } hitcount:          3
982    { child_comm: evolution-sourc                     } hitcount:          4
983    { child_comm: bash                                } hitcount:          5
984    { child_comm: pool                                } hitcount:          5
985    { child_comm: postgres                            } hitcount:          6
986    { child_comm: firefox                             } hitcount:          8
987    { child_comm: dhclient                            } hitcount:         11
988    { child_comm: emacs                               } hitcount:         12
989    { child_comm: dbus-daemon                         } hitcount:         22
990    { child_comm: nm-dispatcher.a                     } hitcount:         22
991    { child_comm: evolution                           } hitcount:         35
992    { child_comm: glib-pacrunner                      } hitcount:         59
993
994    Totals:
995        Hits: 206
996        Entries: 21
997        Dropped: 0
998
999  The previous example showed how to start and stop a hist trigger by
1000  appending 'pause' and 'continue' to the hist trigger command.  A
1001  hist trigger can also be started in a paused state by initially
1002  starting the trigger with ':pause' appended.  This allows you to
1003  start the trigger only when you're ready to start collecting data
1004  and not before.  For example, you could start the trigger in a
1005  paused state, then unpause it and do something you want to measure,
1006  then pause the trigger again when done.
1007
1008  Of course, doing this manually can be difficult and error-prone, but
1009  it is possible to automatically start and stop a hist trigger based
1010  on some condition, via the enable_hist and disable_hist triggers.
1011
1012  For example, suppose we wanted to take a look at the relative
1013  weights in terms of skb length for each callpath that leads to a
1014  netif_receive_skb event when downloading a decent-sized file using
1015  wget.
1016
1017  First we set up an initially paused stacktrace trigger on the
1018  netif_receive_skb event::
1019
1020    # echo 'hist:key=stacktrace:vals=len:pause' > \
1021           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1022
1023  Next, we set up an 'enable_hist' trigger on the sched_process_exec
1024  event, with an 'if filename==/usr/bin/wget' filter.  The effect of
1025  this new trigger is that it will 'unpause' the hist trigger we just
1026  set up on netif_receive_skb if and only if it sees a
1027  sched_process_exec event with a filename of '/usr/bin/wget'.  When
1028  that happens, all netif_receive_skb events are aggregated into a
1029  hash table keyed on stacktrace::
1030
1031    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
1032           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1033
1034  The aggregation continues until the netif_receive_skb is paused
1035  again, which is what the following disable_hist event does by
1036  creating a similar setup on the sched_process_exit event, using the
1037  filter 'comm==wget'::
1038
1039    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
1040           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1041
1042  Whenever a process exits and the comm field of the disable_hist
1043  trigger filter matches 'comm==wget', the netif_receive_skb hist
1044  trigger is disabled.
1045
1046  The overall effect is that netif_receive_skb events are aggregated
1047  into the hash table for only the duration of the wget.  Executing a
1048  wget command and then listing the 'hist' file will display the
1049  output generated by the wget command::
1050
1051    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
1052
1053    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1054    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
1055
1056    { stacktrace:
1057         __netif_receive_skb_core+0x46d/0x990
1058         __netif_receive_skb+0x18/0x60
1059         netif_receive_skb_internal+0x23/0x90
1060         napi_gro_receive+0xc8/0x100
1061         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
1062         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
1063         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
1064         ieee80211_rx+0x31d/0x900 [mac80211]
1065         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
1066         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
1067         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
1068         irq_thread_fn+0x20/0x50
1069         irq_thread+0x11f/0x150
1070         kthread+0xd2/0xf0
1071         ret_from_fork+0x42/0x70
1072    } hitcount:         85  len:      28884
1073    { stacktrace:
1074         __netif_receive_skb_core+0x46d/0x990
1075         __netif_receive_skb+0x18/0x60
1076         netif_receive_skb_internal+0x23/0x90
1077         napi_gro_complete+0xa4/0xe0
1078         dev_gro_receive+0x23a/0x360
1079         napi_gro_receive+0x30/0x100
1080         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
1081         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
1082         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
1083         ieee80211_rx+0x31d/0x900 [mac80211]
1084         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
1085         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
1086         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
1087         irq_thread_fn+0x20/0x50
1088         irq_thread+0x11f/0x150
1089         kthread+0xd2/0xf0
1090    } hitcount:         98  len:     664329
1091    { stacktrace:
1092         __netif_receive_skb_core+0x46d/0x990
1093         __netif_receive_skb+0x18/0x60
1094         process_backlog+0xa8/0x150
1095         net_rx_action+0x15d/0x340
1096         __do_softirq+0x114/0x2c0
1097         do_softirq_own_stack+0x1c/0x30
1098         do_softirq+0x65/0x70
1099         __local_bh_enable_ip+0xb5/0xc0
1100         ip_finish_output+0x1f4/0x840
1101         ip_output+0x6b/0xc0
1102         ip_local_out_sk+0x31/0x40
1103         ip_send_skb+0x1a/0x50
1104         udp_send_skb+0x173/0x2a0
1105         udp_sendmsg+0x2bf/0x9f0
1106         inet_sendmsg+0x64/0xa0
1107         sock_sendmsg+0x3d/0x50
1108    } hitcount:        115  len:      13030
1109    { stacktrace:
1110         __netif_receive_skb_core+0x46d/0x990
1111         __netif_receive_skb+0x18/0x60
1112         netif_receive_skb_internal+0x23/0x90
1113         napi_gro_complete+0xa4/0xe0
1114         napi_gro_flush+0x6d/0x90
1115         iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
1116         irq_thread_fn+0x20/0x50
1117         irq_thread+0x11f/0x150
1118         kthread+0xd2/0xf0
1119         ret_from_fork+0x42/0x70
1120    } hitcount:        934  len:    5512212
1121
1122    Totals:
1123        Hits: 1232
1124        Entries: 4
1125        Dropped: 0
1126
1127  The above shows all the netif_receive_skb callpaths and their total
1128  lengths for the duration of the wget command.
1129
1130  The 'clear' hist trigger param can be used to clear the hash table.
1131  Suppose we wanted to try another run of the previous example but
1132  this time also wanted to see the complete list of events that went
1133  into the histogram.  In order to avoid having to set everything up
1134  again, we can just clear the histogram first::
1135
1136    # echo 'hist:key=stacktrace:vals=len:clear' >> \
1137           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1138
1139  Just to verify that it is in fact cleared, here's what we now see in
1140  the hist file::
1141
1142    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1143    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
1144
1145    Totals:
1146        Hits: 0
1147        Entries: 0
1148        Dropped: 0
1149
1150  Since we want to see the detailed list of every netif_receive_skb
1151  event occurring during the new run, which are in fact the same
1152  events being aggregated into the hash table, we add some additional
1153  'enable_event' events to the triggering sched_process_exec and
1154  sched_process_exit events as such::
1155
1156    # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
1157           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1158
1159    # echo 'disable_event:net:netif_receive_skb if comm==wget' > \
1160           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1161
1162  If you read the trigger files for the sched_process_exec and
1163  sched_process_exit triggers, you should see two triggers for each:
1164  one enabling/disabling the hist aggregation and the other
1165  enabling/disabling the logging of events::
1166
1167    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1168    enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
1169    enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
1170
1171    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1172    enable_event:net:netif_receive_skb:unlimited if comm==wget
1173    disable_hist:net:netif_receive_skb:unlimited if comm==wget
1174
1175  In other words, whenever either of the sched_process_exec or
1176  sched_process_exit events is hit and matches 'wget', it enables or
1177  disables both the histogram and the event log, and what you end up
1178  with is a hash table and set of events just covering the specified
1179  duration.  Run the wget command again::
1180
1181    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
1182
1183  Displaying the 'hist' file should show something similar to what you
1184  saw in the last run, but this time you should also see the
1185  individual events in the trace file::
1186
1187    # cat /sys/kernel/debug/tracing/trace
1188
1189    # tracer: nop
1190    #
1191    # entries-in-buffer/entries-written: 183/1426   #P:4
1192    #
1193    #                              _-----=> irqs-off
1194    #                             / _----=> need-resched
1195    #                            | / _---=> hardirq/softirq
1196    #                            || / _--=> preempt-depth
1197    #                            ||| /     delay
1198    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1199    #              | |       |   ||||       |         |
1200                wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
1201                wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
1202             dnsmasq-1382  [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
1203             dnsmasq-1382  [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
1204    ##### CPU 2 buffer started ####
1205      irq/29-iwlwifi-559   [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
1206      irq/29-iwlwifi-559   [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
1207      irq/29-iwlwifi-559   [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
1208      irq/29-iwlwifi-559   [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
1209      irq/29-iwlwifi-559   [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
1210    .
1211    .
1212    .
1213
1214  The following example demonstrates how multiple hist triggers can be
1215  attached to a given event.  This capability can be useful for
1216  creating a set of different summaries derived from the same set of
1217  events, or for comparing the effects of different filters, among
1218  other things::
1219
1220    # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
1221           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1222    # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
1223           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1224    # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
1225           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1226    # echo 'hist:keys=skbaddr.hex:vals=len' >> \
1227           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1228    # echo 'hist:keys=len:vals=common_preempt_count' >> \
1229           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1230
1231  The above set of commands create four triggers differing only in
1232  their filters, along with a completely different though fairly
1233  nonsensical trigger.  Note that in order to append multiple hist
1234  triggers to the same file, you should use the '>>' operator to
1235  append them ('>' will also add the new hist trigger, but will remove
1236  any existing hist triggers beforehand).
1237
1238  Displaying the contents of the 'hist' file for the event shows the
1239  contents of all five histograms::
1240
1241    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1242
1243    # event histogram
1244    #
1245    # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
1246    #
1247
1248    { len:        176 } hitcount:          1  common_preempt_count:          0
1249    { len:        223 } hitcount:          1  common_preempt_count:          0
1250    { len:       4854 } hitcount:          1  common_preempt_count:          0
1251    { len:        395 } hitcount:          1  common_preempt_count:          0
1252    { len:        177 } hitcount:          1  common_preempt_count:          0
1253    { len:        446 } hitcount:          1  common_preempt_count:          0
1254    { len:       1601 } hitcount:          1  common_preempt_count:          0
1255    .
1256    .
1257    .
1258    { len:       1280 } hitcount:         66  common_preempt_count:          0
1259    { len:        116 } hitcount:         81  common_preempt_count:         40
1260    { len:        708 } hitcount:        112  common_preempt_count:          0
1261    { len:         46 } hitcount:        221  common_preempt_count:          0
1262    { len:       1264 } hitcount:        458  common_preempt_count:          0
1263
1264    Totals:
1265        Hits: 1428
1266        Entries: 147
1267        Dropped: 0
1268
1269
1270    # event histogram
1271    #
1272    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1273    #
1274
1275    { skbaddr: ffff8800baee5e00 } hitcount:          1  len:        130
1276    { skbaddr: ffff88005f3d5600 } hitcount:          1  len:       1280
1277    { skbaddr: ffff88005f3d4900 } hitcount:          1  len:       1280
1278    { skbaddr: ffff88009fed6300 } hitcount:          1  len:        115
1279    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:        115
1280    { skbaddr: ffff88008cdb1900 } hitcount:          1  len:         46
1281    { skbaddr: ffff880064b5ef00 } hitcount:          1  len:        118
1282    { skbaddr: ffff880044e3c700 } hitcount:          1  len:         60
1283    { skbaddr: ffff880100065900 } hitcount:          1  len:         46
1284    { skbaddr: ffff8800d46bd500 } hitcount:          1  len:        116
1285    { skbaddr: ffff88005f3d5f00 } hitcount:          1  len:       1280
1286    { skbaddr: ffff880100064700 } hitcount:          1  len:        365
1287    { skbaddr: ffff8800badb6f00 } hitcount:          1  len:         60
1288    .
1289    .
1290    .
1291    { skbaddr: ffff88009fe0be00 } hitcount:         27  len:      24677
1292    { skbaddr: ffff88009fe0a400 } hitcount:         27  len:      23052
1293    { skbaddr: ffff88009fe0b700 } hitcount:         31  len:      25589
1294    { skbaddr: ffff88009fe0b600 } hitcount:         32  len:      27326
1295    { skbaddr: ffff88006a462800 } hitcount:         68  len:      71678
1296    { skbaddr: ffff88006a463700 } hitcount:         70  len:      72678
1297    { skbaddr: ffff88006a462b00 } hitcount:         71  len:      77589
1298    { skbaddr: ffff88006a463600 } hitcount:         73  len:      71307
1299    { skbaddr: ffff88006a462200 } hitcount:         81  len:      81032
1300
1301    Totals:
1302        Hits: 1451
1303        Entries: 318
1304        Dropped: 0
1305
1306
1307    # event histogram
1308    #
1309    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
1310    #
1311
1312
1313    Totals:
1314        Hits: 0
1315        Entries: 0
1316        Dropped: 0
1317
1318
1319    # event histogram
1320    #
1321    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active]
1322    #
1323
1324    { skbaddr: ffff88009fd2c300 } hitcount:          1  len:       7212
1325    { skbaddr: ffff8800d2bcce00 } hitcount:          1  len:       7212
1326    { skbaddr: ffff8800d2bcd700 } hitcount:          1  len:       7212
1327    { skbaddr: ffff8800d2bcda00 } hitcount:          1  len:      21492
1328    { skbaddr: ffff8800ae2e2d00 } hitcount:          1  len:       7212
1329    { skbaddr: ffff8800d2bcdb00 } hitcount:          1  len:       7212
1330    { skbaddr: ffff88006a4df500 } hitcount:          1  len:       4854
1331    { skbaddr: ffff88008ce47b00 } hitcount:          1  len:      18636
1332    { skbaddr: ffff8800ae2e2200 } hitcount:          1  len:      12924
1333    { skbaddr: ffff88005f3e1000 } hitcount:          1  len:       4356
1334    { skbaddr: ffff8800d2bcdc00 } hitcount:          2  len:      24420
1335    { skbaddr: ffff8800d2bcc200 } hitcount:          2  len:      12996
1336
1337    Totals:
1338        Hits: 14
1339        Entries: 12
1340        Dropped: 0
1341
1342
1343    # event histogram
1344    #
1345    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
1346    #
1347
1348
1349    Totals:
1350        Hits: 0
1351        Entries: 0
1352        Dropped: 0
1353
1354  Named triggers can be used to have triggers share a common set of
1355  histogram data.  This capability is mostly useful for combining the
1356  output of events generated by tracepoints contained inside inline
1357  functions, but names can be used in a hist trigger on any event.
1358  For example, these two triggers when hit will update the same 'len'
1359  field in the shared 'foo' histogram data::
1360
1361    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
1362           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1363    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
1364           /sys/kernel/debug/tracing/events/net/netif_rx/trigger
1365
1366  You can see that they're updating common histogram data by reading
1367  each event's hist files at the same time::
1368
1369    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
1370      cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
1371
1372    # event histogram
1373    #
1374    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1375    #
1376
1377    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
1378    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
1379    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
1380    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
1381    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
1382    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
1383    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
1384    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
1385    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
1386    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
1387    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
1388    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
1389    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
1390    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
1391    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
1392    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
1393    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
1394    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
1395    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
1396    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
1397    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
1398    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
1399    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
1400    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
1401    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
1402    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
1403    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
1404    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
1405    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
1406    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
1407    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
1408    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
1409    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
1410    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
1411    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
1412    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
1413    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
1414    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
1415    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
1416    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
1417    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
1418    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276
1419
1420    Totals:
1421        Hits: 81
1422        Entries: 42
1423        Dropped: 0
1424    # event histogram
1425    #
1426    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1427    #
1428
1429    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
1430    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
1431    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
1432    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
1433    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
1434    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
1435    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
1436    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
1437    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
1438    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
1439    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
1440    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
1441    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
1442    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
1443    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
1444    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
1445    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
1446    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
1447    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
1448    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
1449    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
1450    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
1451    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
1452    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
1453    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
1454    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
1455    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
1456    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
1457    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
1458    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
1459    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
1460    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
1461    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
1462    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
1463    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
1464    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
1465    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
1466    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
1467    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
1468    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
1469    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
1470    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276
1471
1472    Totals:
1473        Hits: 81
1474        Entries: 42
1475        Dropped: 0
1476
1477  And here's an example that shows how to combine histogram data from
1478  any two events even if they don't share any 'compatible' fields
1479  other than 'hitcount' and 'stacktrace'.  These commands create a
1480  couple of triggers named 'bar' using those fields::
1481
1482    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
1483           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
1484    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
1485          /sys/kernel/debug/tracing/events/net/netif_rx/trigger
1486
1487  And displaying the output of either shows some interesting if
1488  somewhat confusing output::
1489
1490    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
1491    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
1492
1493    # event histogram
1494    #
1495    # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
1496    #
1497
1498    { stacktrace:
1499             kernel_clone+0x18e/0x330
1500             kernel_thread+0x29/0x30
1501             kthreadd+0x154/0x1b0
1502             ret_from_fork+0x3f/0x70
1503    } hitcount:          1
1504    { stacktrace:
1505             netif_rx_internal+0xb2/0xd0
1506             netif_rx_ni+0x20/0x70
1507             dev_loopback_xmit+0xaa/0xd0
1508             ip_mc_output+0x126/0x240
1509             ip_local_out_sk+0x31/0x40
1510             igmp_send_report+0x1e9/0x230
1511             igmp_timer_expire+0xe9/0x120
1512             call_timer_fn+0x39/0xf0
1513             run_timer_softirq+0x1e1/0x290
1514             __do_softirq+0xfd/0x290
1515             irq_exit+0x98/0xb0
1516             smp_apic_timer_interrupt+0x4a/0x60
1517             apic_timer_interrupt+0x6d/0x80
1518             cpuidle_enter+0x17/0x20
1519             call_cpuidle+0x3b/0x60
1520             cpu_startup_entry+0x22d/0x310
1521    } hitcount:          1
1522    { stacktrace:
1523             netif_rx_internal+0xb2/0xd0
1524             netif_rx_ni+0x20/0x70
1525             dev_loopback_xmit+0xaa/0xd0
1526             ip_mc_output+0x17f/0x240
1527             ip_local_out_sk+0x31/0x40
1528             ip_send_skb+0x1a/0x50
1529             udp_send_skb+0x13e/0x270
1530             udp_sendmsg+0x2bf/0x980
1531             inet_sendmsg+0x67/0xa0
1532             sock_sendmsg+0x38/0x50
1533             SYSC_sendto+0xef/0x170
1534             SyS_sendto+0xe/0x10
1535             entry_SYSCALL_64_fastpath+0x12/0x6a
1536    } hitcount:          2
1537    { stacktrace:
1538             netif_rx_internal+0xb2/0xd0
1539             netif_rx+0x1c/0x60
1540             loopback_xmit+0x6c/0xb0
1541             dev_hard_start_xmit+0x219/0x3a0
1542             __dev_queue_xmit+0x415/0x4f0
1543             dev_queue_xmit_sk+0x13/0x20
1544             ip_finish_output2+0x237/0x340
1545             ip_finish_output+0x113/0x1d0
1546             ip_output+0x66/0xc0
1547             ip_local_out_sk+0x31/0x40
1548             ip_send_skb+0x1a/0x50
1549             udp_send_skb+0x16d/0x270
1550             udp_sendmsg+0x2bf/0x980
1551             inet_sendmsg+0x67/0xa0
1552             sock_sendmsg+0x38/0x50
1553             ___sys_sendmsg+0x14e/0x270
1554    } hitcount:         76
1555    { stacktrace:
1556             netif_rx_internal+0xb2/0xd0
1557             netif_rx+0x1c/0x60
1558             loopback_xmit+0x6c/0xb0
1559             dev_hard_start_xmit+0x219/0x3a0
1560             __dev_queue_xmit+0x415/0x4f0
1561             dev_queue_xmit_sk+0x13/0x20
1562             ip_finish_output2+0x237/0x340
1563             ip_finish_output+0x113/0x1d0
1564             ip_output+0x66/0xc0
1565             ip_local_out_sk+0x31/0x40
1566             ip_send_skb+0x1a/0x50
1567             udp_send_skb+0x16d/0x270
1568             udp_sendmsg+0x2bf/0x980
1569             inet_sendmsg+0x67/0xa0
1570             sock_sendmsg+0x38/0x50
1571             ___sys_sendmsg+0x269/0x270
1572    } hitcount:         77
1573    { stacktrace:
1574             netif_rx_internal+0xb2/0xd0
1575             netif_rx+0x1c/0x60
1576             loopback_xmit+0x6c/0xb0
1577             dev_hard_start_xmit+0x219/0x3a0
1578             __dev_queue_xmit+0x415/0x4f0
1579             dev_queue_xmit_sk+0x13/0x20
1580             ip_finish_output2+0x237/0x340
1581             ip_finish_output+0x113/0x1d0
1582             ip_output+0x66/0xc0
1583             ip_local_out_sk+0x31/0x40
1584             ip_send_skb+0x1a/0x50
1585             udp_send_skb+0x16d/0x270
1586             udp_sendmsg+0x2bf/0x980
1587             inet_sendmsg+0x67/0xa0
1588             sock_sendmsg+0x38/0x50
1589             SYSC_sendto+0xef/0x170
1590    } hitcount:         88
1591    { stacktrace:
1592             kernel_clone+0x18e/0x330
1593             SyS_clone+0x19/0x20
1594             entry_SYSCALL_64_fastpath+0x12/0x6a
1595    } hitcount:        244
1596
1597    Totals:
1598        Hits: 489
1599        Entries: 7
1600        Dropped: 0
1601
16022.2 Inter-event hist triggers
1603-----------------------------
1604
1605Inter-event hist triggers are hist triggers that combine values from
1606one or more other events and create a histogram using that data.  Data
1607from an inter-event histogram can in turn become the source for
1608further combined histograms, thus providing a chain of related
1609histograms, which is important for some applications.
1610
1611The most important example of an inter-event quantity that can be used
1612in this manner is latency, which is simply a difference in timestamps
1613between two events.  Although latency is the most important
1614inter-event quantity, note that because the support is completely
1615general across the trace event subsystem, any event field can be used
1616in an inter-event quantity.
1617
1618An example of a histogram that combines data from other histograms
1619into a useful chain would be a 'wakeupswitch latency' histogram that
1620combines a 'wakeup latency' histogram and a 'switch latency'
1621histogram.
1622
1623Normally, a hist trigger specification consists of a (possibly
1624compound) key along with one or more numeric values, which are
1625continually updated sums associated with that key.  A histogram
1626specification in this case consists of individual key and value
1627specifications that refer to trace event fields associated with a
1628single event type.
1629
1630The inter-event hist trigger extension allows fields from multiple
1631events to be referenced and combined into a multi-event histogram
1632specification.  In support of this overall goal, a few enabling
1633features have been added to the hist trigger support:
1634
1635  - In order to compute an inter-event quantity, a value from one
1636    event needs to saved and then referenced from another event.  This
1637    requires the introduction of support for histogram 'variables'.
1638
1639  - The computation of inter-event quantities and their combination
1640    require some minimal amount of support for applying simple
1641    expressions to variables (+ and -).
1642
1643  - A histogram consisting of inter-event quantities isn't logically a
1644    histogram on either event (so having the 'hist' file for either
1645    event host the histogram output doesn't really make sense).  To
1646    address the idea that the histogram is associated with a
1647    combination of events, support is added allowing the creation of
1648    'synthetic' events that are events derived from other events.
1649    These synthetic events are full-fledged events just like any other
1650    and can be used as such, as for instance to create the
1651    'combination' histograms mentioned previously.
1652
1653  - A set of 'actions' can be associated with histogram entries -
1654    these can be used to generate the previously mentioned synthetic
1655    events, but can also be used for other purposes, such as for
1656    example saving context when a 'max' latency has been hit.
1657
1658  - Trace events don't have a 'timestamp' associated with them, but
1659    there is an implicit timestamp saved along with an event in the
1660    underlying ftrace ring buffer.  This timestamp is now exposed as a
1661    a synthetic field named 'common_timestamp' which can be used in
1662    histograms as if it were any other event field; it isn't an actual
1663    field in the trace format but rather is a synthesized value that
1664    nonetheless can be used as if it were an actual field.  By default
1665    it is in units of nanoseconds; appending '.usecs' to a
1666    common_timestamp field changes the units to microseconds.
1667
1668A note on inter-event timestamps: If common_timestamp is used in a
1669histogram, the trace buffer is automatically switched over to using
1670absolute timestamps and the "global" trace clock, in order to avoid
1671bogus timestamp differences with other clocks that aren't coherent
1672across CPUs.  This can be overridden by specifying one of the other
1673trace clocks instead, using the "clock=XXX" hist trigger attribute,
1674where XXX is any of the clocks listed in the tracing/trace_clock
1675pseudo-file.
1676
1677These features are described in more detail in the following sections.
1678
16792.2.1 Histogram Variables
1680-------------------------
1681
1682Variables are simply named locations used for saving and retrieving
1683values between matching events.  A 'matching' event is defined as an
1684event that has a matching key - if a variable is saved for a histogram
1685entry corresponding to that key, any subsequent event with a matching
1686key can access that variable.
1687
1688A variable's value is normally available to any subsequent event until
1689it is set to something else by a subsequent event.  The one exception
1690to that rule is that any variable used in an expression is essentially
1691'read-once' - once it's used by an expression in a subsequent event,
1692it's reset to its 'unset' state, which means it can't be used again
1693unless it's set again.  This ensures not only that an event doesn't
1694use an uninitialized variable in a calculation, but that that variable
1695is used only once and not for any unrelated subsequent match.
1696
1697The basic syntax for saving a variable is to simply prefix a unique
1698variable name not corresponding to any keyword along with an '=' sign
1699to any event field.
1700
1701Either keys or values can be saved and retrieved in this way.  This
1702creates a variable named 'ts0' for a histogram entry with the key
1703'next_pid'::
1704
1705  # echo 'hist:keys=next_pid:vals=$ts0:ts0=common_timestamp ... >> \
1706	event/trigger
1707
1708The ts0 variable can be accessed by any subsequent event having the
1709same pid as 'next_pid'.
1710
1711Variable references are formed by prepending the variable name with
1712the '$' sign.  Thus for example, the ts0 variable above would be
1713referenced as '$ts0' in expressions.
1714
1715Because 'vals=' is used, the common_timestamp variable value above
1716will also be summed as a normal histogram value would (though for a
1717timestamp it makes little sense).
1718
1719The below shows that a key value can also be saved in the same way::
1720
1721  # echo 'hist:timer_pid=common_pid:key=timer_pid ...' >> event/trigger
1722
1723If a variable isn't a key variable or prefixed with 'vals=', the
1724associated event field will be saved in a variable but won't be summed
1725as a value::
1726
1727  # echo 'hist:keys=next_pid:ts1=common_timestamp ...' >> event/trigger
1728
1729Multiple variables can be assigned at the same time.  The below would
1730result in both ts0 and b being created as variables, with both
1731common_timestamp and field1 additionally being summed as values::
1732
1733  # echo 'hist:keys=pid:vals=$ts0,$b:ts0=common_timestamp,b=field1 ...' >> \
1734	event/trigger
1735
1736Note that variable assignments can appear either preceding or
1737following their use.  The command below behaves identically to the
1738command above::
1739
1740  # echo 'hist:keys=pid:ts0=common_timestamp,b=field1:vals=$ts0,$b ...' >> \
1741	event/trigger
1742
1743Any number of variables not bound to a 'vals=' prefix can also be
1744assigned by simply separating them with colons.  Below is the same
1745thing but without the values being summed in the histogram::
1746
1747  # echo 'hist:keys=pid:ts0=common_timestamp:b=field1 ...' >> event/trigger
1748
1749Variables set as above can be referenced and used in expressions on
1750another event.
1751
1752For example, here's how a latency can be calculated::
1753
1754  # echo 'hist:keys=pid,prio:ts0=common_timestamp ...' >> event1/trigger
1755  # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...' >> event2/trigger
1756
1757In the first line above, the event's timestamp is saved into the
1758variable ts0.  In the next line, ts0 is subtracted from the second
1759event's timestamp to produce the latency, which is then assigned into
1760yet another variable, 'wakeup_lat'.  The hist trigger below in turn
1761makes use of the wakeup_lat variable to compute a combined latency
1762using the same key and variable from yet another event::
1763
1764  # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
1765
1766Expressions support the use of addition, subtraction, multiplication and
1767division operators (+-\*/).
1768
1769Note if division by zero cannot be detected at parse time (i.e. the
1770divisor is not a constant), the result will be -1.
1771
1772Numeric constants can also be used directly in an expression::
1773
1774  # echo 'hist:keys=next_pid:timestamp_secs=common_timestamp/1000000 ...' >> event/trigger
1775
1776or assigned to a variable and referenced in a subsequent expression::
1777
1778  # echo 'hist:keys=next_pid:us_per_sec=1000000 ...' >> event/trigger
1779  # echo 'hist:keys=next_pid:timestamp_secs=common_timestamp/$us_per_sec ...' >> event/trigger
1780
17812.2.2 Synthetic Events
1782----------------------
1783
1784Synthetic events are user-defined events generated from hist trigger
1785variables or fields associated with one or more other events.  Their
1786purpose is to provide a mechanism for displaying data spanning
1787multiple events consistent with the existing and already familiar
1788usage for normal events.
1789
1790To define a synthetic event, the user writes a simple specification
1791consisting of the name of the new event along with one or more
1792variables and their types, which can be any valid field type,
1793separated by semicolons, to the tracing/synthetic_events file.
1794
1795See synth_field_size() for available types.
1796
1797If field_name contains [n], the field is considered to be a static array.
1798
1799If field_names contains[] (no subscript), the field is considered to
1800be a dynamic array, which will only take as much space in the event as
1801is required to hold the array.
1802
1803A string field can be specified using either the static notation:
1804
1805  char name[32];
1806
1807Or the dynamic:
1808
1809  char name[];
1810
1811The size limit for either is 256.
1812
1813For instance, the following creates a new event named 'wakeup_latency'
1814with 3 fields: lat, pid, and prio.  Each of those fields is simply a
1815variable reference to a variable on another event::
1816
1817  # echo 'wakeup_latency \
1818          u64 lat; \
1819          pid_t pid; \
1820	  int prio' >> \
1821	  /sys/kernel/debug/tracing/synthetic_events
1822
1823Reading the tracing/synthetic_events file lists all the currently
1824defined synthetic events, in this case the event defined above::
1825
1826  # cat /sys/kernel/debug/tracing/synthetic_events
1827    wakeup_latency u64 lat; pid_t pid; int prio
1828
1829An existing synthetic event definition can be removed by prepending
1830the command that defined it with a '!'::
1831
1832  # echo '!wakeup_latency u64 lat pid_t pid int prio' >> \
1833    /sys/kernel/debug/tracing/synthetic_events
1834
1835At this point, there isn't yet an actual 'wakeup_latency' event
1836instantiated in the event subsystem - for this to happen, a 'hist
1837trigger action' needs to be instantiated and bound to actual fields
1838and variables defined on other events (see Section 2.2.3 below on
1839how that is done using hist trigger 'onmatch' action). Once that is
1840done, the 'wakeup_latency' synthetic event instance is created.
1841
1842The new event is created under the tracing/events/synthetic/ directory
1843and looks and behaves just like any other event::
1844
1845  # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
1846        enable  filter  format  hist  id  trigger
1847
1848A histogram can now be defined for the new synthetic event::
1849
1850  # echo 'hist:keys=pid,prio,lat.log2:sort=lat' >> \
1851        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
1852
1853The above shows the latency "lat" in a power of 2 grouping.
1854
1855Like any other event, once a histogram is enabled for the event, the
1856output can be displayed by reading the event's 'hist' file.
1857
1858  # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist
1859
1860  # event histogram
1861  #
1862  # trigger info: hist:keys=pid,prio,lat.log2:vals=hitcount:sort=lat.log2:size=2048 [active]
1863  #
1864
1865  { pid:       2035, prio:          9, lat: ~ 2^2  } hitcount:         43
1866  { pid:       2034, prio:          9, lat: ~ 2^2  } hitcount:         60
1867  { pid:       2029, prio:          9, lat: ~ 2^2  } hitcount:        965
1868  { pid:       2034, prio:        120, lat: ~ 2^2  } hitcount:          9
1869  { pid:       2033, prio:        120, lat: ~ 2^2  } hitcount:          5
1870  { pid:       2030, prio:          9, lat: ~ 2^2  } hitcount:        335
1871  { pid:       2030, prio:        120, lat: ~ 2^2  } hitcount:         10
1872  { pid:       2032, prio:        120, lat: ~ 2^2  } hitcount:          1
1873  { pid:       2035, prio:        120, lat: ~ 2^2  } hitcount:          2
1874  { pid:       2031, prio:          9, lat: ~ 2^2  } hitcount:        176
1875  { pid:       2028, prio:        120, lat: ~ 2^2  } hitcount:         15
1876  { pid:       2033, prio:          9, lat: ~ 2^2  } hitcount:         91
1877  { pid:       2032, prio:          9, lat: ~ 2^2  } hitcount:        125
1878  { pid:       2029, prio:        120, lat: ~ 2^2  } hitcount:          4
1879  { pid:       2031, prio:        120, lat: ~ 2^2  } hitcount:          3
1880  { pid:       2029, prio:        120, lat: ~ 2^3  } hitcount:          2
1881  { pid:       2035, prio:          9, lat: ~ 2^3  } hitcount:         41
1882  { pid:       2030, prio:        120, lat: ~ 2^3  } hitcount:          1
1883  { pid:       2032, prio:          9, lat: ~ 2^3  } hitcount:         32
1884  { pid:       2031, prio:          9, lat: ~ 2^3  } hitcount:         44
1885  { pid:       2034, prio:          9, lat: ~ 2^3  } hitcount:         40
1886  { pid:       2030, prio:          9, lat: ~ 2^3  } hitcount:         29
1887  { pid:       2033, prio:          9, lat: ~ 2^3  } hitcount:         31
1888  { pid:       2029, prio:          9, lat: ~ 2^3  } hitcount:         31
1889  { pid:       2028, prio:        120, lat: ~ 2^3  } hitcount:         18
1890  { pid:       2031, prio:        120, lat: ~ 2^3  } hitcount:          2
1891  { pid:       2028, prio:        120, lat: ~ 2^4  } hitcount:          1
1892  { pid:       2029, prio:          9, lat: ~ 2^4  } hitcount:          4
1893  { pid:       2031, prio:        120, lat: ~ 2^7  } hitcount:          1
1894  { pid:       2032, prio:        120, lat: ~ 2^7  } hitcount:          1
1895
1896  Totals:
1897      Hits: 2122
1898      Entries: 30
1899      Dropped: 0
1900
1901
1902The latency values can also be grouped linearly by a given size with
1903the ".buckets" modifier and specify a size (in this case groups of 10).
1904
1905  # echo 'hist:keys=pid,prio,lat.buckets=10:sort=lat' >> \
1906        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
1907
1908  # event histogram
1909  #
1910  # trigger info: hist:keys=pid,prio,lat.buckets=10:vals=hitcount:sort=lat.buckets=10:size=2048 [active]
1911  #
1912
1913  { pid:       2067, prio:          9, lat: ~ 0-9 } hitcount:        220
1914  { pid:       2068, prio:          9, lat: ~ 0-9 } hitcount:        157
1915  { pid:       2070, prio:          9, lat: ~ 0-9 } hitcount:        100
1916  { pid:       2067, prio:        120, lat: ~ 0-9 } hitcount:          6
1917  { pid:       2065, prio:        120, lat: ~ 0-9 } hitcount:          2
1918  { pid:       2066, prio:        120, lat: ~ 0-9 } hitcount:          2
1919  { pid:       2069, prio:          9, lat: ~ 0-9 } hitcount:        122
1920  { pid:       2069, prio:        120, lat: ~ 0-9 } hitcount:          8
1921  { pid:       2070, prio:        120, lat: ~ 0-9 } hitcount:          1
1922  { pid:       2068, prio:        120, lat: ~ 0-9 } hitcount:          7
1923  { pid:       2066, prio:          9, lat: ~ 0-9 } hitcount:        365
1924  { pid:       2064, prio:        120, lat: ~ 0-9 } hitcount:         35
1925  { pid:       2065, prio:          9, lat: ~ 0-9 } hitcount:        998
1926  { pid:       2071, prio:          9, lat: ~ 0-9 } hitcount:         85
1927  { pid:       2065, prio:          9, lat: ~ 10-19 } hitcount:          2
1928  { pid:       2064, prio:        120, lat: ~ 10-19 } hitcount:          2
1929
1930  Totals:
1931      Hits: 2112
1932      Entries: 16
1933      Dropped: 0
1934
19352.2.3 Hist trigger 'handlers' and 'actions'
1936-------------------------------------------
1937
1938A hist trigger 'action' is a function that's executed (in most cases
1939conditionally) whenever a histogram entry is added or updated.
1940
1941When a histogram entry is added or updated, a hist trigger 'handler'
1942is what decides whether the corresponding action is actually invoked
1943or not.
1944
1945Hist trigger handlers and actions are paired together in the general
1946form:
1947
1948  <handler>.<action>
1949
1950To specify a handler.action pair for a given event, simply specify
1951that handler.action pair between colons in the hist trigger
1952specification.
1953
1954In theory, any handler can be combined with any action, but in
1955practice, not every handler.action combination is currently supported;
1956if a given handler.action combination isn't supported, the hist
1957trigger will fail with -EINVAL;
1958
1959The default 'handler.action' if none is explicitly specified is as it
1960always has been, to simply update the set of values associated with an
1961entry.  Some applications, however, may want to perform additional
1962actions at that point, such as generate another event, or compare and
1963save a maximum.
1964
1965The supported handlers and actions are listed below, and each is
1966described in more detail in the following paragraphs, in the context
1967of descriptions of some common and useful handler.action combinations.
1968
1969The available handlers are:
1970
1971  - onmatch(matching.event)    - invoke action on any addition or update
1972  - onmax(var)                 - invoke action if var exceeds current max
1973  - onchange(var)              - invoke action if var changes
1974
1975The available actions are:
1976
1977  - trace(<synthetic_event_name>,param list)   - generate synthetic event
1978  - save(field,...)                            - save current event fields
1979  - snapshot()                                 - snapshot the trace buffer
1980
1981The following commonly-used handler.action pairs are available:
1982
1983  - onmatch(matching.event).trace(<synthetic_event_name>,param list)
1984
1985    The 'onmatch(matching.event).trace(<synthetic_event_name>,param
1986    list)' hist trigger action is invoked whenever an event matches
1987    and the histogram entry would be added or updated.  It causes the
1988    named synthetic event to be generated with the values given in the
1989    'param list'.  The result is the generation of a synthetic event
1990    that consists of the values contained in those variables at the
1991    time the invoking event was hit.  For example, if the synthetic
1992    event name is 'wakeup_latency', a wakeup_latency event is
1993    generated using onmatch(event).trace(wakeup_latency,arg1,arg2).
1994
1995    There is also an equivalent alternative form available for
1996    generating synthetic events.  In this form, the synthetic event
1997    name is used as if it were a function name.  For example, using
1998    the 'wakeup_latency' synthetic event name again, the
1999    wakeup_latency event would be generated by invoking it as if it
2000    were a function call, with the event field values passed in as
2001    arguments: onmatch(event).wakeup_latency(arg1,arg2).  The syntax
2002    for this form is:
2003
2004      onmatch(matching.event).<synthetic_event_name>(param list)
2005
2006    In either case, the 'param list' consists of one or more
2007    parameters which may be either variables or fields defined on
2008    either the 'matching.event' or the target event.  The variables or
2009    fields specified in the param list may be either fully-qualified
2010    or unqualified.  If a variable is specified as unqualified, it
2011    must be unique between the two events.  A field name used as a
2012    param can be unqualified if it refers to the target event, but
2013    must be fully qualified if it refers to the matching event.  A
2014    fully-qualified name is of the form 'system.event_name.$var_name'
2015    or 'system.event_name.field'.
2016
2017    The 'matching.event' specification is simply the fully qualified
2018    event name of the event that matches the target event for the
2019    onmatch() functionality, in the form 'system.event_name'. Histogram
2020    keys of both events are compared to find if events match. In case
2021    multiple histogram keys are used, they all must match in the specified
2022    order.
2023
2024    Finally, the number and type of variables/fields in the 'param
2025    list' must match the number and types of the fields in the
2026    synthetic event being generated.
2027
2028    As an example the below defines a simple synthetic event and uses
2029    a variable defined on the sched_wakeup_new event as a parameter
2030    when invoking the synthetic event.  Here we define the synthetic
2031    event::
2032
2033      # echo 'wakeup_new_test pid_t pid' >> \
2034             /sys/kernel/debug/tracing/synthetic_events
2035
2036      # cat /sys/kernel/debug/tracing/synthetic_events
2037            wakeup_new_test pid_t pid
2038
2039    The following hist trigger both defines the missing testpid
2040    variable and specifies an onmatch() action that generates a
2041    wakeup_new_test synthetic event whenever a sched_wakeup_new event
2042    occurs, which because of the 'if comm == "cyclictest"' filter only
2043    happens when the executable is cyclictest::
2044
2045      # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
2046              wakeup_new_test($testpid) if comm=="cyclictest"' >> \
2047              /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
2048
2049    Or, equivalently, using the 'trace' keyword syntax:
2050
2051    # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
2052            trace(wakeup_new_test,$testpid) if comm=="cyclictest"' >> \
2053            /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
2054
2055    Creating and displaying a histogram based on those events is now
2056    just a matter of using the fields and new synthetic event in the
2057    tracing/events/synthetic directory, as usual::
2058
2059      # echo 'hist:keys=pid:sort=pid' >> \
2060             /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/trigger
2061
2062    Running 'cyclictest' should cause wakeup_new events to generate
2063    wakeup_new_test synthetic events which should result in histogram
2064    output in the wakeup_new_test event's hist file::
2065
2066      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/hist
2067
2068    A more typical usage would be to use two events to calculate a
2069    latency.  The following example uses a set of hist triggers to
2070    produce a 'wakeup_latency' histogram.
2071
2072    First, we define a 'wakeup_latency' synthetic event::
2073
2074      # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
2075              /sys/kernel/debug/tracing/synthetic_events
2076
2077    Next, we specify that whenever we see a sched_waking event for a
2078    cyclictest thread, save the timestamp in a 'ts0' variable::
2079
2080      # echo 'hist:keys=$saved_pid:saved_pid=pid:ts0=common_timestamp.usecs \
2081              if comm=="cyclictest"' >> \
2082	      /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
2083
2084    Then, when the corresponding thread is actually scheduled onto the
2085    CPU by a sched_switch event (saved_pid matches next_pid), calculate
2086    the latency and use that along with another variable and an event field
2087    to generate a wakeup_latency synthetic event::
2088
2089      # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
2090              onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\
2091	              $saved_pid,next_prio) if next_comm=="cyclictest"' >> \
2092	      /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
2093
2094    We also need to create a histogram on the wakeup_latency synthetic
2095    event in order to aggregate the generated synthetic event data::
2096
2097      # echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
2098              /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
2099
2100    Finally, once we've run cyclictest to actually generate some
2101    events, we can see the output by looking at the wakeup_latency
2102    synthetic event's hist file::
2103
2104      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist
2105
2106  - onmax(var).save(field,..	.)
2107
2108    The 'onmax(var).save(field,...)' hist trigger action is invoked
2109    whenever the value of 'var' associated with a histogram entry
2110    exceeds the current maximum contained in that variable.
2111
2112    The end result is that the trace event fields specified as the
2113    onmax.save() params will be saved if 'var' exceeds the current
2114    maximum for that hist trigger entry.  This allows context from the
2115    event that exhibited the new maximum to be saved for later
2116    reference.  When the histogram is displayed, additional fields
2117    displaying the saved values will be printed.
2118
2119    As an example the below defines a couple of hist triggers, one for
2120    sched_waking and another for sched_switch, keyed on pid.  Whenever
2121    a sched_waking occurs, the timestamp is saved in the entry
2122    corresponding to the current pid, and when the scheduler switches
2123    back to that pid, the timestamp difference is calculated.  If the
2124    resulting latency, stored in wakeup_lat, exceeds the current
2125    maximum latency, the values specified in the save() fields are
2126    recorded::
2127
2128      # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
2129              if comm=="cyclictest"' >> \
2130              /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
2131
2132      # echo 'hist:keys=next_pid:\
2133              wakeup_lat=common_timestamp.usecs-$ts0:\
2134              onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) \
2135              if next_comm=="cyclictest"' >> \
2136              /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
2137
2138    When the histogram is displayed, the max value and the saved
2139    values corresponding to the max are displayed following the rest
2140    of the fields::
2141
2142      # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
2143        { next_pid:       2255 } hitcount:        239
2144          common_timestamp-ts0:          0
2145          max:         27
2146	  next_comm: cyclictest
2147          prev_pid:          0  prev_prio:        120  prev_comm: swapper/1
2148
2149        { next_pid:       2256 } hitcount:       2355
2150          common_timestamp-ts0: 0
2151          max:         49  next_comm: cyclictest
2152          prev_pid:          0  prev_prio:        120  prev_comm: swapper/0
2153
2154        Totals:
2155            Hits: 12970
2156            Entries: 2
2157            Dropped: 0
2158
2159  - onmax(var).snapshot()
2160
2161    The 'onmax(var).snapshot()' hist trigger action is invoked
2162    whenever the value of 'var' associated with a histogram entry
2163    exceeds the current maximum contained in that variable.
2164
2165    The end result is that a global snapshot of the trace buffer will
2166    be saved in the tracing/snapshot file if 'var' exceeds the current
2167    maximum for any hist trigger entry.
2168
2169    Note that in this case the maximum is a global maximum for the
2170    current trace instance, which is the maximum across all buckets of
2171    the histogram.  The key of the specific trace event that caused
2172    the global maximum and the global maximum itself are displayed,
2173    along with a message stating that a snapshot has been taken and
2174    where to find it.  The user can use the key information displayed
2175    to locate the corresponding bucket in the histogram for even more
2176    detail.
2177
2178    As an example the below defines a couple of hist triggers, one for
2179    sched_waking and another for sched_switch, keyed on pid.  Whenever
2180    a sched_waking event occurs, the timestamp is saved in the entry
2181    corresponding to the current pid, and when the scheduler switches
2182    back to that pid, the timestamp difference is calculated.  If the
2183    resulting latency, stored in wakeup_lat, exceeds the current
2184    maximum latency, a snapshot is taken.  As part of the setup, all
2185    the scheduler events are also enabled, which are the events that
2186    will show up in the snapshot when it is taken at some point:
2187
2188    # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
2189
2190    # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
2191            if comm=="cyclictest"' >> \
2192            /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
2193
2194    # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
2195            onmax($wakeup_lat).save(next_prio,next_comm,prev_pid,prev_prio, \
2196	    prev_comm):onmax($wakeup_lat).snapshot() \
2197	    if next_comm=="cyclictest"' >> \
2198	    /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
2199
2200    When the histogram is displayed, for each bucket the max value
2201    and the saved values corresponding to the max are displayed
2202    following the rest of the fields.
2203
2204    If a snapshot was taken, there is also a message indicating that,
2205    along with the value and event that triggered the global maximum:
2206
2207    # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
2208      { next_pid:       2101 } hitcount:        200
2209	max:         52  next_prio:        120  next_comm: cyclictest \
2210        prev_pid:          0  prev_prio:        120  prev_comm: swapper/6
2211
2212      { next_pid:       2103 } hitcount:       1326
2213	max:        572  next_prio:         19  next_comm: cyclictest \
2214        prev_pid:          0  prev_prio:        120  prev_comm: swapper/1
2215
2216      { next_pid:       2102 } hitcount:       1982 \
2217	max:         74  next_prio:         19  next_comm: cyclictest \
2218        prev_pid:          0  prev_prio:        120  prev_comm: swapper/5
2219
2220    Snapshot taken (see tracing/snapshot).  Details:
2221	triggering value { onmax($wakeup_lat) }:        572	\
2222	triggered by event with key: { next_pid:       2103 }
2223
2224    Totals:
2225        Hits: 3508
2226        Entries: 3
2227        Dropped: 0
2228
2229    In the above case, the event that triggered the global maximum has
2230    the key with next_pid == 2103.  If you look at the bucket that has
2231    2103 as the key, you'll find the additional values save()'d along
2232    with the local maximum for that bucket, which should be the same
2233    as the global maximum (since that was the same value that
2234    triggered the global snapshot).
2235
2236    And finally, looking at the snapshot data should show at or near
2237    the end the event that triggered the snapshot (in this case you
2238    can verify the timestamps between the sched_waking and
2239    sched_switch events, which should match the time displayed in the
2240    global maximum)::
2241
2242     # cat /sys/kernel/debug/tracing/snapshot
2243
2244         <...>-2103  [005] d..3   309.873125: sched_switch: prev_comm=cyclictest prev_pid=2103 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
2245         <idle>-0     [005] d.h3   309.873611: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
2246         <idle>-0     [005] dNh4   309.873613: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
2247         <idle>-0     [005] d..3   309.873616: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
2248         <...>-2102  [005] d..3   309.873625: sched_switch: prev_comm=cyclictest prev_pid=2102 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
2249         <idle>-0     [005] d.h3   309.874624: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
2250         <idle>-0     [005] dNh4   309.874626: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
2251         <idle>-0     [005] dNh3   309.874628: sched_waking: comm=cyclictest pid=2103 prio=19 target_cpu=005
2252         <idle>-0     [005] dNh4   309.874630: sched_wakeup: comm=cyclictest pid=2103 prio=19 target_cpu=005
2253         <idle>-0     [005] d..3   309.874633: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
2254         <idle>-0     [004] d.h3   309.874757: sched_waking: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
2255         <idle>-0     [004] dNh4   309.874762: sched_wakeup: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
2256         <idle>-0     [004] d..3   309.874766: sched_switch: prev_comm=swapper/4 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=gnome-terminal- next_pid=1699 next_prio=120
2257     gnome-terminal--1699  [004] d.h2   309.874941: sched_stat_runtime: comm=gnome-terminal- pid=1699 runtime=180706 [ns] vruntime=1126870572 [ns]
2258         <idle>-0     [003] d.s4   309.874956: sched_waking: comm=rcu_sched pid=9 prio=120 target_cpu=007
2259         <idle>-0     [003] d.s5   309.874960: sched_wake_idle_without_ipi: cpu=7
2260         <idle>-0     [003] d.s5   309.874961: sched_wakeup: comm=rcu_sched pid=9 prio=120 target_cpu=007
2261         <idle>-0     [007] d..3   309.874963: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=rcu_sched next_pid=9 next_prio=120
2262      rcu_sched-9     [007] d..3   309.874973: sched_stat_runtime: comm=rcu_sched pid=9 runtime=13646 [ns] vruntime=22531430286 [ns]
2263      rcu_sched-9     [007] d..3   309.874978: sched_switch: prev_comm=rcu_sched prev_pid=9 prev_prio=120 prev_state=R+ ==> next_comm=swapper/7 next_pid=0 next_prio=120
2264          <...>-2102  [005] d..4   309.874994: sched_migrate_task: comm=cyclictest pid=2103 prio=19 orig_cpu=5 dest_cpu=1
2265          <...>-2102  [005] d..4   309.875185: sched_wake_idle_without_ipi: cpu=1
2266         <idle>-0     [001] d..3   309.875200: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2103 next_prio=19
2267
2268  - onchange(var).save(field,..	.)
2269
2270    The 'onchange(var).save(field,...)' hist trigger action is invoked
2271    whenever the value of 'var' associated with a histogram entry
2272    changes.
2273
2274    The end result is that the trace event fields specified as the
2275    onchange.save() params will be saved if 'var' changes for that
2276    hist trigger entry.  This allows context from the event that
2277    changed the value to be saved for later reference.  When the
2278    histogram is displayed, additional fields displaying the saved
2279    values will be printed.
2280
2281  - onchange(var).snapshot()
2282
2283    The 'onchange(var).snapshot()' hist trigger action is invoked
2284    whenever the value of 'var' associated with a histogram entry
2285    changes.
2286
2287    The end result is that a global snapshot of the trace buffer will
2288    be saved in the tracing/snapshot file if 'var' changes for any
2289    hist trigger entry.
2290
2291    Note that in this case the changed value is a global variable
2292    associated with current trace instance.  The key of the specific
2293    trace event that caused the value to change and the global value
2294    itself are displayed, along with a message stating that a snapshot
2295    has been taken and where to find it.  The user can use the key
2296    information displayed to locate the corresponding bucket in the
2297    histogram for even more detail.
2298
2299    As an example the below defines a hist trigger on the tcp_probe
2300    event, keyed on dport.  Whenever a tcp_probe event occurs, the
2301    cwnd field is checked against the current value stored in the
2302    $cwnd variable.  If the value has changed, a snapshot is taken.
2303    As part of the setup, all the scheduler and tcp events are also
2304    enabled, which are the events that will show up in the snapshot
2305    when it is taken at some point:
2306
2307    # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
2308    # echo 1 > /sys/kernel/debug/tracing/events/tcp/enable
2309
2310    # echo 'hist:keys=dport:cwnd=snd_cwnd: \
2311            onchange($cwnd).save(snd_wnd,srtt,rcv_wnd): \
2312	    onchange($cwnd).snapshot()' >> \
2313	    /sys/kernel/debug/tracing/events/tcp/tcp_probe/trigger
2314
2315    When the histogram is displayed, for each bucket the tracked value
2316    and the saved values corresponding to that value are displayed
2317    following the rest of the fields.
2318
2319    If a snapshot was taken, there is also a message indicating that,
2320    along with the value and event that triggered the snapshot::
2321
2322      # cat /sys/kernel/debug/tracing/events/tcp/tcp_probe/hist
2323
2324      { dport:       1521 } hitcount:          8
2325	changed:         10  snd_wnd:      35456  srtt:     154262  rcv_wnd:      42112
2326
2327      { dport:         80 } hitcount:         23
2328	changed:         10  snd_wnd:      28960  srtt:      19604  rcv_wnd:      29312
2329
2330      { dport:       9001 } hitcount:        172
2331	changed:         10  snd_wnd:      48384  srtt:     260444  rcv_wnd:      55168
2332
2333      { dport:        443 } hitcount:        211
2334	changed:         10  snd_wnd:      26960  srtt:      17379  rcv_wnd:      28800
2335
2336    Snapshot taken (see tracing/snapshot).  Details::
2337
2338        triggering value { onchange($cwnd) }:         10
2339        triggered by event with key: { dport:         80 }
2340
2341      Totals:
2342          Hits: 414
2343          Entries: 4
2344          Dropped: 0
2345
2346    In the above case, the event that triggered the snapshot has the
2347    key with dport == 80.  If you look at the bucket that has 80 as
2348    the key, you'll find the additional values save()'d along with the
2349    changed value for that bucket, which should be the same as the
2350    global changed value (since that was the same value that triggered
2351    the global snapshot).
2352
2353    And finally, looking at the snapshot data should show at or near
2354    the end the event that triggered the snapshot::
2355
2356      # cat /sys/kernel/debug/tracing/snapshot
2357
2358         gnome-shell-1261  [006] dN.3    49.823113: sched_stat_runtime: comm=gnome-shell pid=1261 runtime=49347 [ns] vruntime=1835730389 [ns]
2359       kworker/u16:4-773   [003] d..3    49.823114: sched_switch: prev_comm=kworker/u16:4 prev_pid=773 prev_prio=120 prev_state=R+ ==> next_comm=kworker/3:2 next_pid=135 next_prio=120
2360         gnome-shell-1261  [006] d..3    49.823114: sched_switch: prev_comm=gnome-shell prev_pid=1261 prev_prio=120 prev_state=R+ ==> next_comm=kworker/6:2 next_pid=387 next_prio=120
2361         kworker/3:2-135   [003] d..3    49.823118: sched_stat_runtime: comm=kworker/3:2 pid=135 runtime=5339 [ns] vruntime=17815800388 [ns]
2362         kworker/6:2-387   [006] d..3    49.823120: sched_stat_runtime: comm=kworker/6:2 pid=387 runtime=9594 [ns] vruntime=14589605367 [ns]
2363         kworker/6:2-387   [006] d..3    49.823122: sched_switch: prev_comm=kworker/6:2 prev_pid=387 prev_prio=120 prev_state=R+ ==> next_comm=gnome-shell next_pid=1261 next_prio=120
2364         kworker/3:2-135   [003] d..3    49.823123: sched_switch: prev_comm=kworker/3:2 prev_pid=135 prev_prio=120 prev_state=T ==> next_comm=swapper/3 next_pid=0 next_prio=120
2365              <idle>-0     [004] ..s7    49.823798: tcp_probe: src=10.0.0.10:54326 dest=23.215.104.193:80 mark=0x0 length=32 snd_nxt=0xe3ae2ff5 snd_una=0xe3ae2ecd snd_cwnd=10 ssthresh=2147483647 snd_wnd=28960 srtt=19604 rcv_wnd=29312
2366
23673. User space creating a trigger
2368--------------------------------
2369
2370Writing into /sys/kernel/tracing/trace_marker writes into the ftrace
2371ring buffer. This can also act like an event, by writing into the trigger
2372file located in /sys/kernel/tracing/events/ftrace/print/
2373
2374Modifying cyclictest to write into the trace_marker file before it sleeps
2375and after it wakes up, something like this::
2376
2377  static void traceputs(char *str)
2378  {
2379	/* tracemark_fd is the trace_marker file descriptor */
2380	if (tracemark_fd < 0)
2381		return;
2382	/* write the tracemark message */
2383	write(tracemark_fd, str, strlen(str));
2384  }
2385
2386And later add something like::
2387
2388	traceputs("start");
2389	clock_nanosleep(...);
2390	traceputs("end");
2391
2392We can make a histogram from this::
2393
2394 # cd /sys/kernel/tracing
2395 # echo 'latency u64 lat' > synthetic_events
2396 # echo 'hist:keys=common_pid:ts0=common_timestamp.usecs if buf == "start"' > events/ftrace/print/trigger
2397 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(ftrace.print).latency($lat) if buf == "end"' >> events/ftrace/print/trigger
2398 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
2399
2400The above created a synthetic event called "latency" and two histograms
2401against the trace_marker, one gets triggered when "start" is written into the
2402trace_marker file and the other when "end" is written. If the pids match, then
2403it will call the "latency" synthetic event with the calculated latency as its
2404parameter. Finally, a histogram is added to the latency synthetic event to
2405record the calculated latency along with the pid.
2406
2407Now running cyclictest with::
2408
2409 # ./cyclictest -p80 -d0 -i250 -n -a -t --tracemark -b 1000
2410
2411 -p80  : run threads at priority 80
2412 -d0   : have all threads run at the same interval
2413 -i250 : start the interval at 250 microseconds (all threads will do this)
2414 -n    : sleep with nanosleep
2415 -a    : affine all threads to a separate CPU
2416 -t    : one thread per available CPU
2417 --tracemark : enable trace mark writing
2418 -b 1000 : stop if any latency is greater than 1000 microseconds
2419
2420Note, the -b 1000 is used just to make --tracemark available.
2421
2422Then we can see the histogram created by this with::
2423
2424 # cat events/synthetic/latency/hist
2425 # event histogram
2426 #
2427 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
2428 #
2429
2430 { lat:        107, common_pid:       2039 } hitcount:          1
2431 { lat:        122, common_pid:       2041 } hitcount:          1
2432 { lat:        166, common_pid:       2039 } hitcount:          1
2433 { lat:        174, common_pid:       2039 } hitcount:          1
2434 { lat:        194, common_pid:       2041 } hitcount:          1
2435 { lat:        196, common_pid:       2036 } hitcount:          1
2436 { lat:        197, common_pid:       2038 } hitcount:          1
2437 { lat:        198, common_pid:       2039 } hitcount:          1
2438 { lat:        199, common_pid:       2039 } hitcount:          1
2439 { lat:        200, common_pid:       2041 } hitcount:          1
2440 { lat:        201, common_pid:       2039 } hitcount:          2
2441 { lat:        202, common_pid:       2038 } hitcount:          1
2442 { lat:        202, common_pid:       2043 } hitcount:          1
2443 { lat:        203, common_pid:       2039 } hitcount:          1
2444 { lat:        203, common_pid:       2036 } hitcount:          1
2445 { lat:        203, common_pid:       2041 } hitcount:          1
2446 { lat:        206, common_pid:       2038 } hitcount:          2
2447 { lat:        207, common_pid:       2039 } hitcount:          1
2448 { lat:        207, common_pid:       2036 } hitcount:          1
2449 { lat:        208, common_pid:       2040 } hitcount:          1
2450 { lat:        209, common_pid:       2043 } hitcount:          1
2451 { lat:        210, common_pid:       2039 } hitcount:          1
2452 { lat:        211, common_pid:       2039 } hitcount:          4
2453 { lat:        212, common_pid:       2043 } hitcount:          1
2454 { lat:        212, common_pid:       2039 } hitcount:          2
2455 { lat:        213, common_pid:       2039 } hitcount:          1
2456 { lat:        214, common_pid:       2038 } hitcount:          1
2457 { lat:        214, common_pid:       2039 } hitcount:          2
2458 { lat:        214, common_pid:       2042 } hitcount:          1
2459 { lat:        215, common_pid:       2039 } hitcount:          1
2460 { lat:        217, common_pid:       2036 } hitcount:          1
2461 { lat:        217, common_pid:       2040 } hitcount:          1
2462 { lat:        217, common_pid:       2039 } hitcount:          1
2463 { lat:        218, common_pid:       2039 } hitcount:          6
2464 { lat:        219, common_pid:       2039 } hitcount:          9
2465 { lat:        220, common_pid:       2039 } hitcount:         11
2466 { lat:        221, common_pid:       2039 } hitcount:          5
2467 { lat:        221, common_pid:       2042 } hitcount:          1
2468 { lat:        222, common_pid:       2039 } hitcount:          7
2469 { lat:        223, common_pid:       2036 } hitcount:          1
2470 { lat:        223, common_pid:       2039 } hitcount:          3
2471 { lat:        224, common_pid:       2039 } hitcount:          4
2472 { lat:        224, common_pid:       2037 } hitcount:          1
2473 { lat:        224, common_pid:       2036 } hitcount:          2
2474 { lat:        225, common_pid:       2039 } hitcount:          5
2475 { lat:        225, common_pid:       2042 } hitcount:          1
2476 { lat:        226, common_pid:       2039 } hitcount:          7
2477 { lat:        226, common_pid:       2036 } hitcount:          4
2478 { lat:        227, common_pid:       2039 } hitcount:          6
2479 { lat:        227, common_pid:       2036 } hitcount:         12
2480 { lat:        227, common_pid:       2043 } hitcount:          1
2481 { lat:        228, common_pid:       2039 } hitcount:          7
2482 { lat:        228, common_pid:       2036 } hitcount:         14
2483 { lat:        229, common_pid:       2039 } hitcount:          9
2484 { lat:        229, common_pid:       2036 } hitcount:          8
2485 { lat:        229, common_pid:       2038 } hitcount:          1
2486 { lat:        230, common_pid:       2039 } hitcount:         11
2487 { lat:        230, common_pid:       2036 } hitcount:          6
2488 { lat:        230, common_pid:       2043 } hitcount:          1
2489 { lat:        230, common_pid:       2042 } hitcount:          2
2490 { lat:        231, common_pid:       2041 } hitcount:          1
2491 { lat:        231, common_pid:       2036 } hitcount:          6
2492 { lat:        231, common_pid:       2043 } hitcount:          1
2493 { lat:        231, common_pid:       2039 } hitcount:          8
2494 { lat:        232, common_pid:       2037 } hitcount:          1
2495 { lat:        232, common_pid:       2039 } hitcount:          6
2496 { lat:        232, common_pid:       2040 } hitcount:          2
2497 { lat:        232, common_pid:       2036 } hitcount:          5
2498 { lat:        232, common_pid:       2043 } hitcount:          1
2499 { lat:        233, common_pid:       2036 } hitcount:          5
2500 { lat:        233, common_pid:       2039 } hitcount:         11
2501 { lat:        234, common_pid:       2039 } hitcount:          4
2502 { lat:        234, common_pid:       2038 } hitcount:          2
2503 { lat:        234, common_pid:       2043 } hitcount:          2
2504 { lat:        234, common_pid:       2036 } hitcount:         11
2505 { lat:        234, common_pid:       2040 } hitcount:          1
2506 { lat:        235, common_pid:       2037 } hitcount:          2
2507 { lat:        235, common_pid:       2036 } hitcount:          8
2508 { lat:        235, common_pid:       2043 } hitcount:          2
2509 { lat:        235, common_pid:       2039 } hitcount:          5
2510 { lat:        235, common_pid:       2042 } hitcount:          2
2511 { lat:        235, common_pid:       2040 } hitcount:          4
2512 { lat:        235, common_pid:       2041 } hitcount:          1
2513 { lat:        236, common_pid:       2036 } hitcount:          7
2514 { lat:        236, common_pid:       2037 } hitcount:          1
2515 { lat:        236, common_pid:       2041 } hitcount:          5
2516 { lat:        236, common_pid:       2039 } hitcount:          3
2517 { lat:        236, common_pid:       2043 } hitcount:          9
2518 { lat:        236, common_pid:       2040 } hitcount:          7
2519 { lat:        237, common_pid:       2037 } hitcount:          1
2520 { lat:        237, common_pid:       2040 } hitcount:          1
2521 { lat:        237, common_pid:       2036 } hitcount:          9
2522 { lat:        237, common_pid:       2039 } hitcount:          3
2523 { lat:        237, common_pid:       2043 } hitcount:          8
2524 { lat:        237, common_pid:       2042 } hitcount:          2
2525 { lat:        237, common_pid:       2041 } hitcount:          2
2526 { lat:        238, common_pid:       2043 } hitcount:         10
2527 { lat:        238, common_pid:       2040 } hitcount:          1
2528 { lat:        238, common_pid:       2037 } hitcount:          9
2529 { lat:        238, common_pid:       2038 } hitcount:          1
2530 { lat:        238, common_pid:       2039 } hitcount:          1
2531 { lat:        238, common_pid:       2042 } hitcount:          3
2532 { lat:        238, common_pid:       2036 } hitcount:          7
2533 { lat:        239, common_pid:       2041 } hitcount:          1
2534 { lat:        239, common_pid:       2043 } hitcount:         11
2535 { lat:        239, common_pid:       2037 } hitcount:         11
2536 { lat:        239, common_pid:       2038 } hitcount:          6
2537 { lat:        239, common_pid:       2036 } hitcount:          7
2538 { lat:        239, common_pid:       2040 } hitcount:          1
2539 { lat:        239, common_pid:       2042 } hitcount:          9
2540 { lat:        240, common_pid:       2037 } hitcount:         29
2541 { lat:        240, common_pid:       2043 } hitcount:         15
2542 { lat:        240, common_pid:       2040 } hitcount:         44
2543 { lat:        240, common_pid:       2039 } hitcount:          1
2544 { lat:        240, common_pid:       2041 } hitcount:          2
2545 { lat:        240, common_pid:       2038 } hitcount:          1
2546 { lat:        240, common_pid:       2036 } hitcount:         10
2547 { lat:        240, common_pid:       2042 } hitcount:         13
2548 { lat:        241, common_pid:       2036 } hitcount:         21
2549 { lat:        241, common_pid:       2041 } hitcount:         36
2550 { lat:        241, common_pid:       2037 } hitcount:         34
2551 { lat:        241, common_pid:       2042 } hitcount:         14
2552 { lat:        241, common_pid:       2040 } hitcount:         94
2553 { lat:        241, common_pid:       2039 } hitcount:         12
2554 { lat:        241, common_pid:       2038 } hitcount:          2
2555 { lat:        241, common_pid:       2043 } hitcount:         28
2556 { lat:        242, common_pid:       2040 } hitcount:        109
2557 { lat:        242, common_pid:       2041 } hitcount:        506
2558 { lat:        242, common_pid:       2039 } hitcount:        155
2559 { lat:        242, common_pid:       2042 } hitcount:         21
2560 { lat:        242, common_pid:       2037 } hitcount:         52
2561 { lat:        242, common_pid:       2043 } hitcount:         21
2562 { lat:        242, common_pid:       2036 } hitcount:         16
2563 { lat:        242, common_pid:       2038 } hitcount:        156
2564 { lat:        243, common_pid:       2037 } hitcount:         46
2565 { lat:        243, common_pid:       2039 } hitcount:         40
2566 { lat:        243, common_pid:       2042 } hitcount:        119
2567 { lat:        243, common_pid:       2041 } hitcount:        611
2568 { lat:        243, common_pid:       2036 } hitcount:         69
2569 { lat:        243, common_pid:       2038 } hitcount:        784
2570 { lat:        243, common_pid:       2040 } hitcount:        323
2571 { lat:        243, common_pid:       2043 } hitcount:         14
2572 { lat:        244, common_pid:       2043 } hitcount:         35
2573 { lat:        244, common_pid:       2042 } hitcount:        305
2574 { lat:        244, common_pid:       2039 } hitcount:          8
2575 { lat:        244, common_pid:       2040 } hitcount:       4515
2576 { lat:        244, common_pid:       2038 } hitcount:        371
2577 { lat:        244, common_pid:       2037 } hitcount:         31
2578 { lat:        244, common_pid:       2036 } hitcount:        114
2579 { lat:        244, common_pid:       2041 } hitcount:       3396
2580 { lat:        245, common_pid:       2036 } hitcount:        700
2581 { lat:        245, common_pid:       2041 } hitcount:       2772
2582 { lat:        245, common_pid:       2037 } hitcount:        268
2583 { lat:        245, common_pid:       2039 } hitcount:        472
2584 { lat:        245, common_pid:       2038 } hitcount:       2758
2585 { lat:        245, common_pid:       2042 } hitcount:       3833
2586 { lat:        245, common_pid:       2040 } hitcount:       3105
2587 { lat:        245, common_pid:       2043 } hitcount:        645
2588 { lat:        246, common_pid:       2038 } hitcount:       3451
2589 { lat:        246, common_pid:       2041 } hitcount:        142
2590 { lat:        246, common_pid:       2037 } hitcount:       5101
2591 { lat:        246, common_pid:       2040 } hitcount:         68
2592 { lat:        246, common_pid:       2043 } hitcount:       5099
2593 { lat:        246, common_pid:       2039 } hitcount:       5608
2594 { lat:        246, common_pid:       2042 } hitcount:       3723
2595 { lat:        246, common_pid:       2036 } hitcount:       4738
2596 { lat:        247, common_pid:       2042 } hitcount:        312
2597 { lat:        247, common_pid:       2043 } hitcount:       2385
2598 { lat:        247, common_pid:       2041 } hitcount:        452
2599 { lat:        247, common_pid:       2038 } hitcount:        792
2600 { lat:        247, common_pid:       2040 } hitcount:         78
2601 { lat:        247, common_pid:       2036 } hitcount:       2375
2602 { lat:        247, common_pid:       2039 } hitcount:       1834
2603 { lat:        247, common_pid:       2037 } hitcount:       2655
2604 { lat:        248, common_pid:       2037 } hitcount:         36
2605 { lat:        248, common_pid:       2042 } hitcount:         11
2606 { lat:        248, common_pid:       2038 } hitcount:        122
2607 { lat:        248, common_pid:       2036 } hitcount:        135
2608 { lat:        248, common_pid:       2039 } hitcount:         26
2609 { lat:        248, common_pid:       2041 } hitcount:        503
2610 { lat:        248, common_pid:       2043 } hitcount:         66
2611 { lat:        248, common_pid:       2040 } hitcount:         46
2612 { lat:        249, common_pid:       2037 } hitcount:         29
2613 { lat:        249, common_pid:       2038 } hitcount:          1
2614 { lat:        249, common_pid:       2043 } hitcount:         29
2615 { lat:        249, common_pid:       2039 } hitcount:          8
2616 { lat:        249, common_pid:       2042 } hitcount:         56
2617 { lat:        249, common_pid:       2040 } hitcount:         27
2618 { lat:        249, common_pid:       2041 } hitcount:         11
2619 { lat:        249, common_pid:       2036 } hitcount:         27
2620 { lat:        250, common_pid:       2038 } hitcount:          1
2621 { lat:        250, common_pid:       2036 } hitcount:         30
2622 { lat:        250, common_pid:       2040 } hitcount:         19
2623 { lat:        250, common_pid:       2043 } hitcount:         22
2624 { lat:        250, common_pid:       2042 } hitcount:         20
2625 { lat:        250, common_pid:       2041 } hitcount:          1
2626 { lat:        250, common_pid:       2039 } hitcount:          6
2627 { lat:        250, common_pid:       2037 } hitcount:         48
2628 { lat:        251, common_pid:       2037 } hitcount:         43
2629 { lat:        251, common_pid:       2039 } hitcount:          1
2630 { lat:        251, common_pid:       2036 } hitcount:         12
2631 { lat:        251, common_pid:       2042 } hitcount:          2
2632 { lat:        251, common_pid:       2041 } hitcount:          1
2633 { lat:        251, common_pid:       2043 } hitcount:         15
2634 { lat:        251, common_pid:       2040 } hitcount:          3
2635 { lat:        252, common_pid:       2040 } hitcount:          1
2636 { lat:        252, common_pid:       2036 } hitcount:         12
2637 { lat:        252, common_pid:       2037 } hitcount:         21
2638 { lat:        252, common_pid:       2043 } hitcount:         14
2639 { lat:        253, common_pid:       2037 } hitcount:         21
2640 { lat:        253, common_pid:       2039 } hitcount:          2
2641 { lat:        253, common_pid:       2036 } hitcount:          9
2642 { lat:        253, common_pid:       2043 } hitcount:          6
2643 { lat:        253, common_pid:       2040 } hitcount:          1
2644 { lat:        254, common_pid:       2036 } hitcount:          8
2645 { lat:        254, common_pid:       2043 } hitcount:          3
2646 { lat:        254, common_pid:       2041 } hitcount:          1
2647 { lat:        254, common_pid:       2042 } hitcount:          1
2648 { lat:        254, common_pid:       2039 } hitcount:          1
2649 { lat:        254, common_pid:       2037 } hitcount:         12
2650 { lat:        255, common_pid:       2043 } hitcount:          1
2651 { lat:        255, common_pid:       2037 } hitcount:          2
2652 { lat:        255, common_pid:       2036 } hitcount:          2
2653 { lat:        255, common_pid:       2039 } hitcount:          8
2654 { lat:        256, common_pid:       2043 } hitcount:          1
2655 { lat:        256, common_pid:       2036 } hitcount:          4
2656 { lat:        256, common_pid:       2039 } hitcount:          6
2657 { lat:        257, common_pid:       2039 } hitcount:          5
2658 { lat:        257, common_pid:       2036 } hitcount:          4
2659 { lat:        258, common_pid:       2039 } hitcount:          5
2660 { lat:        258, common_pid:       2036 } hitcount:          2
2661 { lat:        259, common_pid:       2036 } hitcount:          7
2662 { lat:        259, common_pid:       2039 } hitcount:          7
2663 { lat:        260, common_pid:       2036 } hitcount:          8
2664 { lat:        260, common_pid:       2039 } hitcount:          6
2665 { lat:        261, common_pid:       2036 } hitcount:          5
2666 { lat:        261, common_pid:       2039 } hitcount:          7
2667 { lat:        262, common_pid:       2039 } hitcount:          5
2668 { lat:        262, common_pid:       2036 } hitcount:          5
2669 { lat:        263, common_pid:       2039 } hitcount:          7
2670 { lat:        263, common_pid:       2036 } hitcount:          7
2671 { lat:        264, common_pid:       2039 } hitcount:          9
2672 { lat:        264, common_pid:       2036 } hitcount:          9
2673 { lat:        265, common_pid:       2036 } hitcount:          5
2674 { lat:        265, common_pid:       2039 } hitcount:          1
2675 { lat:        266, common_pid:       2036 } hitcount:          1
2676 { lat:        266, common_pid:       2039 } hitcount:          3
2677 { lat:        267, common_pid:       2036 } hitcount:          1
2678 { lat:        267, common_pid:       2039 } hitcount:          3
2679 { lat:        268, common_pid:       2036 } hitcount:          1
2680 { lat:        268, common_pid:       2039 } hitcount:          6
2681 { lat:        269, common_pid:       2036 } hitcount:          1
2682 { lat:        269, common_pid:       2043 } hitcount:          1
2683 { lat:        269, common_pid:       2039 } hitcount:          2
2684 { lat:        270, common_pid:       2040 } hitcount:          1
2685 { lat:        270, common_pid:       2039 } hitcount:          6
2686 { lat:        271, common_pid:       2041 } hitcount:          1
2687 { lat:        271, common_pid:       2039 } hitcount:          5
2688 { lat:        272, common_pid:       2039 } hitcount:         10
2689 { lat:        273, common_pid:       2039 } hitcount:          8
2690 { lat:        274, common_pid:       2039 } hitcount:          2
2691 { lat:        275, common_pid:       2039 } hitcount:          1
2692 { lat:        276, common_pid:       2039 } hitcount:          2
2693 { lat:        276, common_pid:       2037 } hitcount:          1
2694 { lat:        276, common_pid:       2038 } hitcount:          1
2695 { lat:        277, common_pid:       2039 } hitcount:          1
2696 { lat:        277, common_pid:       2042 } hitcount:          1
2697 { lat:        278, common_pid:       2039 } hitcount:          1
2698 { lat:        279, common_pid:       2039 } hitcount:          4
2699 { lat:        279, common_pid:       2043 } hitcount:          1
2700 { lat:        280, common_pid:       2039 } hitcount:          3
2701 { lat:        283, common_pid:       2036 } hitcount:          2
2702 { lat:        284, common_pid:       2039 } hitcount:          1
2703 { lat:        284, common_pid:       2043 } hitcount:          1
2704 { lat:        288, common_pid:       2039 } hitcount:          1
2705 { lat:        289, common_pid:       2039 } hitcount:          1
2706 { lat:        300, common_pid:       2039 } hitcount:          1
2707 { lat:        384, common_pid:       2039 } hitcount:          1
2708
2709 Totals:
2710     Hits: 67625
2711     Entries: 278
2712     Dropped: 0
2713
2714Note, the writes are around the sleep, so ideally they will all be of 250
2715microseconds. If you are wondering how there are several that are under
2716250 microseconds, that is because the way cyclictest works, is if one
2717iteration comes in late, the next one will set the timer to wake up less that
2718250. That is, if an iteration came in 50 microseconds late, the next wake up
2719will be at 200 microseconds.
2720
2721But this could easily be done in userspace. To make this even more
2722interesting, we can mix the histogram between events that happened in the
2723kernel with trace_marker::
2724
2725 # cd /sys/kernel/tracing
2726 # echo 'latency u64 lat' > synthetic_events
2727 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' > events/sched/sched_waking/trigger
2728 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).latency($lat) if buf == "end"' > events/ftrace/print/trigger
2729 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
2730
2731The difference this time is that instead of using the trace_marker to start
2732the latency, the sched_waking event is used, matching the common_pid for the
2733trace_marker write with the pid that is being woken by sched_waking.
2734
2735After running cyclictest again with the same parameters, we now have::
2736
2737 # cat events/synthetic/latency/hist
2738 # event histogram
2739 #
2740 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
2741 #
2742
2743 { lat:          7, common_pid:       2302 } hitcount:        640
2744 { lat:          7, common_pid:       2299 } hitcount:         42
2745 { lat:          7, common_pid:       2303 } hitcount:         18
2746 { lat:          7, common_pid:       2305 } hitcount:        166
2747 { lat:          7, common_pid:       2306 } hitcount:          1
2748 { lat:          7, common_pid:       2301 } hitcount:         91
2749 { lat:          7, common_pid:       2300 } hitcount:         17
2750 { lat:          8, common_pid:       2303 } hitcount:       8296
2751 { lat:          8, common_pid:       2304 } hitcount:       6864
2752 { lat:          8, common_pid:       2305 } hitcount:       9464
2753 { lat:          8, common_pid:       2301 } hitcount:       9213
2754 { lat:          8, common_pid:       2306 } hitcount:       6246
2755 { lat:          8, common_pid:       2302 } hitcount:       8797
2756 { lat:          8, common_pid:       2299 } hitcount:       8771
2757 { lat:          8, common_pid:       2300 } hitcount:       8119
2758 { lat:          9, common_pid:       2305 } hitcount:       1519
2759 { lat:          9, common_pid:       2299 } hitcount:       2346
2760 { lat:          9, common_pid:       2303 } hitcount:       2841
2761 { lat:          9, common_pid:       2301 } hitcount:       1846
2762 { lat:          9, common_pid:       2304 } hitcount:       3861
2763 { lat:          9, common_pid:       2302 } hitcount:       1210
2764 { lat:          9, common_pid:       2300 } hitcount:       2762
2765 { lat:          9, common_pid:       2306 } hitcount:       4247
2766 { lat:         10, common_pid:       2299 } hitcount:         16
2767 { lat:         10, common_pid:       2306 } hitcount:        333
2768 { lat:         10, common_pid:       2303 } hitcount:         16
2769 { lat:         10, common_pid:       2304 } hitcount:        168
2770 { lat:         10, common_pid:       2302 } hitcount:        240
2771 { lat:         10, common_pid:       2301 } hitcount:         28
2772 { lat:         10, common_pid:       2300 } hitcount:         95
2773 { lat:         10, common_pid:       2305 } hitcount:         18
2774 { lat:         11, common_pid:       2303 } hitcount:          5
2775 { lat:         11, common_pid:       2305 } hitcount:          8
2776 { lat:         11, common_pid:       2306 } hitcount:        221
2777 { lat:         11, common_pid:       2302 } hitcount:         76
2778 { lat:         11, common_pid:       2304 } hitcount:         26
2779 { lat:         11, common_pid:       2300 } hitcount:        125
2780 { lat:         11, common_pid:       2299 } hitcount:          2
2781 { lat:         12, common_pid:       2305 } hitcount:          3
2782 { lat:         12, common_pid:       2300 } hitcount:          6
2783 { lat:         12, common_pid:       2306 } hitcount:         90
2784 { lat:         12, common_pid:       2302 } hitcount:          4
2785 { lat:         12, common_pid:       2303 } hitcount:          1
2786 { lat:         12, common_pid:       2304 } hitcount:        122
2787 { lat:         13, common_pid:       2300 } hitcount:         12
2788 { lat:         13, common_pid:       2301 } hitcount:          1
2789 { lat:         13, common_pid:       2306 } hitcount:         32
2790 { lat:         13, common_pid:       2302 } hitcount:          5
2791 { lat:         13, common_pid:       2305 } hitcount:          1
2792 { lat:         13, common_pid:       2303 } hitcount:          1
2793 { lat:         13, common_pid:       2304 } hitcount:         61
2794 { lat:         14, common_pid:       2303 } hitcount:          4
2795 { lat:         14, common_pid:       2306 } hitcount:          5
2796 { lat:         14, common_pid:       2305 } hitcount:          4
2797 { lat:         14, common_pid:       2304 } hitcount:         62
2798 { lat:         14, common_pid:       2302 } hitcount:         19
2799 { lat:         14, common_pid:       2300 } hitcount:         33
2800 { lat:         14, common_pid:       2299 } hitcount:          1
2801 { lat:         14, common_pid:       2301 } hitcount:          4
2802 { lat:         15, common_pid:       2305 } hitcount:          1
2803 { lat:         15, common_pid:       2302 } hitcount:         25
2804 { lat:         15, common_pid:       2300 } hitcount:         11
2805 { lat:         15, common_pid:       2299 } hitcount:          5
2806 { lat:         15, common_pid:       2301 } hitcount:          1
2807 { lat:         15, common_pid:       2304 } hitcount:          8
2808 { lat:         15, common_pid:       2303 } hitcount:          1
2809 { lat:         15, common_pid:       2306 } hitcount:          6
2810 { lat:         16, common_pid:       2302 } hitcount:         31
2811 { lat:         16, common_pid:       2306 } hitcount:          3
2812 { lat:         16, common_pid:       2300 } hitcount:          5
2813 { lat:         17, common_pid:       2302 } hitcount:          6
2814 { lat:         17, common_pid:       2303 } hitcount:          1
2815 { lat:         18, common_pid:       2304 } hitcount:          1
2816 { lat:         18, common_pid:       2302 } hitcount:          8
2817 { lat:         18, common_pid:       2299 } hitcount:          1
2818 { lat:         18, common_pid:       2301 } hitcount:          1
2819 { lat:         19, common_pid:       2303 } hitcount:          4
2820 { lat:         19, common_pid:       2304 } hitcount:          5
2821 { lat:         19, common_pid:       2302 } hitcount:          4
2822 { lat:         19, common_pid:       2299 } hitcount:          3
2823 { lat:         19, common_pid:       2306 } hitcount:          1
2824 { lat:         19, common_pid:       2300 } hitcount:          4
2825 { lat:         19, common_pid:       2305 } hitcount:          5
2826 { lat:         20, common_pid:       2299 } hitcount:          2
2827 { lat:         20, common_pid:       2302 } hitcount:          3
2828 { lat:         20, common_pid:       2305 } hitcount:          1
2829 { lat:         20, common_pid:       2300 } hitcount:          2
2830 { lat:         20, common_pid:       2301 } hitcount:          2
2831 { lat:         20, common_pid:       2303 } hitcount:          3
2832 { lat:         21, common_pid:       2305 } hitcount:          1
2833 { lat:         21, common_pid:       2299 } hitcount:          5
2834 { lat:         21, common_pid:       2303 } hitcount:          4
2835 { lat:         21, common_pid:       2302 } hitcount:          7
2836 { lat:         21, common_pid:       2300 } hitcount:          1
2837 { lat:         21, common_pid:       2301 } hitcount:          5
2838 { lat:         21, common_pid:       2304 } hitcount:          2
2839 { lat:         22, common_pid:       2302 } hitcount:          5
2840 { lat:         22, common_pid:       2303 } hitcount:          1
2841 { lat:         22, common_pid:       2306 } hitcount:          3
2842 { lat:         22, common_pid:       2301 } hitcount:          2
2843 { lat:         22, common_pid:       2300 } hitcount:          1
2844 { lat:         22, common_pid:       2299 } hitcount:          1
2845 { lat:         22, common_pid:       2305 } hitcount:          1
2846 { lat:         22, common_pid:       2304 } hitcount:          1
2847 { lat:         23, common_pid:       2299 } hitcount:          1
2848 { lat:         23, common_pid:       2306 } hitcount:          2
2849 { lat:         23, common_pid:       2302 } hitcount:          6
2850 { lat:         24, common_pid:       2302 } hitcount:          3
2851 { lat:         24, common_pid:       2300 } hitcount:          1
2852 { lat:         24, common_pid:       2306 } hitcount:          2
2853 { lat:         24, common_pid:       2305 } hitcount:          1
2854 { lat:         24, common_pid:       2299 } hitcount:          1
2855 { lat:         25, common_pid:       2300 } hitcount:          1
2856 { lat:         25, common_pid:       2302 } hitcount:          4
2857 { lat:         26, common_pid:       2302 } hitcount:          2
2858 { lat:         27, common_pid:       2305 } hitcount:          1
2859 { lat:         27, common_pid:       2300 } hitcount:          1
2860 { lat:         27, common_pid:       2302 } hitcount:          3
2861 { lat:         28, common_pid:       2306 } hitcount:          1
2862 { lat:         28, common_pid:       2302 } hitcount:          4
2863 { lat:         29, common_pid:       2302 } hitcount:          1
2864 { lat:         29, common_pid:       2300 } hitcount:          2
2865 { lat:         29, common_pid:       2306 } hitcount:          1
2866 { lat:         29, common_pid:       2304 } hitcount:          1
2867 { lat:         30, common_pid:       2302 } hitcount:          4
2868 { lat:         31, common_pid:       2302 } hitcount:          6
2869 { lat:         32, common_pid:       2302 } hitcount:          1
2870 { lat:         33, common_pid:       2299 } hitcount:          1
2871 { lat:         33, common_pid:       2302 } hitcount:          3
2872 { lat:         34, common_pid:       2302 } hitcount:          2
2873 { lat:         35, common_pid:       2302 } hitcount:          1
2874 { lat:         35, common_pid:       2304 } hitcount:          1
2875 { lat:         36, common_pid:       2302 } hitcount:          4
2876 { lat:         37, common_pid:       2302 } hitcount:          6
2877 { lat:         38, common_pid:       2302 } hitcount:          2
2878 { lat:         39, common_pid:       2302 } hitcount:          2
2879 { lat:         39, common_pid:       2304 } hitcount:          1
2880 { lat:         40, common_pid:       2304 } hitcount:          2
2881 { lat:         40, common_pid:       2302 } hitcount:          5
2882 { lat:         41, common_pid:       2304 } hitcount:          1
2883 { lat:         41, common_pid:       2302 } hitcount:          8
2884 { lat:         42, common_pid:       2302 } hitcount:          6
2885 { lat:         42, common_pid:       2304 } hitcount:          1
2886 { lat:         43, common_pid:       2302 } hitcount:          3
2887 { lat:         43, common_pid:       2304 } hitcount:          4
2888 { lat:         44, common_pid:       2302 } hitcount:          6
2889 { lat:         45, common_pid:       2302 } hitcount:          5
2890 { lat:         46, common_pid:       2302 } hitcount:          5
2891 { lat:         47, common_pid:       2302 } hitcount:          7
2892 { lat:         48, common_pid:       2301 } hitcount:          1
2893 { lat:         48, common_pid:       2302 } hitcount:          9
2894 { lat:         49, common_pid:       2302 } hitcount:          3
2895 { lat:         50, common_pid:       2302 } hitcount:          1
2896 { lat:         50, common_pid:       2301 } hitcount:          1
2897 { lat:         51, common_pid:       2302 } hitcount:          2
2898 { lat:         51, common_pid:       2301 } hitcount:          1
2899 { lat:         61, common_pid:       2302 } hitcount:          1
2900 { lat:        110, common_pid:       2302 } hitcount:          1
2901
2902 Totals:
2903     Hits: 89565
2904     Entries: 158
2905     Dropped: 0
2906
2907This doesn't tell us any information about how late cyclictest may have
2908woken up, but it does show us a nice histogram of how long it took from
2909the time that cyclictest was woken to the time it made it into user space.
2910