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] [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	.usecs      display a common_timestamp in microseconds
81	=========== ==========================================
82
83  Note that in general the semantics of a given field aren't
84  interpreted when applying a modifier to it, but there are some
85  restrictions to be aware of in this regard:
86
87    - only the 'hex' modifier can be used for values (because values
88      are essentially sums, and the other modifiers don't make sense
89      in that context).
90    - the 'execname' modifier can only be used on a 'common_pid'.  The
91      reason for this is that the execname is simply the 'comm' value
92      saved for the 'current' process when an event was triggered,
93      which is the same as the common_pid value saved by the event
94      tracing code.  Trying to apply that comm value to other pid
95      values wouldn't be correct, and typically events that care save
96      pid-specific comm fields in the event itself.
97
98  A typical usage scenario would be the following to enable a hist
99  trigger, read its current contents, and then turn it off::
100
101    # echo 'hist:keys=skbaddr.hex:vals=len' > \
102      /sys/kernel/debug/tracing/events/net/netif_rx/trigger
103
104    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
105
106    # echo '!hist:keys=skbaddr.hex:vals=len' > \
107      /sys/kernel/debug/tracing/events/net/netif_rx/trigger
108
109  The trigger file itself can be read to show the details of the
110  currently attached hist trigger.  This information is also displayed
111  at the top of the 'hist' file when read.
112
113  By default, the size of the hash table is 2048 entries.  The 'size'
114  parameter can be used to specify more or fewer than that.  The units
115  are in terms of hashtable entries - if a run uses more entries than
116  specified, the results will show the number of 'drops', the number
117  of hits that were ignored.  The size should be a power of 2 between
118  128 and 131072 (any non- power-of-2 number specified will be rounded
119  up).
120
121  The 'sort' parameter can be used to specify a value field to sort
122  on.  The default if unspecified is 'hitcount' and the default sort
123  order is 'ascending'.  To sort in the opposite direction, append
124  .descending' to the sort key.
125
126  The 'pause' parameter can be used to pause an existing hist trigger
127  or to start a hist trigger but not log any events until told to do
128  so.  'continue' or 'cont' can be used to start or restart a paused
129  hist trigger.
130
131  The 'clear' parameter will clear the contents of a running hist
132  trigger and leave its current paused/active state.
133
134  Note that the 'pause', 'cont', and 'clear' parameters should be
135  applied using 'append' shell operator ('>>') if applied to an
136  existing trigger, rather than via the '>' operator, which will cause
137  the trigger to be removed through truncation.
138
139- enable_hist/disable_hist
140
141  The enable_hist and disable_hist triggers can be used to have one
142  event conditionally start and stop another event's already-attached
143  hist trigger.  Any number of enable_hist and disable_hist triggers
144  can be attached to a given event, allowing that event to kick off
145  and stop aggregations on a host of other events.
146
147  The format is very similar to the enable/disable_event triggers::
148
149      enable_hist:<system>:<event>[:count]
150      disable_hist:<system>:<event>[:count]
151
152  Instead of enabling or disabling the tracing of the target event
153  into the trace buffer as the enable/disable_event triggers do, the
154  enable/disable_hist triggers enable or disable the aggregation of
155  the target event into a hash table.
156
157  A typical usage scenario for the enable_hist/disable_hist triggers
158  would be to first set up a paused hist trigger on some event,
159  followed by an enable_hist/disable_hist pair that turns the hist
160  aggregation on and off when conditions of interest are hit::
161
162   # echo 'hist:keys=skbaddr.hex:vals=len:pause' > \
163      /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
164
165    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
166      /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
167
168    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
169      /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
170
171  The above sets up an initially paused hist trigger which is unpaused
172  and starts aggregating events when a given program is executed, and
173  which stops aggregating when the process exits and the hist trigger
174  is paused again.
175
176  The examples below provide a more concrete illustration of the
177  concepts and typical usage patterns discussed above.
178
179'special' event fields
180------------------------
181
182  There are a number of 'special event fields' available for use as
183  keys or values in a hist trigger.  These look like and behave as if
184  they were actual event fields, but aren't really part of the event's
185  field definition or format file.  They are however available for any
186  event, and can be used anywhere an actual event field could be.
187  They are:
188
189    ====================== ==== =======================================
190    common_timestamp       u64  timestamp (from ring buffer) associated
191                                with the event, in nanoseconds.  May be
192			        modified by .usecs to have timestamps
193			        interpreted as microseconds.
194    cpu                    int  the cpu on which the event occurred.
195    ====================== ==== =======================================
196
197Extended error information
198--------------------------
199
200  For some error conditions encountered when invoking a hist trigger
201  command, extended error information is available via the
202  corresponding event's 'hist' file.  Reading the hist file after an
203  error will display more detailed information about what went wrong,
204  if information is available.  This extended error information will
205  be available until the next hist trigger command for that event.
206
207  If available for a given error condition, the extended error
208  information and usage takes the following form::
209
210    # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
211    echo: write error: Invalid argument
212
213    # cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/hist
214    ERROR: Couldn't yyy: zzz
215      Last command: xxx
216
2176.2 'hist' trigger examples
218---------------------------
219
220  The first set of examples creates aggregations using the kmalloc
221  event.  The fields that can be used for the hist trigger are listed
222  in the kmalloc event's format file::
223
224    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
225    name: kmalloc
226    ID: 374
227    format:
228	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
229	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
230	field:unsigned char common_preempt_count;		offset:3;	size:1;	signed:0;
231	field:int common_pid;					offset:4;	size:4;	signed:1;
232
233	field:unsigned long call_site;				offset:8;	size:8;	signed:0;
234	field:const void * ptr;					offset:16;	size:8;	signed:0;
235	field:size_t bytes_req;					offset:24;	size:8;	signed:0;
236	field:size_t bytes_alloc;				offset:32;	size:8;	signed:0;
237	field:gfp_t gfp_flags;					offset:40;	size:4;	signed:0;
238
239  We'll start by creating a hist trigger that generates a simple table
240  that lists the total number of bytes requested for each function in
241  the kernel that made one or more calls to kmalloc::
242
243    # echo 'hist:key=call_site:val=bytes_req' > \
244            /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
245
246  This tells the tracing system to create a 'hist' trigger using the
247  call_site field of the kmalloc event as the key for the table, which
248  just means that each unique call_site address will have an entry
249  created for it in the table.  The 'val=bytes_req' parameter tells
250  the hist trigger that for each unique entry (call_site) in the
251  table, it should keep a running total of the number of bytes
252  requested by that call_site.
253
254  We'll let it run for awhile and then dump the contents of the 'hist'
255  file in the kmalloc event's subdirectory (for readability, a number
256  of entries have been omitted)::
257
258    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
259    # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
260
261    { call_site: 18446744072106379007 } hitcount:          1  bytes_req:        176
262    { call_site: 18446744071579557049 } hitcount:          1  bytes_req:       1024
263    { call_site: 18446744071580608289 } hitcount:          1  bytes_req:      16384
264    { call_site: 18446744071581827654 } hitcount:          1  bytes_req:         24
265    { call_site: 18446744071580700980 } hitcount:          1  bytes_req:          8
266    { call_site: 18446744071579359876 } hitcount:          1  bytes_req:        152
267    { call_site: 18446744071580795365 } hitcount:          3  bytes_req:        144
268    { call_site: 18446744071581303129 } hitcount:          3  bytes_req:        144
269    { call_site: 18446744071580713234 } hitcount:          4  bytes_req:       2560
270    { call_site: 18446744071580933750 } hitcount:          4  bytes_req:        736
271    .
272    .
273    .
274    { call_site: 18446744072106047046 } hitcount:         69  bytes_req:       5576
275    { call_site: 18446744071582116407 } hitcount:         73  bytes_req:       2336
276    { call_site: 18446744072106054684 } hitcount:        136  bytes_req:     140504
277    { call_site: 18446744072106224230 } hitcount:        136  bytes_req:      19584
278    { call_site: 18446744072106078074 } hitcount:        153  bytes_req:       2448
279    { call_site: 18446744072106062406 } hitcount:        153  bytes_req:      36720
280    { call_site: 18446744071582507929 } hitcount:        153  bytes_req:      37088
281    { call_site: 18446744072102520590 } hitcount:        273  bytes_req:      10920
282    { call_site: 18446744071582143559 } hitcount:        358  bytes_req:        716
283    { call_site: 18446744072106465852 } hitcount:        417  bytes_req:      56712
284    { call_site: 18446744072102523378 } hitcount:        485  bytes_req:      27160
285    { call_site: 18446744072099568646 } hitcount:       1676  bytes_req:      33520
286
287    Totals:
288        Hits: 4610
289        Entries: 45
290        Dropped: 0
291
292  The output displays a line for each entry, beginning with the key
293  specified in the trigger, followed by the value(s) also specified in
294  the trigger.  At the beginning of the output is a line that displays
295  the trigger info, which can also be displayed by reading the
296  'trigger' file::
297
298    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
299    hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
300
301  At the end of the output are a few lines that display the overall
302  totals for the run.  The 'Hits' field shows the total number of
303  times the event trigger was hit, the 'Entries' field shows the total
304  number of used entries in the hash table, and the 'Dropped' field
305  shows the number of hits that were dropped because the number of
306  used entries for the run exceeded the maximum number of entries
307  allowed for the table (normally 0, but if not a hint that you may
308  want to increase the size of the table using the 'size' parameter).
309
310  Notice in the above output that there's an extra field, 'hitcount',
311  which wasn't specified in the trigger.  Also notice that in the
312  trigger info output, there's a parameter, 'sort=hitcount', which
313  wasn't specified in the trigger either.  The reason for that is that
314  every trigger implicitly keeps a count of the total number of hits
315  attributed to a given entry, called the 'hitcount'.  That hitcount
316  information is explicitly displayed in the output, and in the
317  absence of a user-specified sort parameter, is used as the default
318  sort field.
319
320  The value 'hitcount' can be used in place of an explicit value in
321  the 'values' parameter if you don't really need to have any
322  particular field summed and are mainly interested in hit
323  frequencies.
324
325  To turn the hist trigger off, simply call up the trigger in the
326  command history and re-execute it with a '!' prepended::
327
328    # echo '!hist:key=call_site:val=bytes_req' > \
329           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
330
331  Finally, notice that the call_site as displayed in the output above
332  isn't really very useful.  It's an address, but normally addresses
333  are displayed in hex.  To have a numeric field displayed as a hex
334  value, simply append '.hex' to the field name in the trigger::
335
336    # echo 'hist:key=call_site.hex:val=bytes_req' > \
337           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
338
339    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
340    # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]
341
342    { call_site: ffffffffa026b291 } hitcount:          1  bytes_req:        433
343    { call_site: ffffffffa07186ff } hitcount:          1  bytes_req:        176
344    { call_site: ffffffff811ae721 } hitcount:          1  bytes_req:      16384
345    { call_site: ffffffff811c5134 } hitcount:          1  bytes_req:          8
346    { call_site: ffffffffa04a9ebb } hitcount:          1  bytes_req:        511
347    { call_site: ffffffff8122e0a6 } hitcount:          1  bytes_req:         12
348    { call_site: ffffffff8107da84 } hitcount:          1  bytes_req:        152
349    { call_site: ffffffff812d8246 } hitcount:          1  bytes_req:         24
350    { call_site: ffffffff811dc1e5 } hitcount:          3  bytes_req:        144
351    { call_site: ffffffffa02515e8 } hitcount:          3  bytes_req:        648
352    { call_site: ffffffff81258159 } hitcount:          3  bytes_req:        144
353    { call_site: ffffffff811c80f4 } hitcount:          4  bytes_req:        544
354    .
355    .
356    .
357    { call_site: ffffffffa06c7646 } hitcount:        106  bytes_req:       8024
358    { call_site: ffffffffa06cb246 } hitcount:        132  bytes_req:      31680
359    { call_site: ffffffffa06cef7a } hitcount:        132  bytes_req:       2112
360    { call_site: ffffffff8137e399 } hitcount:        132  bytes_req:      23232
361    { call_site: ffffffffa06c941c } hitcount:        185  bytes_req:     171360
362    { call_site: ffffffffa06f2a66 } hitcount:        185  bytes_req:      26640
363    { call_site: ffffffffa036a70e } hitcount:        265  bytes_req:      10600
364    { call_site: ffffffff81325447 } hitcount:        292  bytes_req:        584
365    { call_site: ffffffffa072da3c } hitcount:        446  bytes_req:      60656
366    { call_site: ffffffffa036b1f2 } hitcount:        526  bytes_req:      29456
367    { call_site: ffffffffa0099c06 } hitcount:       1780  bytes_req:      35600
368
369    Totals:
370        Hits: 4775
371        Entries: 46
372        Dropped: 0
373
374  Even that's only marginally more useful - while hex values do look
375  more like addresses, what users are typically more interested in
376  when looking at text addresses are the corresponding symbols
377  instead.  To have an address displayed as symbolic value instead,
378  simply append '.sym' or '.sym-offset' to the field name in the
379  trigger::
380
381    # echo 'hist:key=call_site.sym:val=bytes_req' > \
382           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
383
384    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
385    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]
386
387    { call_site: [ffffffff810adcb9] syslog_print_all                              } hitcount:          1  bytes_req:       1024
388    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
389    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
390    { call_site: [ffffffff8154acbe] usb_alloc_urb                                 } hitcount:          1  bytes_req:        192
391    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
392    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
393    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
394    { call_site: [ffffffff811febd5] fsnotify_alloc_group                          } hitcount:          2  bytes_req:        528
395    { call_site: [ffffffff81440f58] __tty_buffer_request_room                     } hitcount:          2  bytes_req:       2624
396    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          2  bytes_req:         96
397    { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211]      } hitcount:          2  bytes_req:        464
398    { call_site: [ffffffff81672406] tcp_get_metrics                               } hitcount:          2  bytes_req:        304
399    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
400    { call_site: [ffffffff81089b05] sched_create_group                            } hitcount:          2  bytes_req:       1424
401    .
402    .
403    .
404    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1185  bytes_req:     123240
405    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:       1185  bytes_req:     104280
406    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       1402  bytes_req:     190672
407    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       1518  bytes_req:     146208
408    { call_site: [ffffffffa029070e] drm_vma_node_allow [drm]                      } hitcount:       1746  bytes_req:      69840
409    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       2021  bytes_req:     792312
410    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       2592  bytes_req:     145152
411    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2629  bytes_req:     378576
412    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2629  bytes_req:    3783248
413    { call_site: [ffffffff81325607] apparmor_file_alloc_security                  } hitcount:       5192  bytes_req:      10384
414    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       5529  bytes_req:     110584
415    { call_site: [ffffffff8131ebf7] aa_alloc_task_context                         } hitcount:      21943  bytes_req:     702176
416    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:      55759  bytes_req:    5074265
417
418    Totals:
419        Hits: 109928
420        Entries: 71
421        Dropped: 0
422
423  Because the default sort key above is 'hitcount', the above shows a
424  the list of call_sites by increasing hitcount, so that at the bottom
425  we see the functions that made the most kmalloc calls during the
426  run.  If instead we we wanted to see the top kmalloc callers in
427  terms of the number of bytes requested rather than the number of
428  calls, and we wanted the top caller to appear at the top, we can use
429  the 'sort' parameter, along with the 'descending' modifier::
430
431    # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
432           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
433
434    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
435    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
436
437    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2186  bytes_req:    3397464
438    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1790  bytes_req:     712176
439    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       8132  bytes_req:     513135
440    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        106  bytes_req:     440128
441    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2186  bytes_req:     314784
442    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       2174  bytes_req:     208992
443    { call_site: [ffffffff811ae8e1] __kmalloc                                     } hitcount:          8  bytes_req:     131072
444    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:        859  bytes_req:     116824
445    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       1834  bytes_req:     102704
446    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:        972  bytes_req:     101088
447    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:        972  bytes_req:      85536
448    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       3333  bytes_req:      66664
449    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        209  bytes_req:      61632
450    .
451    .
452    .
453    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
454    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
455    { call_site: [ffffffff812d8406] copy_semundo                                  } hitcount:          2  bytes_req:         48
456    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          1  bytes_req:         48
457    { call_site: [ffffffffa027121a] drm_getmagic [drm]                            } hitcount:          1  bytes_req:         48
458    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
459    { call_site: [ffffffff811c52f4] bprm_change_interp                            } hitcount:          2  bytes_req:         16
460    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
461    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
462    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
463
464    Totals:
465        Hits: 32133
466        Entries: 81
467        Dropped: 0
468
469  To display the offset and size information in addition to the symbol
470  name, just use 'sym-offset' instead::
471
472    # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
473           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
474
475    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
476    # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
477
478    { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915]                  } hitcount:       4569  bytes_req:    3163720
479    { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915]                      } hitcount:       4569  bytes_req:     657936
480    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915]      } hitcount:       1519  bytes_req:     472936
481    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915]      } hitcount:       3050  bytes_req:     211832
482    { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50                                 } hitcount:         34  bytes_req:     148384
483    { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915]                  } hitcount:       1385  bytes_req:     144040
484    { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0                                   } hitcount:          8  bytes_req:     131072
485    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm]              } hitcount:       1385  bytes_req:     121880
486    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm]                  } hitcount:       1848  bytes_req:     103488
487    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915]            } hitcount:        461  bytes_req:      62696
488    { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm]                      } hitcount:       1541  bytes_req:      61640
489    { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0                                } hitcount:         57  bytes_req:      57456
490    .
491    .
492    .
493    { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0                       } hitcount:          2  bytes_req:        128
494    { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm]                      } hitcount:          3  bytes_req:         96
495    { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0                         } hitcount:          8  bytes_req:         96
496    { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650                            } hitcount:          3  bytes_req:         84
497    { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110                              } hitcount:          1  bytes_req:          8
498    { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid]                     } hitcount:          1  bytes_req:          7
499    { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid]                    } hitcount:          1  bytes_req:          7
500
501    Totals:
502        Hits: 26098
503        Entries: 64
504        Dropped: 0
505
506  We can also add multiple fields to the 'values' parameter.  For
507  example, we might want to see the total number of bytes allocated
508  alongside bytes requested, and display the result sorted by bytes
509  allocated in a descending order::
510
511    # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
512           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
513
514    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
515    # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]
516
517    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       7403  bytes_req:    4084360  bytes_alloc:    5958016
518    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        541  bytes_req:    2213968  bytes_alloc:    2228224
519    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       7404  bytes_req:    1066176  bytes_alloc:    1421568
520    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1565  bytes_req:     557368  bytes_alloc:    1037760
521    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       9557  bytes_req:     595778  bytes_alloc:     695744
522    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       5839  bytes_req:     430680  bytes_alloc:     470400
523    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       2388  bytes_req:     324768  bytes_alloc:     458496
524    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       3911  bytes_req:     219016  bytes_alloc:     250304
525    { call_site: [ffffffff815f8d7b] sk_prot_alloc                                 } hitcount:        235  bytes_req:     236880  bytes_alloc:     240640
526    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        557  bytes_req:     169024  bytes_alloc:     221760
527    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       9378  bytes_req:     187548  bytes_alloc:     206312
528    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1519  bytes_req:     157976  bytes_alloc:     194432
529    .
530    .
531    .
532    { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach                 } hitcount:          2  bytes_req:        144  bytes_alloc:        192
533    { call_site: [ffffffff81097ee8] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
534    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
535    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
536    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
537    { call_site: [ffffffff81213e80] load_elf_binary                               } hitcount:          3  bytes_req:         84  bytes_alloc:         96
538    { call_site: [ffffffff81079a2e] kthread_create_on_node                        } hitcount:          1  bytes_req:         56  bytes_alloc:         64
539    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7  bytes_alloc:          8
540    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8  bytes_alloc:          8
541    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7  bytes_alloc:          8
542
543    Totals:
544        Hits: 66598
545        Entries: 65
546        Dropped: 0
547
548  Finally, to finish off our kmalloc example, instead of simply having
549  the hist trigger display symbolic call_sites, we can have the hist
550  trigger additionally display the complete set of kernel stack traces
551  that led to each call_site.  To do that, we simply use the special
552  value 'stacktrace' for the key parameter::
553
554    # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
555           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
556
557  The above trigger will use the kernel stack trace in effect when an
558  event is triggered as the key for the hash table.  This allows the
559  enumeration of every kernel callpath that led up to a particular
560  event, along with a running total of any of the event fields for
561  that event.  Here we tally bytes requested and bytes allocated for
562  every callpath in the system that led up to a kmalloc (in this case
563  every callpath to a kmalloc for a kernel compile)::
564
565    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
566    # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]
567
568    { stacktrace:
569         __kmalloc_track_caller+0x10b/0x1a0
570         kmemdup+0x20/0x50
571         hidraw_report_event+0x8a/0x120 [hid]
572         hid_report_raw_event+0x3ea/0x440 [hid]
573         hid_input_report+0x112/0x190 [hid]
574         hid_irq_in+0xc2/0x260 [usbhid]
575         __usb_hcd_giveback_urb+0x72/0x120
576         usb_giveback_urb_bh+0x9e/0xe0
577         tasklet_hi_action+0xf8/0x100
578         __do_softirq+0x114/0x2c0
579         irq_exit+0xa5/0xb0
580         do_IRQ+0x5a/0xf0
581         ret_from_intr+0x0/0x30
582         cpuidle_enter+0x17/0x20
583         cpu_startup_entry+0x315/0x3e0
584         rest_init+0x7c/0x80
585    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
586    { stacktrace:
587         __kmalloc_track_caller+0x10b/0x1a0
588         kmemdup+0x20/0x50
589         hidraw_report_event+0x8a/0x120 [hid]
590         hid_report_raw_event+0x3ea/0x440 [hid]
591         hid_input_report+0x112/0x190 [hid]
592         hid_irq_in+0xc2/0x260 [usbhid]
593         __usb_hcd_giveback_urb+0x72/0x120
594         usb_giveback_urb_bh+0x9e/0xe0
595         tasklet_hi_action+0xf8/0x100
596         __do_softirq+0x114/0x2c0
597         irq_exit+0xa5/0xb0
598         do_IRQ+0x5a/0xf0
599         ret_from_intr+0x0/0x30
600    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
601    { stacktrace:
602         kmem_cache_alloc_trace+0xeb/0x150
603         aa_alloc_task_context+0x27/0x40
604         apparmor_cred_prepare+0x1f/0x50
605         security_prepare_creds+0x16/0x20
606         prepare_creds+0xdf/0x1a0
607         SyS_capset+0xb5/0x200
608         system_call_fastpath+0x12/0x6a
609    } hitcount:          1  bytes_req:         32  bytes_alloc:         32
610    .
611    .
612    .
613    { stacktrace:
614         __kmalloc+0x11b/0x1b0
615         i915_gem_execbuffer2+0x6c/0x2c0 [i915]
616         drm_ioctl+0x349/0x670 [drm]
617         do_vfs_ioctl+0x2f0/0x4f0
618         SyS_ioctl+0x81/0xa0
619         system_call_fastpath+0x12/0x6a
620    } hitcount:      17726  bytes_req:   13944120  bytes_alloc:   19593808
621    { stacktrace:
622         __kmalloc+0x11b/0x1b0
623         load_elf_phdrs+0x76/0xa0
624         load_elf_binary+0x102/0x1650
625         search_binary_handler+0x97/0x1d0
626         do_execveat_common.isra.34+0x551/0x6e0
627         SyS_execve+0x3a/0x50
628         return_from_execve+0x0/0x23
629    } hitcount:      33348  bytes_req:   17152128  bytes_alloc:   20226048
630    { stacktrace:
631         kmem_cache_alloc_trace+0xeb/0x150
632         apparmor_file_alloc_security+0x27/0x40
633         security_file_alloc+0x16/0x20
634         get_empty_filp+0x93/0x1c0
635         path_openat+0x31/0x5f0
636         do_filp_open+0x3a/0x90
637         do_sys_open+0x128/0x220
638         SyS_open+0x1e/0x20
639         system_call_fastpath+0x12/0x6a
640    } hitcount:    4766422  bytes_req:    9532844  bytes_alloc:   38131376
641    { stacktrace:
642         __kmalloc+0x11b/0x1b0
643         seq_buf_alloc+0x1b/0x50
644         seq_read+0x2cc/0x370
645         proc_reg_read+0x3d/0x80
646         __vfs_read+0x28/0xe0
647         vfs_read+0x86/0x140
648         SyS_read+0x46/0xb0
649         system_call_fastpath+0x12/0x6a
650    } hitcount:      19133  bytes_req:   78368768  bytes_alloc:   78368768
651
652    Totals:
653        Hits: 6085872
654        Entries: 253
655        Dropped: 0
656
657  If you key a hist trigger on common_pid, in order for example to
658  gather and display sorted totals for each process, you can use the
659  special .execname modifier to display the executable names for the
660  processes in the table rather than raw pids.  The example below
661  keeps a per-process sum of total bytes read::
662
663    # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
664           /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger
665
666    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
667    # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]
668
669    { common_pid: gnome-terminal  [      3196] } hitcount:        280  count:    1093512
670    { common_pid: Xorg            [      1309] } hitcount:        525  count:     256640
671    { common_pid: compiz          [      2889] } hitcount:         59  count:     254400
672    { common_pid: bash            [      8710] } hitcount:          3  count:      66369
673    { common_pid: dbus-daemon-lau [      8703] } hitcount:         49  count:      47739
674    { common_pid: irqbalance      [      1252] } hitcount:         27  count:      27648
675    { common_pid: 01ifupdown      [      8705] } hitcount:          3  count:      17216
676    { common_pid: dbus-daemon     [       772] } hitcount:         10  count:      12396
677    { common_pid: Socket Thread   [      8342] } hitcount:         11  count:      11264
678    { common_pid: nm-dhcp-client. [      8701] } hitcount:          6  count:       7424
679    { common_pid: gmain           [      1315] } hitcount:         18  count:       6336
680    .
681    .
682    .
683    { common_pid: postgres        [      1892] } hitcount:          2  count:         32
684    { common_pid: postgres        [      1891] } hitcount:          2  count:         32
685    { common_pid: gmain           [      8704] } hitcount:          2  count:         32
686    { common_pid: upstart-dbus-br [      2740] } hitcount:         21  count:         21
687    { common_pid: nm-dispatcher.a [      8696] } hitcount:          1  count:         16
688    { common_pid: indicator-datet [      2904] } hitcount:          1  count:         16
689    { common_pid: gdbus           [      2998] } hitcount:          1  count:         16
690    { common_pid: rtkit-daemon    [      2052] } hitcount:          1  count:          8
691    { common_pid: init            [         1] } hitcount:          2  count:          2
692
693    Totals:
694        Hits: 2116
695        Entries: 51
696        Dropped: 0
697
698  Similarly, if you key a hist trigger on syscall id, for example to
699  gather and display a list of systemwide syscall hits, you can use
700  the special .syscall modifier to display the syscall names rather
701  than raw ids.  The example below keeps a running total of syscall
702  counts for the system during the run::
703
704    # echo 'hist:key=id.syscall:val=hitcount' > \
705           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
706
707    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
708    # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]
709
710    { id: sys_fsync                     [ 74] } hitcount:          1
711    { id: sys_newuname                  [ 63] } hitcount:          1
712    { id: sys_prctl                     [157] } hitcount:          1
713    { id: sys_statfs                    [137] } hitcount:          1
714    { id: sys_symlink                   [ 88] } hitcount:          1
715    { id: sys_sendmmsg                  [307] } hitcount:          1
716    { id: sys_semctl                    [ 66] } hitcount:          1
717    { id: sys_readlink                  [ 89] } hitcount:          3
718    { id: sys_bind                      [ 49] } hitcount:          3
719    { id: sys_getsockname               [ 51] } hitcount:          3
720    { id: sys_unlink                    [ 87] } hitcount:          3
721    { id: sys_rename                    [ 82] } hitcount:          4
722    { id: unknown_syscall               [ 58] } hitcount:          4
723    { id: sys_connect                   [ 42] } hitcount:          4
724    { id: sys_getpid                    [ 39] } hitcount:          4
725    .
726    .
727    .
728    { id: sys_rt_sigprocmask            [ 14] } hitcount:        952
729    { id: sys_futex                     [202] } hitcount:       1534
730    { id: sys_write                     [  1] } hitcount:       2689
731    { id: sys_setitimer                 [ 38] } hitcount:       2797
732    { id: sys_read                      [  0] } hitcount:       3202
733    { id: sys_select                    [ 23] } hitcount:       3773
734    { id: sys_writev                    [ 20] } hitcount:       4531
735    { id: sys_poll                      [  7] } hitcount:       8314
736    { id: sys_recvmsg                   [ 47] } hitcount:      13738
737    { id: sys_ioctl                     [ 16] } hitcount:      21843
738
739    Totals:
740        Hits: 67612
741        Entries: 72
742        Dropped: 0
743
744  The syscall counts above provide a rough overall picture of system
745  call activity on the system; we can see for example that the most
746  popular system call on this system was the 'sys_ioctl' system call.
747
748  We can use 'compound' keys to refine that number and provide some
749  further insight as to which processes exactly contribute to the
750  overall ioctl count.
751
752  The command below keeps a hitcount for every unique combination of
753  system call id and pid - the end result is essentially a table
754  that keeps a per-pid sum of system call hits.  The results are
755  sorted using the system call id as the primary key, and the
756  hitcount sum as the secondary key::
757
758    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
759           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
760
761    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
762    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]
763
764    { id: sys_read                      [  0], common_pid: rtkit-daemon    [      1877] } hitcount:          1
765    { id: sys_read                      [  0], common_pid: gdbus           [      2976] } hitcount:          1
766    { id: sys_read                      [  0], common_pid: console-kit-dae [      3400] } hitcount:          1
767    { id: sys_read                      [  0], common_pid: postgres        [      1865] } hitcount:          1
768    { id: sys_read                      [  0], common_pid: deja-dup-monito [      3543] } hitcount:          2
769    { id: sys_read                      [  0], common_pid: NetworkManager  [       890] } hitcount:          2
770    { id: sys_read                      [  0], common_pid: evolution-calen [      3048] } hitcount:          2
771    { id: sys_read                      [  0], common_pid: postgres        [      1864] } hitcount:          2
772    { id: sys_read                      [  0], common_pid: nm-applet       [      3022] } hitcount:          2
773    { id: sys_read                      [  0], common_pid: whoopsie        [      1212] } hitcount:          2
774    .
775    .
776    .
777    { id: sys_ioctl                     [ 16], common_pid: bash            [      8479] } hitcount:          1
778    { id: sys_ioctl                     [ 16], common_pid: bash            [      3472] } hitcount:         12
779    { id: sys_ioctl                     [ 16], common_pid: gnome-terminal  [      3199] } hitcount:         16
780    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:       1808
781    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:       5580
782    .
783    .
784    .
785    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2690] } hitcount:          3
786    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2688] } hitcount:         16
787    { id: sys_inotify_add_watch         [254], common_pid: gmain           [       975] } hitcount:          2
788    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3204] } hitcount:          4
789    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2888] } hitcount:          4
790    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3003] } hitcount:          4
791    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2873] } hitcount:          4
792    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3196] } hitcount:          6
793    { id: sys_openat                    [257], common_pid: java            [      2623] } hitcount:          2
794    { id: sys_eventfd2                  [290], common_pid: ibus-ui-gtk3    [      2760] } hitcount:          4
795    { id: sys_eventfd2                  [290], common_pid: compiz          [      2994] } hitcount:          6
796
797    Totals:
798        Hits: 31536
799        Entries: 323
800        Dropped: 0
801
802  The above list does give us a breakdown of the ioctl syscall by
803  pid, but it also gives us quite a bit more than that, which we
804  don't really care about at the moment.  Since we know the syscall
805  id for sys_ioctl (16, displayed next to the sys_ioctl name), we
806  can use that to filter out all the other syscalls::
807
808    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
809           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
810
811    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
812    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]
813
814    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2769] } hitcount:          1
815    { id: sys_ioctl                     [ 16], common_pid: evolution-addre [      8571] } hitcount:          1
816    { id: sys_ioctl                     [ 16], common_pid: gmain           [      3003] } hitcount:          1
817    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2781] } hitcount:          1
818    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2829] } hitcount:          1
819    { id: sys_ioctl                     [ 16], common_pid: bash            [      8726] } hitcount:          1
820    { id: sys_ioctl                     [ 16], common_pid: bash            [      8508] } hitcount:          1
821    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2970] } hitcount:          1
822    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2768] } hitcount:          1
823    .
824    .
825    .
826    { id: sys_ioctl                     [ 16], common_pid: pool            [      8559] } hitcount:         45
827    { id: sys_ioctl                     [ 16], common_pid: pool            [      8555] } hitcount:         48
828    { id: sys_ioctl                     [ 16], common_pid: pool            [      8551] } hitcount:         48
829    { id: sys_ioctl                     [ 16], common_pid: avahi-daemon    [       896] } hitcount:         66
830    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:      26674
831    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:      73443
832
833    Totals:
834        Hits: 101162
835        Entries: 103
836        Dropped: 0
837
838  The above output shows that 'compiz' and 'Xorg' are far and away
839  the heaviest ioctl callers (which might lead to questions about
840  whether they really need to be making all those calls and to
841  possible avenues for further investigation.)
842
843  The compound key examples used a key and a sum value (hitcount) to
844  sort the output, but we can just as easily use two keys instead.
845  Here's an example where we use a compound key composed of the the
846  common_pid and size event fields.  Sorting with pid as the primary
847  key and 'size' as the secondary key allows us to display an
848  ordered summary of the recvfrom sizes, with counts, received by
849  each process::
850
851    # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
852           /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger
853
854    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
855    # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]
856
857    { common_pid: smbd            [       784], size:          4 } hitcount:          1
858    { common_pid: dnsmasq         [      1412], size:       4096 } hitcount:        672
859    { common_pid: postgres        [      1796], size:       1000 } hitcount:          6
860    { common_pid: postgres        [      1867], size:       1000 } hitcount:         10
861    { common_pid: bamfdaemon      [      2787], size:         28 } hitcount:          2
862    { common_pid: bamfdaemon      [      2787], size:      14360 } hitcount:          1
863    { common_pid: compiz          [      2994], size:          8 } hitcount:          1
864    { common_pid: compiz          [      2994], size:         20 } hitcount:         11
865    { common_pid: gnome-terminal  [      3199], size:          4 } hitcount:          2
866    { common_pid: firefox         [      8817], size:          4 } hitcount:          1
867    { common_pid: firefox         [      8817], size:          8 } hitcount:          5
868    { common_pid: firefox         [      8817], size:        588 } hitcount:          2
869    { common_pid: firefox         [      8817], size:        628 } hitcount:          1
870    { common_pid: firefox         [      8817], size:       6944 } hitcount:          1
871    { common_pid: firefox         [      8817], size:     408880 } hitcount:          2
872    { common_pid: firefox         [      8822], size:          8 } hitcount:          2
873    { common_pid: firefox         [      8822], size:        160 } hitcount:          2
874    { common_pid: firefox         [      8822], size:        320 } hitcount:          2
875    { common_pid: firefox         [      8822], size:        352 } hitcount:          1
876    .
877    .
878    .
879    { common_pid: pool            [      8923], size:       1960 } hitcount:         10
880    { common_pid: pool            [      8923], size:       2048 } hitcount:         10
881    { common_pid: pool            [      8924], size:       1960 } hitcount:         10
882    { common_pid: pool            [      8924], size:       2048 } hitcount:         10
883    { common_pid: pool            [      8928], size:       1964 } hitcount:          4
884    { common_pid: pool            [      8928], size:       1965 } hitcount:          2
885    { common_pid: pool            [      8928], size:       2048 } hitcount:          6
886    { common_pid: pool            [      8929], size:       1982 } hitcount:          1
887    { common_pid: pool            [      8929], size:       2048 } hitcount:          1
888
889    Totals:
890        Hits: 2016
891        Entries: 224
892        Dropped: 0
893
894  The above example also illustrates the fact that although a compound
895  key is treated as a single entity for hashing purposes, the sub-keys
896  it's composed of can be accessed independently.
897
898  The next example uses a string field as the hash key and
899  demonstrates how you can manually pause and continue a hist trigger.
900  In this example, we'll aggregate fork counts and don't expect a
901  large number of entries in the hash table, so we'll drop it to a
902  much smaller number, say 256::
903
904    # echo 'hist:key=child_comm:val=hitcount:size=256' > \
905           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
906
907    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
908    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
909
910    { child_comm: dconf worker                        } hitcount:          1
911    { child_comm: ibus-daemon                         } hitcount:          1
912    { child_comm: whoopsie                            } hitcount:          1
913    { child_comm: smbd                                } hitcount:          1
914    { child_comm: gdbus                               } hitcount:          1
915    { child_comm: kthreadd                            } hitcount:          1
916    { child_comm: dconf worker                        } hitcount:          1
917    { child_comm: evolution-alarm                     } hitcount:          2
918    { child_comm: Socket Thread                       } hitcount:          2
919    { child_comm: postgres                            } hitcount:          2
920    { child_comm: bash                                } hitcount:          3
921    { child_comm: compiz                              } hitcount:          3
922    { child_comm: evolution-sourc                     } hitcount:          4
923    { child_comm: dhclient                            } hitcount:          4
924    { child_comm: pool                                } hitcount:          5
925    { child_comm: nm-dispatcher.a                     } hitcount:          8
926    { child_comm: firefox                             } hitcount:          8
927    { child_comm: dbus-daemon                         } hitcount:          8
928    { child_comm: glib-pacrunner                      } hitcount:         10
929    { child_comm: evolution                           } hitcount:         23
930
931    Totals:
932        Hits: 89
933        Entries: 20
934        Dropped: 0
935
936  If we want to pause the hist trigger, we can simply append :pause to
937  the command that started the trigger.  Notice that the trigger info
938  displays as [paused]::
939
940    # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
941           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
942
943    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
944    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]
945
946    { child_comm: dconf worker                        } hitcount:          1
947    { child_comm: kthreadd                            } hitcount:          1
948    { child_comm: dconf worker                        } hitcount:          1
949    { child_comm: gdbus                               } hitcount:          1
950    { child_comm: ibus-daemon                         } hitcount:          1
951    { child_comm: Socket Thread                       } hitcount:          2
952    { child_comm: evolution-alarm                     } hitcount:          2
953    { child_comm: smbd                                } hitcount:          2
954    { child_comm: bash                                } hitcount:          3
955    { child_comm: whoopsie                            } hitcount:          3
956    { child_comm: compiz                              } hitcount:          3
957    { child_comm: evolution-sourc                     } hitcount:          4
958    { child_comm: pool                                } hitcount:          5
959    { child_comm: postgres                            } hitcount:          6
960    { child_comm: firefox                             } hitcount:          8
961    { child_comm: dhclient                            } hitcount:         10
962    { child_comm: emacs                               } hitcount:         12
963    { child_comm: dbus-daemon                         } hitcount:         20
964    { child_comm: nm-dispatcher.a                     } hitcount:         20
965    { child_comm: evolution                           } hitcount:         35
966    { child_comm: glib-pacrunner                      } hitcount:         59
967
968    Totals:
969        Hits: 199
970        Entries: 21
971        Dropped: 0
972
973  To manually continue having the trigger aggregate events, append
974  :cont instead.  Notice that the trigger info displays as [active]
975  again, and the data has changed::
976
977    # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
978           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
979
980    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
981    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
982
983    { child_comm: dconf worker                        } hitcount:          1
984    { child_comm: dconf worker                        } hitcount:          1
985    { child_comm: kthreadd                            } hitcount:          1
986    { child_comm: gdbus                               } hitcount:          1
987    { child_comm: ibus-daemon                         } hitcount:          1
988    { child_comm: Socket Thread                       } hitcount:          2
989    { child_comm: evolution-alarm                     } hitcount:          2
990    { child_comm: smbd                                } hitcount:          2
991    { child_comm: whoopsie                            } hitcount:          3
992    { child_comm: compiz                              } hitcount:          3
993    { child_comm: evolution-sourc                     } hitcount:          4
994    { child_comm: bash                                } hitcount:          5
995    { child_comm: pool                                } hitcount:          5
996    { child_comm: postgres                            } hitcount:          6
997    { child_comm: firefox                             } hitcount:          8
998    { child_comm: dhclient                            } hitcount:         11
999    { child_comm: emacs                               } hitcount:         12
1000    { child_comm: dbus-daemon                         } hitcount:         22
1001    { child_comm: nm-dispatcher.a                     } hitcount:         22
1002    { child_comm: evolution                           } hitcount:         35
1003    { child_comm: glib-pacrunner                      } hitcount:         59
1004
1005    Totals:
1006        Hits: 206
1007        Entries: 21
1008        Dropped: 0
1009
1010  The previous example showed how to start and stop a hist trigger by
1011  appending 'pause' and 'continue' to the hist trigger command.  A
1012  hist trigger can also be started in a paused state by initially
1013  starting the trigger with ':pause' appended.  This allows you to
1014  start the trigger only when you're ready to start collecting data
1015  and not before.  For example, you could start the trigger in a
1016  paused state, then unpause it and do something you want to measure,
1017  then pause the trigger again when done.
1018
1019  Of course, doing this manually can be difficult and error-prone, but
1020  it is possible to automatically start and stop a hist trigger based
1021  on some condition, via the enable_hist and disable_hist triggers.
1022
1023  For example, suppose we wanted to take a look at the relative
1024  weights in terms of skb length for each callpath that leads to a
1025  netif_receieve_skb event when downloading a decent-sized file using
1026  wget.
1027
1028  First we set up an initially paused stacktrace trigger on the
1029  netif_receive_skb event::
1030
1031    # echo 'hist:key=stacktrace:vals=len:pause' > \
1032           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1033
1034  Next, we set up an 'enable_hist' trigger on the sched_process_exec
1035  event, with an 'if filename==/usr/bin/wget' filter.  The effect of
1036  this new trigger is that it will 'unpause' the hist trigger we just
1037  set up on netif_receive_skb if and only if it sees a
1038  sched_process_exec event with a filename of '/usr/bin/wget'.  When
1039  that happens, all netif_receive_skb events are aggregated into a
1040  hash table keyed on stacktrace::
1041
1042    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
1043           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1044
1045  The aggregation continues until the netif_receive_skb is paused
1046  again, which is what the following disable_hist event does by
1047  creating a similar setup on the sched_process_exit event, using the
1048  filter 'comm==wget'::
1049
1050    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
1051           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1052
1053  Whenever a process exits and the comm field of the disable_hist
1054  trigger filter matches 'comm==wget', the netif_receive_skb hist
1055  trigger is disabled.
1056
1057  The overall effect is that netif_receive_skb events are aggregated
1058  into the hash table for only the duration of the wget.  Executing a
1059  wget command and then listing the 'hist' file will display the
1060  output generated by the wget command::
1061
1062    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
1063
1064    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1065    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
1066
1067    { stacktrace:
1068         __netif_receive_skb_core+0x46d/0x990
1069         __netif_receive_skb+0x18/0x60
1070         netif_receive_skb_internal+0x23/0x90
1071         napi_gro_receive+0xc8/0x100
1072         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
1073         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
1074         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
1075         ieee80211_rx+0x31d/0x900 [mac80211]
1076         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
1077         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
1078         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
1079         irq_thread_fn+0x20/0x50
1080         irq_thread+0x11f/0x150
1081         kthread+0xd2/0xf0
1082         ret_from_fork+0x42/0x70
1083    } hitcount:         85  len:      28884
1084    { stacktrace:
1085         __netif_receive_skb_core+0x46d/0x990
1086         __netif_receive_skb+0x18/0x60
1087         netif_receive_skb_internal+0x23/0x90
1088         napi_gro_complete+0xa4/0xe0
1089         dev_gro_receive+0x23a/0x360
1090         napi_gro_receive+0x30/0x100
1091         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
1092         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
1093         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
1094         ieee80211_rx+0x31d/0x900 [mac80211]
1095         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
1096         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
1097         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
1098         irq_thread_fn+0x20/0x50
1099         irq_thread+0x11f/0x150
1100         kthread+0xd2/0xf0
1101    } hitcount:         98  len:     664329
1102    { stacktrace:
1103         __netif_receive_skb_core+0x46d/0x990
1104         __netif_receive_skb+0x18/0x60
1105         process_backlog+0xa8/0x150
1106         net_rx_action+0x15d/0x340
1107         __do_softirq+0x114/0x2c0
1108         do_softirq_own_stack+0x1c/0x30
1109         do_softirq+0x65/0x70
1110         __local_bh_enable_ip+0xb5/0xc0
1111         ip_finish_output+0x1f4/0x840
1112         ip_output+0x6b/0xc0
1113         ip_local_out_sk+0x31/0x40
1114         ip_send_skb+0x1a/0x50
1115         udp_send_skb+0x173/0x2a0
1116         udp_sendmsg+0x2bf/0x9f0
1117         inet_sendmsg+0x64/0xa0
1118         sock_sendmsg+0x3d/0x50
1119    } hitcount:        115  len:      13030
1120    { stacktrace:
1121         __netif_receive_skb_core+0x46d/0x990
1122         __netif_receive_skb+0x18/0x60
1123         netif_receive_skb_internal+0x23/0x90
1124         napi_gro_complete+0xa4/0xe0
1125         napi_gro_flush+0x6d/0x90
1126         iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
1127         irq_thread_fn+0x20/0x50
1128         irq_thread+0x11f/0x150
1129         kthread+0xd2/0xf0
1130         ret_from_fork+0x42/0x70
1131    } hitcount:        934  len:    5512212
1132
1133    Totals:
1134        Hits: 1232
1135        Entries: 4
1136        Dropped: 0
1137
1138  The above shows all the netif_receive_skb callpaths and their total
1139  lengths for the duration of the wget command.
1140
1141  The 'clear' hist trigger param can be used to clear the hash table.
1142  Suppose we wanted to try another run of the previous example but
1143  this time also wanted to see the complete list of events that went
1144  into the histogram.  In order to avoid having to set everything up
1145  again, we can just clear the histogram first::
1146
1147    # echo 'hist:key=stacktrace:vals=len:clear' >> \
1148           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1149
1150  Just to verify that it is in fact cleared, here's what we now see in
1151  the hist file::
1152
1153    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1154    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
1155
1156    Totals:
1157        Hits: 0
1158        Entries: 0
1159        Dropped: 0
1160
1161  Since we want to see the detailed list of every netif_receive_skb
1162  event occurring during the new run, which are in fact the same
1163  events being aggregated into the hash table, we add some additional
1164  'enable_event' events to the triggering sched_process_exec and
1165  sched_process_exit events as such::
1166
1167    # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
1168           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1169
1170    # echo 'disable_event:net:netif_receive_skb if comm==wget' > \
1171           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1172
1173  If you read the trigger files for the sched_process_exec and
1174  sched_process_exit triggers, you should see two triggers for each:
1175  one enabling/disabling the hist aggregation and the other
1176  enabling/disabling the logging of events::
1177
1178    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1179    enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
1180    enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
1181
1182    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1183    enable_event:net:netif_receive_skb:unlimited if comm==wget
1184    disable_hist:net:netif_receive_skb:unlimited if comm==wget
1185
1186  In other words, whenever either of the sched_process_exec or
1187  sched_process_exit events is hit and matches 'wget', it enables or
1188  disables both the histogram and the event log, and what you end up
1189  with is a hash table and set of events just covering the specified
1190  duration.  Run the wget command again::
1191
1192    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
1193
1194  Displaying the 'hist' file should show something similar to what you
1195  saw in the last run, but this time you should also see the
1196  individual events in the trace file::
1197
1198    # cat /sys/kernel/debug/tracing/trace
1199
1200    # tracer: nop
1201    #
1202    # entries-in-buffer/entries-written: 183/1426   #P:4
1203    #
1204    #                              _-----=> irqs-off
1205    #                             / _----=> need-resched
1206    #                            | / _---=> hardirq/softirq
1207    #                            || / _--=> preempt-depth
1208    #                            ||| /     delay
1209    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1210    #              | |       |   ||||       |         |
1211                wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
1212                wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
1213             dnsmasq-1382  [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
1214             dnsmasq-1382  [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
1215    ##### CPU 2 buffer started ####
1216      irq/29-iwlwifi-559   [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
1217      irq/29-iwlwifi-559   [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
1218      irq/29-iwlwifi-559   [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
1219      irq/29-iwlwifi-559   [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
1220      irq/29-iwlwifi-559   [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
1221    .
1222    .
1223    .
1224
1225  The following example demonstrates how multiple hist triggers can be
1226  attached to a given event.  This capability can be useful for
1227  creating a set of different summaries derived from the same set of
1228  events, or for comparing the effects of different filters, among
1229  other things::
1230
1231    # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
1232           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1233    # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
1234           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1235    # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
1236           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1237    # echo 'hist:keys=skbaddr.hex:vals=len' >> \
1238           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1239    # echo 'hist:keys=len:vals=common_preempt_count' >> \
1240           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1241
1242  The above set of commands create four triggers differing only in
1243  their filters, along with a completely different though fairly
1244  nonsensical trigger.  Note that in order to append multiple hist
1245  triggers to the same file, you should use the '>>' operator to
1246  append them ('>' will also add the new hist trigger, but will remove
1247  any existing hist triggers beforehand).
1248
1249  Displaying the contents of the 'hist' file for the event shows the
1250  contents of all five histograms::
1251
1252    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1253
1254    # event histogram
1255    #
1256    # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
1257    #
1258
1259    { len:        176 } hitcount:          1  common_preempt_count:          0
1260    { len:        223 } hitcount:          1  common_preempt_count:          0
1261    { len:       4854 } hitcount:          1  common_preempt_count:          0
1262    { len:        395 } hitcount:          1  common_preempt_count:          0
1263    { len:        177 } hitcount:          1  common_preempt_count:          0
1264    { len:        446 } hitcount:          1  common_preempt_count:          0
1265    { len:       1601 } hitcount:          1  common_preempt_count:          0
1266    .
1267    .
1268    .
1269    { len:       1280 } hitcount:         66  common_preempt_count:          0
1270    { len:        116 } hitcount:         81  common_preempt_count:         40
1271    { len:        708 } hitcount:        112  common_preempt_count:          0
1272    { len:         46 } hitcount:        221  common_preempt_count:          0
1273    { len:       1264 } hitcount:        458  common_preempt_count:          0
1274
1275    Totals:
1276        Hits: 1428
1277        Entries: 147
1278        Dropped: 0
1279
1280
1281    # event histogram
1282    #
1283    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1284    #
1285
1286    { skbaddr: ffff8800baee5e00 } hitcount:          1  len:        130
1287    { skbaddr: ffff88005f3d5600 } hitcount:          1  len:       1280
1288    { skbaddr: ffff88005f3d4900 } hitcount:          1  len:       1280
1289    { skbaddr: ffff88009fed6300 } hitcount:          1  len:        115
1290    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:        115
1291    { skbaddr: ffff88008cdb1900 } hitcount:          1  len:         46
1292    { skbaddr: ffff880064b5ef00 } hitcount:          1  len:        118
1293    { skbaddr: ffff880044e3c700 } hitcount:          1  len:         60
1294    { skbaddr: ffff880100065900 } hitcount:          1  len:         46
1295    { skbaddr: ffff8800d46bd500 } hitcount:          1  len:        116
1296    { skbaddr: ffff88005f3d5f00 } hitcount:          1  len:       1280
1297    { skbaddr: ffff880100064700 } hitcount:          1  len:        365
1298    { skbaddr: ffff8800badb6f00 } hitcount:          1  len:         60
1299    .
1300    .
1301    .
1302    { skbaddr: ffff88009fe0be00 } hitcount:         27  len:      24677
1303    { skbaddr: ffff88009fe0a400 } hitcount:         27  len:      23052
1304    { skbaddr: ffff88009fe0b700 } hitcount:         31  len:      25589
1305    { skbaddr: ffff88009fe0b600 } hitcount:         32  len:      27326
1306    { skbaddr: ffff88006a462800 } hitcount:         68  len:      71678
1307    { skbaddr: ffff88006a463700 } hitcount:         70  len:      72678
1308    { skbaddr: ffff88006a462b00 } hitcount:         71  len:      77589
1309    { skbaddr: ffff88006a463600 } hitcount:         73  len:      71307
1310    { skbaddr: ffff88006a462200 } hitcount:         81  len:      81032
1311
1312    Totals:
1313        Hits: 1451
1314        Entries: 318
1315        Dropped: 0
1316
1317
1318    # event histogram
1319    #
1320    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
1321    #
1322
1323
1324    Totals:
1325        Hits: 0
1326        Entries: 0
1327        Dropped: 0
1328
1329
1330    # event histogram
1331    #
1332    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active]
1333    #
1334
1335    { skbaddr: ffff88009fd2c300 } hitcount:          1  len:       7212
1336    { skbaddr: ffff8800d2bcce00 } hitcount:          1  len:       7212
1337    { skbaddr: ffff8800d2bcd700 } hitcount:          1  len:       7212
1338    { skbaddr: ffff8800d2bcda00 } hitcount:          1  len:      21492
1339    { skbaddr: ffff8800ae2e2d00 } hitcount:          1  len:       7212
1340    { skbaddr: ffff8800d2bcdb00 } hitcount:          1  len:       7212
1341    { skbaddr: ffff88006a4df500 } hitcount:          1  len:       4854
1342    { skbaddr: ffff88008ce47b00 } hitcount:          1  len:      18636
1343    { skbaddr: ffff8800ae2e2200 } hitcount:          1  len:      12924
1344    { skbaddr: ffff88005f3e1000 } hitcount:          1  len:       4356
1345    { skbaddr: ffff8800d2bcdc00 } hitcount:          2  len:      24420
1346    { skbaddr: ffff8800d2bcc200 } hitcount:          2  len:      12996
1347
1348    Totals:
1349        Hits: 14
1350        Entries: 12
1351        Dropped: 0
1352
1353
1354    # event histogram
1355    #
1356    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
1357    #
1358
1359
1360    Totals:
1361        Hits: 0
1362        Entries: 0
1363        Dropped: 0
1364
1365  Named triggers can be used to have triggers share a common set of
1366  histogram data.  This capability is mostly useful for combining the
1367  output of events generated by tracepoints contained inside inline
1368  functions, but names can be used in a hist trigger on any event.
1369  For example, these two triggers when hit will update the same 'len'
1370  field in the shared 'foo' histogram data::
1371
1372    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
1373           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1374    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
1375           /sys/kernel/debug/tracing/events/net/netif_rx/trigger
1376
1377  You can see that they're updating common histogram data by reading
1378  each event's hist files at the same time::
1379
1380    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
1381      cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
1382
1383    # event histogram
1384    #
1385    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1386    #
1387
1388    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
1389    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
1390    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
1391    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
1392    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
1393    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
1394    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
1395    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
1396    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
1397    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
1398    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
1399    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
1400    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
1401    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
1402    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
1403    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
1404    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
1405    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
1406    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
1407    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
1408    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
1409    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
1410    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
1411    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
1412    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
1413    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
1414    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
1415    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
1416    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
1417    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
1418    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
1419    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
1420    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
1421    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
1422    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
1423    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
1424    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
1425    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
1426    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
1427    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
1428    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
1429    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276
1430
1431    Totals:
1432        Hits: 81
1433        Entries: 42
1434        Dropped: 0
1435    # event histogram
1436    #
1437    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1438    #
1439
1440    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
1441    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
1442    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
1443    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
1444    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
1445    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
1446    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
1447    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
1448    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
1449    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
1450    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
1451    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
1452    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
1453    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
1454    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
1455    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
1456    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
1457    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
1458    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
1459    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
1460    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
1461    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
1462    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
1463    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
1464    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
1465    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
1466    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
1467    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
1468    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
1469    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
1470    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
1471    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
1472    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
1473    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
1474    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
1475    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
1476    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
1477    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
1478    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
1479    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
1480    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
1481    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276
1482
1483    Totals:
1484        Hits: 81
1485        Entries: 42
1486        Dropped: 0
1487
1488  And here's an example that shows how to combine histogram data from
1489  any two events even if they don't share any 'compatible' fields
1490  other than 'hitcount' and 'stacktrace'.  These commands create a
1491  couple of triggers named 'bar' using those fields::
1492
1493    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
1494           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
1495    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
1496          /sys/kernel/debug/tracing/events/net/netif_rx/trigger
1497
1498  And displaying the output of either shows some interesting if
1499  somewhat confusing output::
1500
1501    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
1502    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
1503
1504    # event histogram
1505    #
1506    # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
1507    #
1508
1509    { stacktrace:
1510             _do_fork+0x18e/0x330
1511             kernel_thread+0x29/0x30
1512             kthreadd+0x154/0x1b0
1513             ret_from_fork+0x3f/0x70
1514    } hitcount:          1
1515    { stacktrace:
1516             netif_rx_internal+0xb2/0xd0
1517             netif_rx_ni+0x20/0x70
1518             dev_loopback_xmit+0xaa/0xd0
1519             ip_mc_output+0x126/0x240
1520             ip_local_out_sk+0x31/0x40
1521             igmp_send_report+0x1e9/0x230
1522             igmp_timer_expire+0xe9/0x120
1523             call_timer_fn+0x39/0xf0
1524             run_timer_softirq+0x1e1/0x290
1525             __do_softirq+0xfd/0x290
1526             irq_exit+0x98/0xb0
1527             smp_apic_timer_interrupt+0x4a/0x60
1528             apic_timer_interrupt+0x6d/0x80
1529             cpuidle_enter+0x17/0x20
1530             call_cpuidle+0x3b/0x60
1531             cpu_startup_entry+0x22d/0x310
1532    } hitcount:          1
1533    { stacktrace:
1534             netif_rx_internal+0xb2/0xd0
1535             netif_rx_ni+0x20/0x70
1536             dev_loopback_xmit+0xaa/0xd0
1537             ip_mc_output+0x17f/0x240
1538             ip_local_out_sk+0x31/0x40
1539             ip_send_skb+0x1a/0x50
1540             udp_send_skb+0x13e/0x270
1541             udp_sendmsg+0x2bf/0x980
1542             inet_sendmsg+0x67/0xa0
1543             sock_sendmsg+0x38/0x50
1544             SYSC_sendto+0xef/0x170
1545             SyS_sendto+0xe/0x10
1546             entry_SYSCALL_64_fastpath+0x12/0x6a
1547    } hitcount:          2
1548    { stacktrace:
1549             netif_rx_internal+0xb2/0xd0
1550             netif_rx+0x1c/0x60
1551             loopback_xmit+0x6c/0xb0
1552             dev_hard_start_xmit+0x219/0x3a0
1553             __dev_queue_xmit+0x415/0x4f0
1554             dev_queue_xmit_sk+0x13/0x20
1555             ip_finish_output2+0x237/0x340
1556             ip_finish_output+0x113/0x1d0
1557             ip_output+0x66/0xc0
1558             ip_local_out_sk+0x31/0x40
1559             ip_send_skb+0x1a/0x50
1560             udp_send_skb+0x16d/0x270
1561             udp_sendmsg+0x2bf/0x980
1562             inet_sendmsg+0x67/0xa0
1563             sock_sendmsg+0x38/0x50
1564             ___sys_sendmsg+0x14e/0x270
1565    } hitcount:         76
1566    { stacktrace:
1567             netif_rx_internal+0xb2/0xd0
1568             netif_rx+0x1c/0x60
1569             loopback_xmit+0x6c/0xb0
1570             dev_hard_start_xmit+0x219/0x3a0
1571             __dev_queue_xmit+0x415/0x4f0
1572             dev_queue_xmit_sk+0x13/0x20
1573             ip_finish_output2+0x237/0x340
1574             ip_finish_output+0x113/0x1d0
1575             ip_output+0x66/0xc0
1576             ip_local_out_sk+0x31/0x40
1577             ip_send_skb+0x1a/0x50
1578             udp_send_skb+0x16d/0x270
1579             udp_sendmsg+0x2bf/0x980
1580             inet_sendmsg+0x67/0xa0
1581             sock_sendmsg+0x38/0x50
1582             ___sys_sendmsg+0x269/0x270
1583    } hitcount:         77
1584    { stacktrace:
1585             netif_rx_internal+0xb2/0xd0
1586             netif_rx+0x1c/0x60
1587             loopback_xmit+0x6c/0xb0
1588             dev_hard_start_xmit+0x219/0x3a0
1589             __dev_queue_xmit+0x415/0x4f0
1590             dev_queue_xmit_sk+0x13/0x20
1591             ip_finish_output2+0x237/0x340
1592             ip_finish_output+0x113/0x1d0
1593             ip_output+0x66/0xc0
1594             ip_local_out_sk+0x31/0x40
1595             ip_send_skb+0x1a/0x50
1596             udp_send_skb+0x16d/0x270
1597             udp_sendmsg+0x2bf/0x980
1598             inet_sendmsg+0x67/0xa0
1599             sock_sendmsg+0x38/0x50
1600             SYSC_sendto+0xef/0x170
1601    } hitcount:         88
1602    { stacktrace:
1603             _do_fork+0x18e/0x330
1604             SyS_clone+0x19/0x20
1605             entry_SYSCALL_64_fastpath+0x12/0x6a
1606    } hitcount:        244
1607
1608    Totals:
1609        Hits: 489
1610        Entries: 7
1611        Dropped: 0
1612
16132.2 Inter-event hist triggers
1614-----------------------------
1615
1616Inter-event hist triggers are hist triggers that combine values from
1617one or more other events and create a histogram using that data.  Data
1618from an inter-event histogram can in turn become the source for
1619further combined histograms, thus providing a chain of related
1620histograms, which is important for some applications.
1621
1622The most important example of an inter-event quantity that can be used
1623in this manner is latency, which is simply a difference in timestamps
1624between two events.  Although latency is the most important
1625inter-event quantity, note that because the support is completely
1626general across the trace event subsystem, any event field can be used
1627in an inter-event quantity.
1628
1629An example of a histogram that combines data from other histograms
1630into a useful chain would be a 'wakeupswitch latency' histogram that
1631combines a 'wakeup latency' histogram and a 'switch latency'
1632histogram.
1633
1634Normally, a hist trigger specification consists of a (possibly
1635compound) key along with one or more numeric values, which are
1636continually updated sums associated with that key.  A histogram
1637specification in this case consists of individual key and value
1638specifications that refer to trace event fields associated with a
1639single event type.
1640
1641The inter-event hist trigger extension allows fields from multiple
1642events to be referenced and combined into a multi-event histogram
1643specification.  In support of this overall goal, a few enabling
1644features have been added to the hist trigger support:
1645
1646  - In order to compute an inter-event quantity, a value from one
1647    event needs to saved and then referenced from another event.  This
1648    requires the introduction of support for histogram 'variables'.
1649
1650  - The computation of inter-event quantities and their combination
1651    require some minimal amount of support for applying simple
1652    expressions to variables (+ and -).
1653
1654  - A histogram consisting of inter-event quantities isn't logically a
1655    histogram on either event (so having the 'hist' file for either
1656    event host the histogram output doesn't really make sense).  To
1657    address the idea that the histogram is associated with a
1658    combination of events, support is added allowing the creation of
1659    'synthetic' events that are events derived from other events.
1660    These synthetic events are full-fledged events just like any other
1661    and can be used as such, as for instance to create the
1662    'combination' histograms mentioned previously.
1663
1664  - A set of 'actions' can be associated with histogram entries -
1665    these can be used to generate the previously mentioned synthetic
1666    events, but can also be used for other purposes, such as for
1667    example saving context when a 'max' latency has been hit.
1668
1669  - Trace events don't have a 'timestamp' associated with them, but
1670    there is an implicit timestamp saved along with an event in the
1671    underlying ftrace ring buffer.  This timestamp is now exposed as a
1672    a synthetic field named 'common_timestamp' which can be used in
1673    histograms as if it were any other event field; it isn't an actual
1674    field in the trace format but rather is a synthesized value that
1675    nonetheless can be used as if it were an actual field.  By default
1676    it is in units of nanoseconds; appending '.usecs' to a
1677    common_timestamp field changes the units to microseconds.
1678
1679A note on inter-event timestamps: If common_timestamp is used in a
1680histogram, the trace buffer is automatically switched over to using
1681absolute timestamps and the "global" trace clock, in order to avoid
1682bogus timestamp differences with other clocks that aren't coherent
1683across CPUs.  This can be overridden by specifying one of the other
1684trace clocks instead, using the "clock=XXX" hist trigger attribute,
1685where XXX is any of the clocks listed in the tracing/trace_clock
1686pseudo-file.
1687
1688These features are described in more detail in the following sections.
1689
16902.2.1 Histogram Variables
1691-------------------------
1692
1693Variables are simply named locations used for saving and retrieving
1694values between matching events.  A 'matching' event is defined as an
1695event that has a matching key - if a variable is saved for a histogram
1696entry corresponding to that key, any subsequent event with a matching
1697key can access that variable.
1698
1699A variable's value is normally available to any subsequent event until
1700it is set to something else by a subsequent event.  The one exception
1701to that rule is that any variable used in an expression is essentially
1702'read-once' - once it's used by an expression in a subsequent event,
1703it's reset to its 'unset' state, which means it can't be used again
1704unless it's set again.  This ensures not only that an event doesn't
1705use an uninitialized variable in a calculation, but that that variable
1706is used only once and not for any unrelated subsequent match.
1707
1708The basic syntax for saving a variable is to simply prefix a unique
1709variable name not corresponding to any keyword along with an '=' sign
1710to any event field.
1711
1712Either keys or values can be saved and retrieved in this way.  This
1713creates a variable named 'ts0' for a histogram entry with the key
1714'next_pid'::
1715
1716  # echo 'hist:keys=next_pid:vals=$ts0:ts0=common_timestamp ... >> \
1717	event/trigger
1718
1719The ts0 variable can be accessed by any subsequent event having the
1720same pid as 'next_pid'.
1721
1722Variable references are formed by prepending the variable name with
1723the '$' sign.  Thus for example, the ts0 variable above would be
1724referenced as '$ts0' in expressions.
1725
1726Because 'vals=' is used, the common_timestamp variable value above
1727will also be summed as a normal histogram value would (though for a
1728timestamp it makes little sense).
1729
1730The below shows that a key value can also be saved in the same way::
1731
1732  # echo 'hist:timer_pid=common_pid:key=timer_pid ...' >> event/trigger
1733
1734If a variable isn't a key variable or prefixed with 'vals=', the
1735associated event field will be saved in a variable but won't be summed
1736as a value::
1737
1738  # echo 'hist:keys=next_pid:ts1=common_timestamp ...' >> event/trigger
1739
1740Multiple variables can be assigned at the same time.  The below would
1741result in both ts0 and b being created as variables, with both
1742common_timestamp and field1 additionally being summed as values::
1743
1744  # echo 'hist:keys=pid:vals=$ts0,$b:ts0=common_timestamp,b=field1 ...' >> \
1745	event/trigger
1746
1747Note that variable assignments can appear either preceding or
1748following their use.  The command below behaves identically to the
1749command above::
1750
1751  # echo 'hist:keys=pid:ts0=common_timestamp,b=field1:vals=$ts0,$b ...' >> \
1752	event/trigger
1753
1754Any number of variables not bound to a 'vals=' prefix can also be
1755assigned by simply separating them with colons.  Below is the same
1756thing but without the values being summed in the histogram::
1757
1758  # echo 'hist:keys=pid:ts0=common_timestamp:b=field1 ...' >> event/trigger
1759
1760Variables set as above can be referenced and used in expressions on
1761another event.
1762
1763For example, here's how a latency can be calculated::
1764
1765  # echo 'hist:keys=pid,prio:ts0=common_timestamp ...' >> event1/trigger
1766  # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...' >> event2/trigger
1767
1768In the first line above, the event's timestamp is saved into the
1769variable ts0.  In the next line, ts0 is subtracted from the second
1770event's timestamp to produce the latency, which is then assigned into
1771yet another variable, 'wakeup_lat'.  The hist trigger below in turn
1772makes use of the wakeup_lat variable to compute a combined latency
1773using the same key and variable from yet another event::
1774
1775  # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
1776
17772.2.2 Synthetic Events
1778----------------------
1779
1780Synthetic events are user-defined events generated from hist trigger
1781variables or fields associated with one or more other events.  Their
1782purpose is to provide a mechanism for displaying data spanning
1783multiple events consistent with the existing and already familiar
1784usage for normal events.
1785
1786To define a synthetic event, the user writes a simple specification
1787consisting of the name of the new event along with one or more
1788variables and their types, which can be any valid field type,
1789separated by semicolons, to the tracing/synthetic_events file.
1790
1791For instance, the following creates a new event named 'wakeup_latency'
1792with 3 fields: lat, pid, and prio.  Each of those fields is simply a
1793variable reference to a variable on another event::
1794
1795  # echo 'wakeup_latency \
1796          u64 lat; \
1797          pid_t pid; \
1798	  int prio' >> \
1799	  /sys/kernel/debug/tracing/synthetic_events
1800
1801Reading the tracing/synthetic_events file lists all the currently
1802defined synthetic events, in this case the event defined above::
1803
1804  # cat /sys/kernel/debug/tracing/synthetic_events
1805    wakeup_latency u64 lat; pid_t pid; int prio
1806
1807An existing synthetic event definition can be removed by prepending
1808the command that defined it with a '!'::
1809
1810  # echo '!wakeup_latency u64 lat pid_t pid int prio' >> \
1811    /sys/kernel/debug/tracing/synthetic_events
1812
1813At this point, there isn't yet an actual 'wakeup_latency' event
1814instantiated in the event subsystem - for this to happen, a 'hist
1815trigger action' needs to be instantiated and bound to actual fields
1816and variables defined on other events (see Section 2.2.3 below on
1817how that is done using hist trigger 'onmatch' action). Once that is
1818done, the 'wakeup_latency' synthetic event instance is created.
1819
1820A histogram can now be defined for the new synthetic event::
1821
1822  # echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
1823        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
1824
1825The new event is created under the tracing/events/synthetic/ directory
1826and looks and behaves just like any other event::
1827
1828  # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
1829        enable  filter  format  hist  id  trigger
1830
1831Like any other event, once a histogram is enabled for the event, the
1832output can be displayed by reading the event's 'hist' file.
1833
18342.2.3 Hist trigger 'actions'
1835----------------------------
1836
1837A hist trigger 'action' is a function that's executed whenever a
1838histogram entry is added or updated.
1839
1840The default 'action' if no special function is explicitly specified is
1841as it always has been, to simply update the set of values associated
1842with an entry.  Some applications, however, may want to perform
1843additional actions at that point, such as generate another event, or
1844compare and save a maximum.
1845
1846The following additional actions are available.  To specify an action
1847for a given event, simply specify the action between colons in the
1848hist trigger specification.
1849
1850  - onmatch(matching.event).<synthetic_event_name>(param list)
1851
1852    The 'onmatch(matching.event).<synthetic_event_name>(params)' hist
1853    trigger action is invoked whenever an event matches and the
1854    histogram entry would be added or updated.  It causes the named
1855    synthetic event to be generated with the values given in the
1856    'param list'.  The result is the generation of a synthetic event
1857    that consists of the values contained in those variables at the
1858    time the invoking event was hit.
1859
1860    The 'param list' consists of one or more parameters which may be
1861    either variables or fields defined on either the 'matching.event'
1862    or the target event.  The variables or fields specified in the
1863    param list may be either fully-qualified or unqualified.  If a
1864    variable is specified as unqualified, it must be unique between
1865    the two events.  A field name used as a param can be unqualified
1866    if it refers to the target event, but must be fully qualified if
1867    it refers to the matching event.  A fully-qualified name is of the
1868    form 'system.event_name.$var_name' or 'system.event_name.field'.
1869
1870    The 'matching.event' specification is simply the fully qualified
1871    event name of the event that matches the target event for the
1872    onmatch() functionality, in the form 'system.event_name'.
1873
1874    Finally, the number and type of variables/fields in the 'param
1875    list' must match the number and types of the fields in the
1876    synthetic event being generated.
1877
1878    As an example the below defines a simple synthetic event and uses
1879    a variable defined on the sched_wakeup_new event as a parameter
1880    when invoking the synthetic event.  Here we define the synthetic
1881    event::
1882
1883      # echo 'wakeup_new_test pid_t pid' >> \
1884             /sys/kernel/debug/tracing/synthetic_events
1885
1886      # cat /sys/kernel/debug/tracing/synthetic_events
1887            wakeup_new_test pid_t pid
1888
1889    The following hist trigger both defines the missing testpid
1890    variable and specifies an onmatch() action that generates a
1891    wakeup_new_test synthetic event whenever a sched_wakeup_new event
1892    occurs, which because of the 'if comm == "cyclictest"' filter only
1893    happens when the executable is cyclictest::
1894
1895      # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
1896              wakeup_new_test($testpid) if comm=="cyclictest"' >> \
1897              /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
1898
1899    Creating and displaying a histogram based on those events is now
1900    just a matter of using the fields and new synthetic event in the
1901    tracing/events/synthetic directory, as usual::
1902
1903      # echo 'hist:keys=pid:sort=pid' >> \
1904             /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/trigger
1905
1906    Running 'cyclictest' should cause wakeup_new events to generate
1907    wakeup_new_test synthetic events which should result in histogram
1908    output in the wakeup_new_test event's hist file::
1909
1910      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/hist
1911
1912    A more typical usage would be to use two events to calculate a
1913    latency.  The following example uses a set of hist triggers to
1914    produce a 'wakeup_latency' histogram.
1915
1916    First, we define a 'wakeup_latency' synthetic event::
1917
1918      # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
1919              /sys/kernel/debug/tracing/synthetic_events
1920
1921    Next, we specify that whenever we see a sched_waking event for a
1922    cyclictest thread, save the timestamp in a 'ts0' variable::
1923
1924      # echo 'hist:keys=$saved_pid:saved_pid=pid:ts0=common_timestamp.usecs \
1925              if comm=="cyclictest"' >> \
1926	      /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
1927
1928    Then, when the corresponding thread is actually scheduled onto the
1929    CPU by a sched_switch event, calculate the latency and use that
1930    along with another variable and an event field to generate a
1931    wakeup_latency synthetic event::
1932
1933      # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
1934              onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\
1935	              $saved_pid,next_prio) if next_comm=="cyclictest"' >> \
1936	      /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
1937
1938    We also need to create a histogram on the wakeup_latency synthetic
1939    event in order to aggregate the generated synthetic event data::
1940
1941      # echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
1942              /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
1943
1944    Finally, once we've run cyclictest to actually generate some
1945    events, we can see the output by looking at the wakeup_latency
1946    synthetic event's hist file::
1947
1948      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist
1949
1950  - onmax(var).save(field,..	.)
1951
1952    The 'onmax(var).save(field,...)' hist trigger action is invoked
1953    whenever the value of 'var' associated with a histogram entry
1954    exceeds the current maximum contained in that variable.
1955
1956    The end result is that the trace event fields specified as the
1957    onmax.save() params will be saved if 'var' exceeds the current
1958    maximum for that hist trigger entry.  This allows context from the
1959    event that exhibited the new maximum to be saved for later
1960    reference.  When the histogram is displayed, additional fields
1961    displaying the saved values will be printed.
1962
1963    As an example the below defines a couple of hist triggers, one for
1964    sched_waking and another for sched_switch, keyed on pid.  Whenever
1965    a sched_waking occurs, the timestamp is saved in the entry
1966    corresponding to the current pid, and when the scheduler switches
1967    back to that pid, the timestamp difference is calculated.  If the
1968    resulting latency, stored in wakeup_lat, exceeds the current
1969    maximum latency, the values specified in the save() fields are
1970    recorded::
1971
1972      # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
1973              if comm=="cyclictest"' >> \
1974              /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
1975
1976      # echo 'hist:keys=next_pid:\
1977              wakeup_lat=common_timestamp.usecs-$ts0:\
1978              onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) \
1979              if next_comm=="cyclictest"' >> \
1980              /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
1981
1982    When the histogram is displayed, the max value and the saved
1983    values corresponding to the max are displayed following the rest
1984    of the fields::
1985
1986      # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
1987        { next_pid:       2255 } hitcount:        239
1988          common_timestamp-ts0:          0
1989          max:         27
1990	  next_comm: cyclictest
1991          prev_pid:          0  prev_prio:        120  prev_comm: swapper/1
1992
1993        { next_pid:       2256 } hitcount:       2355
1994          common_timestamp-ts0: 0
1995          max:         49  next_comm: cyclictest
1996          prev_pid:          0  prev_prio:        120  prev_comm: swapper/0
1997
1998        Totals:
1999            Hits: 12970
2000            Entries: 2
2001            Dropped: 0
2002
20033. User space creating a trigger
2004--------------------------------
2005
2006Writing into /sys/kernel/tracing/trace_marker writes into the ftrace
2007ring buffer. This can also act like an event, by writing into the trigger
2008file located in /sys/kernel/tracing/events/ftrace/print/
2009
2010Modifying cyclictest to write into the trace_marker file before it sleeps
2011and after it wakes up, something like this::
2012
2013  static void traceputs(char *str)
2014  {
2015	/* tracemark_fd is the trace_marker file descriptor */
2016	if (tracemark_fd < 0)
2017		return;
2018	/* write the tracemark message */
2019	write(tracemark_fd, str, strlen(str));
2020  }
2021
2022And later add something like::
2023
2024	traceputs("start");
2025	clock_nanosleep(...);
2026	traceputs("end");
2027
2028We can make a histogram from this::
2029
2030 # cd /sys/kernel/tracing
2031 # echo 'latency u64 lat' > synthetic_events
2032 # echo 'hist:keys=common_pid:ts0=common_timestamp.usecs if buf == "start"' > events/ftrace/print/trigger
2033 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(ftrace.print).latency($lat) if buf == "end"' >> events/ftrace/print/trigger
2034 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
2035
2036The above created a synthetic event called "latency" and two histograms
2037against the trace_marker, one gets triggered when "start" is written into the
2038trace_marker file and the other when "end" is written. If the pids match, then
2039it will call the "latency" synthetic event with the calculated latency as its
2040parameter. Finally, a histogram is added to the latency synthetic event to
2041record the calculated latency along with the pid.
2042
2043Now running cyclictest with::
2044
2045 # ./cyclictest -p80 -d0 -i250 -n -a -t --tracemark -b 1000
2046
2047 -p80  : run threads at priority 80
2048 -d0   : have all threads run at the same interval
2049 -i250 : start the interval at 250 microseconds (all threads will do this)
2050 -n    : sleep with nanosleep
2051 -a    : affine all threads to a separate CPU
2052 -t    : one thread per available CPU
2053 --tracemark : enable trace mark writing
2054 -b 1000 : stop if any latency is greater than 1000 microseconds
2055
2056Note, the -b 1000 is used just to make --tracemark available.
2057
2058Then we can see the histogram created by this with::
2059
2060 # cat events/synthetic/latency/hist
2061 # event histogram
2062 #
2063 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
2064 #
2065
2066 { lat:        107, common_pid:       2039 } hitcount:          1
2067 { lat:        122, common_pid:       2041 } hitcount:          1
2068 { lat:        166, common_pid:       2039 } hitcount:          1
2069 { lat:        174, common_pid:       2039 } hitcount:          1
2070 { lat:        194, common_pid:       2041 } hitcount:          1
2071 { lat:        196, common_pid:       2036 } hitcount:          1
2072 { lat:        197, common_pid:       2038 } hitcount:          1
2073 { lat:        198, common_pid:       2039 } hitcount:          1
2074 { lat:        199, common_pid:       2039 } hitcount:          1
2075 { lat:        200, common_pid:       2041 } hitcount:          1
2076 { lat:        201, common_pid:       2039 } hitcount:          2
2077 { lat:        202, common_pid:       2038 } hitcount:          1
2078 { lat:        202, common_pid:       2043 } hitcount:          1
2079 { lat:        203, common_pid:       2039 } hitcount:          1
2080 { lat:        203, common_pid:       2036 } hitcount:          1
2081 { lat:        203, common_pid:       2041 } hitcount:          1
2082 { lat:        206, common_pid:       2038 } hitcount:          2
2083 { lat:        207, common_pid:       2039 } hitcount:          1
2084 { lat:        207, common_pid:       2036 } hitcount:          1
2085 { lat:        208, common_pid:       2040 } hitcount:          1
2086 { lat:        209, common_pid:       2043 } hitcount:          1
2087 { lat:        210, common_pid:       2039 } hitcount:          1
2088 { lat:        211, common_pid:       2039 } hitcount:          4
2089 { lat:        212, common_pid:       2043 } hitcount:          1
2090 { lat:        212, common_pid:       2039 } hitcount:          2
2091 { lat:        213, common_pid:       2039 } hitcount:          1
2092 { lat:        214, common_pid:       2038 } hitcount:          1
2093 { lat:        214, common_pid:       2039 } hitcount:          2
2094 { lat:        214, common_pid:       2042 } hitcount:          1
2095 { lat:        215, common_pid:       2039 } hitcount:          1
2096 { lat:        217, common_pid:       2036 } hitcount:          1
2097 { lat:        217, common_pid:       2040 } hitcount:          1
2098 { lat:        217, common_pid:       2039 } hitcount:          1
2099 { lat:        218, common_pid:       2039 } hitcount:          6
2100 { lat:        219, common_pid:       2039 } hitcount:          9
2101 { lat:        220, common_pid:       2039 } hitcount:         11
2102 { lat:        221, common_pid:       2039 } hitcount:          5
2103 { lat:        221, common_pid:       2042 } hitcount:          1
2104 { lat:        222, common_pid:       2039 } hitcount:          7
2105 { lat:        223, common_pid:       2036 } hitcount:          1
2106 { lat:        223, common_pid:       2039 } hitcount:          3
2107 { lat:        224, common_pid:       2039 } hitcount:          4
2108 { lat:        224, common_pid:       2037 } hitcount:          1
2109 { lat:        224, common_pid:       2036 } hitcount:          2
2110 { lat:        225, common_pid:       2039 } hitcount:          5
2111 { lat:        225, common_pid:       2042 } hitcount:          1
2112 { lat:        226, common_pid:       2039 } hitcount:          7
2113 { lat:        226, common_pid:       2036 } hitcount:          4
2114 { lat:        227, common_pid:       2039 } hitcount:          6
2115 { lat:        227, common_pid:       2036 } hitcount:         12
2116 { lat:        227, common_pid:       2043 } hitcount:          1
2117 { lat:        228, common_pid:       2039 } hitcount:          7
2118 { lat:        228, common_pid:       2036 } hitcount:         14
2119 { lat:        229, common_pid:       2039 } hitcount:          9
2120 { lat:        229, common_pid:       2036 } hitcount:          8
2121 { lat:        229, common_pid:       2038 } hitcount:          1
2122 { lat:        230, common_pid:       2039 } hitcount:         11
2123 { lat:        230, common_pid:       2036 } hitcount:          6
2124 { lat:        230, common_pid:       2043 } hitcount:          1
2125 { lat:        230, common_pid:       2042 } hitcount:          2
2126 { lat:        231, common_pid:       2041 } hitcount:          1
2127 { lat:        231, common_pid:       2036 } hitcount:          6
2128 { lat:        231, common_pid:       2043 } hitcount:          1
2129 { lat:        231, common_pid:       2039 } hitcount:          8
2130 { lat:        232, common_pid:       2037 } hitcount:          1
2131 { lat:        232, common_pid:       2039 } hitcount:          6
2132 { lat:        232, common_pid:       2040 } hitcount:          2
2133 { lat:        232, common_pid:       2036 } hitcount:          5
2134 { lat:        232, common_pid:       2043 } hitcount:          1
2135 { lat:        233, common_pid:       2036 } hitcount:          5
2136 { lat:        233, common_pid:       2039 } hitcount:         11
2137 { lat:        234, common_pid:       2039 } hitcount:          4
2138 { lat:        234, common_pid:       2038 } hitcount:          2
2139 { lat:        234, common_pid:       2043 } hitcount:          2
2140 { lat:        234, common_pid:       2036 } hitcount:         11
2141 { lat:        234, common_pid:       2040 } hitcount:          1
2142 { lat:        235, common_pid:       2037 } hitcount:          2
2143 { lat:        235, common_pid:       2036 } hitcount:          8
2144 { lat:        235, common_pid:       2043 } hitcount:          2
2145 { lat:        235, common_pid:       2039 } hitcount:          5
2146 { lat:        235, common_pid:       2042 } hitcount:          2
2147 { lat:        235, common_pid:       2040 } hitcount:          4
2148 { lat:        235, common_pid:       2041 } hitcount:          1
2149 { lat:        236, common_pid:       2036 } hitcount:          7
2150 { lat:        236, common_pid:       2037 } hitcount:          1
2151 { lat:        236, common_pid:       2041 } hitcount:          5
2152 { lat:        236, common_pid:       2039 } hitcount:          3
2153 { lat:        236, common_pid:       2043 } hitcount:          9
2154 { lat:        236, common_pid:       2040 } hitcount:          7
2155 { lat:        237, common_pid:       2037 } hitcount:          1
2156 { lat:        237, common_pid:       2040 } hitcount:          1
2157 { lat:        237, common_pid:       2036 } hitcount:          9
2158 { lat:        237, common_pid:       2039 } hitcount:          3
2159 { lat:        237, common_pid:       2043 } hitcount:          8
2160 { lat:        237, common_pid:       2042 } hitcount:          2
2161 { lat:        237, common_pid:       2041 } hitcount:          2
2162 { lat:        238, common_pid:       2043 } hitcount:         10
2163 { lat:        238, common_pid:       2040 } hitcount:          1
2164 { lat:        238, common_pid:       2037 } hitcount:          9
2165 { lat:        238, common_pid:       2038 } hitcount:          1
2166 { lat:        238, common_pid:       2039 } hitcount:          1
2167 { lat:        238, common_pid:       2042 } hitcount:          3
2168 { lat:        238, common_pid:       2036 } hitcount:          7
2169 { lat:        239, common_pid:       2041 } hitcount:          1
2170 { lat:        239, common_pid:       2043 } hitcount:         11
2171 { lat:        239, common_pid:       2037 } hitcount:         11
2172 { lat:        239, common_pid:       2038 } hitcount:          6
2173 { lat:        239, common_pid:       2036 } hitcount:          7
2174 { lat:        239, common_pid:       2040 } hitcount:          1
2175 { lat:        239, common_pid:       2042 } hitcount:          9
2176 { lat:        240, common_pid:       2037 } hitcount:         29
2177 { lat:        240, common_pid:       2043 } hitcount:         15
2178 { lat:        240, common_pid:       2040 } hitcount:         44
2179 { lat:        240, common_pid:       2039 } hitcount:          1
2180 { lat:        240, common_pid:       2041 } hitcount:          2
2181 { lat:        240, common_pid:       2038 } hitcount:          1
2182 { lat:        240, common_pid:       2036 } hitcount:         10
2183 { lat:        240, common_pid:       2042 } hitcount:         13
2184 { lat:        241, common_pid:       2036 } hitcount:         21
2185 { lat:        241, common_pid:       2041 } hitcount:         36
2186 { lat:        241, common_pid:       2037 } hitcount:         34
2187 { lat:        241, common_pid:       2042 } hitcount:         14
2188 { lat:        241, common_pid:       2040 } hitcount:         94
2189 { lat:        241, common_pid:       2039 } hitcount:         12
2190 { lat:        241, common_pid:       2038 } hitcount:          2
2191 { lat:        241, common_pid:       2043 } hitcount:         28
2192 { lat:        242, common_pid:       2040 } hitcount:        109
2193 { lat:        242, common_pid:       2041 } hitcount:        506
2194 { lat:        242, common_pid:       2039 } hitcount:        155
2195 { lat:        242, common_pid:       2042 } hitcount:         21
2196 { lat:        242, common_pid:       2037 } hitcount:         52
2197 { lat:        242, common_pid:       2043 } hitcount:         21
2198 { lat:        242, common_pid:       2036 } hitcount:         16
2199 { lat:        242, common_pid:       2038 } hitcount:        156
2200 { lat:        243, common_pid:       2037 } hitcount:         46
2201 { lat:        243, common_pid:       2039 } hitcount:         40
2202 { lat:        243, common_pid:       2042 } hitcount:        119
2203 { lat:        243, common_pid:       2041 } hitcount:        611
2204 { lat:        243, common_pid:       2036 } hitcount:         69
2205 { lat:        243, common_pid:       2038 } hitcount:        784
2206 { lat:        243, common_pid:       2040 } hitcount:        323
2207 { lat:        243, common_pid:       2043 } hitcount:         14
2208 { lat:        244, common_pid:       2043 } hitcount:         35
2209 { lat:        244, common_pid:       2042 } hitcount:        305
2210 { lat:        244, common_pid:       2039 } hitcount:          8
2211 { lat:        244, common_pid:       2040 } hitcount:       4515
2212 { lat:        244, common_pid:       2038 } hitcount:        371
2213 { lat:        244, common_pid:       2037 } hitcount:         31
2214 { lat:        244, common_pid:       2036 } hitcount:        114
2215 { lat:        244, common_pid:       2041 } hitcount:       3396
2216 { lat:        245, common_pid:       2036 } hitcount:        700
2217 { lat:        245, common_pid:       2041 } hitcount:       2772
2218 { lat:        245, common_pid:       2037 } hitcount:        268
2219 { lat:        245, common_pid:       2039 } hitcount:        472
2220 { lat:        245, common_pid:       2038 } hitcount:       2758
2221 { lat:        245, common_pid:       2042 } hitcount:       3833
2222 { lat:        245, common_pid:       2040 } hitcount:       3105
2223 { lat:        245, common_pid:       2043 } hitcount:        645
2224 { lat:        246, common_pid:       2038 } hitcount:       3451
2225 { lat:        246, common_pid:       2041 } hitcount:        142
2226 { lat:        246, common_pid:       2037 } hitcount:       5101
2227 { lat:        246, common_pid:       2040 } hitcount:         68
2228 { lat:        246, common_pid:       2043 } hitcount:       5099
2229 { lat:        246, common_pid:       2039 } hitcount:       5608
2230 { lat:        246, common_pid:       2042 } hitcount:       3723
2231 { lat:        246, common_pid:       2036 } hitcount:       4738
2232 { lat:        247, common_pid:       2042 } hitcount:        312
2233 { lat:        247, common_pid:       2043 } hitcount:       2385
2234 { lat:        247, common_pid:       2041 } hitcount:        452
2235 { lat:        247, common_pid:       2038 } hitcount:        792
2236 { lat:        247, common_pid:       2040 } hitcount:         78
2237 { lat:        247, common_pid:       2036 } hitcount:       2375
2238 { lat:        247, common_pid:       2039 } hitcount:       1834
2239 { lat:        247, common_pid:       2037 } hitcount:       2655
2240 { lat:        248, common_pid:       2037 } hitcount:         36
2241 { lat:        248, common_pid:       2042 } hitcount:         11
2242 { lat:        248, common_pid:       2038 } hitcount:        122
2243 { lat:        248, common_pid:       2036 } hitcount:        135
2244 { lat:        248, common_pid:       2039 } hitcount:         26
2245 { lat:        248, common_pid:       2041 } hitcount:        503
2246 { lat:        248, common_pid:       2043 } hitcount:         66
2247 { lat:        248, common_pid:       2040 } hitcount:         46
2248 { lat:        249, common_pid:       2037 } hitcount:         29
2249 { lat:        249, common_pid:       2038 } hitcount:          1
2250 { lat:        249, common_pid:       2043 } hitcount:         29
2251 { lat:        249, common_pid:       2039 } hitcount:          8
2252 { lat:        249, common_pid:       2042 } hitcount:         56
2253 { lat:        249, common_pid:       2040 } hitcount:         27
2254 { lat:        249, common_pid:       2041 } hitcount:         11
2255 { lat:        249, common_pid:       2036 } hitcount:         27
2256 { lat:        250, common_pid:       2038 } hitcount:          1
2257 { lat:        250, common_pid:       2036 } hitcount:         30
2258 { lat:        250, common_pid:       2040 } hitcount:         19
2259 { lat:        250, common_pid:       2043 } hitcount:         22
2260 { lat:        250, common_pid:       2042 } hitcount:         20
2261 { lat:        250, common_pid:       2041 } hitcount:          1
2262 { lat:        250, common_pid:       2039 } hitcount:          6
2263 { lat:        250, common_pid:       2037 } hitcount:         48
2264 { lat:        251, common_pid:       2037 } hitcount:         43
2265 { lat:        251, common_pid:       2039 } hitcount:          1
2266 { lat:        251, common_pid:       2036 } hitcount:         12
2267 { lat:        251, common_pid:       2042 } hitcount:          2
2268 { lat:        251, common_pid:       2041 } hitcount:          1
2269 { lat:        251, common_pid:       2043 } hitcount:         15
2270 { lat:        251, common_pid:       2040 } hitcount:          3
2271 { lat:        252, common_pid:       2040 } hitcount:          1
2272 { lat:        252, common_pid:       2036 } hitcount:         12
2273 { lat:        252, common_pid:       2037 } hitcount:         21
2274 { lat:        252, common_pid:       2043 } hitcount:         14
2275 { lat:        253, common_pid:       2037 } hitcount:         21
2276 { lat:        253, common_pid:       2039 } hitcount:          2
2277 { lat:        253, common_pid:       2036 } hitcount:          9
2278 { lat:        253, common_pid:       2043 } hitcount:          6
2279 { lat:        253, common_pid:       2040 } hitcount:          1
2280 { lat:        254, common_pid:       2036 } hitcount:          8
2281 { lat:        254, common_pid:       2043 } hitcount:          3
2282 { lat:        254, common_pid:       2041 } hitcount:          1
2283 { lat:        254, common_pid:       2042 } hitcount:          1
2284 { lat:        254, common_pid:       2039 } hitcount:          1
2285 { lat:        254, common_pid:       2037 } hitcount:         12
2286 { lat:        255, common_pid:       2043 } hitcount:          1
2287 { lat:        255, common_pid:       2037 } hitcount:          2
2288 { lat:        255, common_pid:       2036 } hitcount:          2
2289 { lat:        255, common_pid:       2039 } hitcount:          8
2290 { lat:        256, common_pid:       2043 } hitcount:          1
2291 { lat:        256, common_pid:       2036 } hitcount:          4
2292 { lat:        256, common_pid:       2039 } hitcount:          6
2293 { lat:        257, common_pid:       2039 } hitcount:          5
2294 { lat:        257, common_pid:       2036 } hitcount:          4
2295 { lat:        258, common_pid:       2039 } hitcount:          5
2296 { lat:        258, common_pid:       2036 } hitcount:          2
2297 { lat:        259, common_pid:       2036 } hitcount:          7
2298 { lat:        259, common_pid:       2039 } hitcount:          7
2299 { lat:        260, common_pid:       2036 } hitcount:          8
2300 { lat:        260, common_pid:       2039 } hitcount:          6
2301 { lat:        261, common_pid:       2036 } hitcount:          5
2302 { lat:        261, common_pid:       2039 } hitcount:          7
2303 { lat:        262, common_pid:       2039 } hitcount:          5
2304 { lat:        262, common_pid:       2036 } hitcount:          5
2305 { lat:        263, common_pid:       2039 } hitcount:          7
2306 { lat:        263, common_pid:       2036 } hitcount:          7
2307 { lat:        264, common_pid:       2039 } hitcount:          9
2308 { lat:        264, common_pid:       2036 } hitcount:          9
2309 { lat:        265, common_pid:       2036 } hitcount:          5
2310 { lat:        265, common_pid:       2039 } hitcount:          1
2311 { lat:        266, common_pid:       2036 } hitcount:          1
2312 { lat:        266, common_pid:       2039 } hitcount:          3
2313 { lat:        267, common_pid:       2036 } hitcount:          1
2314 { lat:        267, common_pid:       2039 } hitcount:          3
2315 { lat:        268, common_pid:       2036 } hitcount:          1
2316 { lat:        268, common_pid:       2039 } hitcount:          6
2317 { lat:        269, common_pid:       2036 } hitcount:          1
2318 { lat:        269, common_pid:       2043 } hitcount:          1
2319 { lat:        269, common_pid:       2039 } hitcount:          2
2320 { lat:        270, common_pid:       2040 } hitcount:          1
2321 { lat:        270, common_pid:       2039 } hitcount:          6
2322 { lat:        271, common_pid:       2041 } hitcount:          1
2323 { lat:        271, common_pid:       2039 } hitcount:          5
2324 { lat:        272, common_pid:       2039 } hitcount:         10
2325 { lat:        273, common_pid:       2039 } hitcount:          8
2326 { lat:        274, common_pid:       2039 } hitcount:          2
2327 { lat:        275, common_pid:       2039 } hitcount:          1
2328 { lat:        276, common_pid:       2039 } hitcount:          2
2329 { lat:        276, common_pid:       2037 } hitcount:          1
2330 { lat:        276, common_pid:       2038 } hitcount:          1
2331 { lat:        277, common_pid:       2039 } hitcount:          1
2332 { lat:        277, common_pid:       2042 } hitcount:          1
2333 { lat:        278, common_pid:       2039 } hitcount:          1
2334 { lat:        279, common_pid:       2039 } hitcount:          4
2335 { lat:        279, common_pid:       2043 } hitcount:          1
2336 { lat:        280, common_pid:       2039 } hitcount:          3
2337 { lat:        283, common_pid:       2036 } hitcount:          2
2338 { lat:        284, common_pid:       2039 } hitcount:          1
2339 { lat:        284, common_pid:       2043 } hitcount:          1
2340 { lat:        288, common_pid:       2039 } hitcount:          1
2341 { lat:        289, common_pid:       2039 } hitcount:          1
2342 { lat:        300, common_pid:       2039 } hitcount:          1
2343 { lat:        384, common_pid:       2039 } hitcount:          1
2344
2345 Totals:
2346     Hits: 67625
2347     Entries: 278
2348     Dropped: 0
2349
2350Note, the writes are around the sleep, so ideally they will all be of 250
2351microseconds. If you are wondering how there are several that are under
2352250 microseconds, that is because the way cyclictest works, is if one
2353iteration comes in late, the next one will set the timer to wake up less that
2354250. That is, if an iteration came in 50 microseconds late, the next wake up
2355will be at 200 microseconds.
2356
2357But this could easily be done in userspace. To make this even more
2358interesting, we can mix the histogram between events that happened in the
2359kernel with trace_marker::
2360
2361 # cd /sys/kernel/tracing
2362 # echo 'latency u64 lat' > synthetic_events
2363 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' > events/sched/sched_waking/trigger
2364 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).latency($lat) if buf == "end"' > events/ftrace/print/trigger
2365 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
2366
2367The difference this time is that instead of using the trace_marker to start
2368the latency, the sched_waking event is used, matching the common_pid for the
2369trace_marker write with the pid that is being woken by sched_waking.
2370
2371After running cyclictest again with the same parameters, we now have::
2372
2373 # cat events/synthetic/latency/hist
2374 # event histogram
2375 #
2376 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
2377 #
2378
2379 { lat:          7, common_pid:       2302 } hitcount:        640
2380 { lat:          7, common_pid:       2299 } hitcount:         42
2381 { lat:          7, common_pid:       2303 } hitcount:         18
2382 { lat:          7, common_pid:       2305 } hitcount:        166
2383 { lat:          7, common_pid:       2306 } hitcount:          1
2384 { lat:          7, common_pid:       2301 } hitcount:         91
2385 { lat:          7, common_pid:       2300 } hitcount:         17
2386 { lat:          8, common_pid:       2303 } hitcount:       8296
2387 { lat:          8, common_pid:       2304 } hitcount:       6864
2388 { lat:          8, common_pid:       2305 } hitcount:       9464
2389 { lat:          8, common_pid:       2301 } hitcount:       9213
2390 { lat:          8, common_pid:       2306 } hitcount:       6246
2391 { lat:          8, common_pid:       2302 } hitcount:       8797
2392 { lat:          8, common_pid:       2299 } hitcount:       8771
2393 { lat:          8, common_pid:       2300 } hitcount:       8119
2394 { lat:          9, common_pid:       2305 } hitcount:       1519
2395 { lat:          9, common_pid:       2299 } hitcount:       2346
2396 { lat:          9, common_pid:       2303 } hitcount:       2841
2397 { lat:          9, common_pid:       2301 } hitcount:       1846
2398 { lat:          9, common_pid:       2304 } hitcount:       3861
2399 { lat:          9, common_pid:       2302 } hitcount:       1210
2400 { lat:          9, common_pid:       2300 } hitcount:       2762
2401 { lat:          9, common_pid:       2306 } hitcount:       4247
2402 { lat:         10, common_pid:       2299 } hitcount:         16
2403 { lat:         10, common_pid:       2306 } hitcount:        333
2404 { lat:         10, common_pid:       2303 } hitcount:         16
2405 { lat:         10, common_pid:       2304 } hitcount:        168
2406 { lat:         10, common_pid:       2302 } hitcount:        240
2407 { lat:         10, common_pid:       2301 } hitcount:         28
2408 { lat:         10, common_pid:       2300 } hitcount:         95
2409 { lat:         10, common_pid:       2305 } hitcount:         18
2410 { lat:         11, common_pid:       2303 } hitcount:          5
2411 { lat:         11, common_pid:       2305 } hitcount:          8
2412 { lat:         11, common_pid:       2306 } hitcount:        221
2413 { lat:         11, common_pid:       2302 } hitcount:         76
2414 { lat:         11, common_pid:       2304 } hitcount:         26
2415 { lat:         11, common_pid:       2300 } hitcount:        125
2416 { lat:         11, common_pid:       2299 } hitcount:          2
2417 { lat:         12, common_pid:       2305 } hitcount:          3
2418 { lat:         12, common_pid:       2300 } hitcount:          6
2419 { lat:         12, common_pid:       2306 } hitcount:         90
2420 { lat:         12, common_pid:       2302 } hitcount:          4
2421 { lat:         12, common_pid:       2303 } hitcount:          1
2422 { lat:         12, common_pid:       2304 } hitcount:        122
2423 { lat:         13, common_pid:       2300 } hitcount:         12
2424 { lat:         13, common_pid:       2301 } hitcount:          1
2425 { lat:         13, common_pid:       2306 } hitcount:         32
2426 { lat:         13, common_pid:       2302 } hitcount:          5
2427 { lat:         13, common_pid:       2305 } hitcount:          1
2428 { lat:         13, common_pid:       2303 } hitcount:          1
2429 { lat:         13, common_pid:       2304 } hitcount:         61
2430 { lat:         14, common_pid:       2303 } hitcount:          4
2431 { lat:         14, common_pid:       2306 } hitcount:          5
2432 { lat:         14, common_pid:       2305 } hitcount:          4
2433 { lat:         14, common_pid:       2304 } hitcount:         62
2434 { lat:         14, common_pid:       2302 } hitcount:         19
2435 { lat:         14, common_pid:       2300 } hitcount:         33
2436 { lat:         14, common_pid:       2299 } hitcount:          1
2437 { lat:         14, common_pid:       2301 } hitcount:          4
2438 { lat:         15, common_pid:       2305 } hitcount:          1
2439 { lat:         15, common_pid:       2302 } hitcount:         25
2440 { lat:         15, common_pid:       2300 } hitcount:         11
2441 { lat:         15, common_pid:       2299 } hitcount:          5
2442 { lat:         15, common_pid:       2301 } hitcount:          1
2443 { lat:         15, common_pid:       2304 } hitcount:          8
2444 { lat:         15, common_pid:       2303 } hitcount:          1
2445 { lat:         15, common_pid:       2306 } hitcount:          6
2446 { lat:         16, common_pid:       2302 } hitcount:         31
2447 { lat:         16, common_pid:       2306 } hitcount:          3
2448 { lat:         16, common_pid:       2300 } hitcount:          5
2449 { lat:         17, common_pid:       2302 } hitcount:          6
2450 { lat:         17, common_pid:       2303 } hitcount:          1
2451 { lat:         18, common_pid:       2304 } hitcount:          1
2452 { lat:         18, common_pid:       2302 } hitcount:          8
2453 { lat:         18, common_pid:       2299 } hitcount:          1
2454 { lat:         18, common_pid:       2301 } hitcount:          1
2455 { lat:         19, common_pid:       2303 } hitcount:          4
2456 { lat:         19, common_pid:       2304 } hitcount:          5
2457 { lat:         19, common_pid:       2302 } hitcount:          4
2458 { lat:         19, common_pid:       2299 } hitcount:          3
2459 { lat:         19, common_pid:       2306 } hitcount:          1
2460 { lat:         19, common_pid:       2300 } hitcount:          4
2461 { lat:         19, common_pid:       2305 } hitcount:          5
2462 { lat:         20, common_pid:       2299 } hitcount:          2
2463 { lat:         20, common_pid:       2302 } hitcount:          3
2464 { lat:         20, common_pid:       2305 } hitcount:          1
2465 { lat:         20, common_pid:       2300 } hitcount:          2
2466 { lat:         20, common_pid:       2301 } hitcount:          2
2467 { lat:         20, common_pid:       2303 } hitcount:          3
2468 { lat:         21, common_pid:       2305 } hitcount:          1
2469 { lat:         21, common_pid:       2299 } hitcount:          5
2470 { lat:         21, common_pid:       2303 } hitcount:          4
2471 { lat:         21, common_pid:       2302 } hitcount:          7
2472 { lat:         21, common_pid:       2300 } hitcount:          1
2473 { lat:         21, common_pid:       2301 } hitcount:          5
2474 { lat:         21, common_pid:       2304 } hitcount:          2
2475 { lat:         22, common_pid:       2302 } hitcount:          5
2476 { lat:         22, common_pid:       2303 } hitcount:          1
2477 { lat:         22, common_pid:       2306 } hitcount:          3
2478 { lat:         22, common_pid:       2301 } hitcount:          2
2479 { lat:         22, common_pid:       2300 } hitcount:          1
2480 { lat:         22, common_pid:       2299 } hitcount:          1
2481 { lat:         22, common_pid:       2305 } hitcount:          1
2482 { lat:         22, common_pid:       2304 } hitcount:          1
2483 { lat:         23, common_pid:       2299 } hitcount:          1
2484 { lat:         23, common_pid:       2306 } hitcount:          2
2485 { lat:         23, common_pid:       2302 } hitcount:          6
2486 { lat:         24, common_pid:       2302 } hitcount:          3
2487 { lat:         24, common_pid:       2300 } hitcount:          1
2488 { lat:         24, common_pid:       2306 } hitcount:          2
2489 { lat:         24, common_pid:       2305 } hitcount:          1
2490 { lat:         24, common_pid:       2299 } hitcount:          1
2491 { lat:         25, common_pid:       2300 } hitcount:          1
2492 { lat:         25, common_pid:       2302 } hitcount:          4
2493 { lat:         26, common_pid:       2302 } hitcount:          2
2494 { lat:         27, common_pid:       2305 } hitcount:          1
2495 { lat:         27, common_pid:       2300 } hitcount:          1
2496 { lat:         27, common_pid:       2302 } hitcount:          3
2497 { lat:         28, common_pid:       2306 } hitcount:          1
2498 { lat:         28, common_pid:       2302 } hitcount:          4
2499 { lat:         29, common_pid:       2302 } hitcount:          1
2500 { lat:         29, common_pid:       2300 } hitcount:          2
2501 { lat:         29, common_pid:       2306 } hitcount:          1
2502 { lat:         29, common_pid:       2304 } hitcount:          1
2503 { lat:         30, common_pid:       2302 } hitcount:          4
2504 { lat:         31, common_pid:       2302 } hitcount:          6
2505 { lat:         32, common_pid:       2302 } hitcount:          1
2506 { lat:         33, common_pid:       2299 } hitcount:          1
2507 { lat:         33, common_pid:       2302 } hitcount:          3
2508 { lat:         34, common_pid:       2302 } hitcount:          2
2509 { lat:         35, common_pid:       2302 } hitcount:          1
2510 { lat:         35, common_pid:       2304 } hitcount:          1
2511 { lat:         36, common_pid:       2302 } hitcount:          4
2512 { lat:         37, common_pid:       2302 } hitcount:          6
2513 { lat:         38, common_pid:       2302 } hitcount:          2
2514 { lat:         39, common_pid:       2302 } hitcount:          2
2515 { lat:         39, common_pid:       2304 } hitcount:          1
2516 { lat:         40, common_pid:       2304 } hitcount:          2
2517 { lat:         40, common_pid:       2302 } hitcount:          5
2518 { lat:         41, common_pid:       2304 } hitcount:          1
2519 { lat:         41, common_pid:       2302 } hitcount:          8
2520 { lat:         42, common_pid:       2302 } hitcount:          6
2521 { lat:         42, common_pid:       2304 } hitcount:          1
2522 { lat:         43, common_pid:       2302 } hitcount:          3
2523 { lat:         43, common_pid:       2304 } hitcount:          4
2524 { lat:         44, common_pid:       2302 } hitcount:          6
2525 { lat:         45, common_pid:       2302 } hitcount:          5
2526 { lat:         46, common_pid:       2302 } hitcount:          5
2527 { lat:         47, common_pid:       2302 } hitcount:          7
2528 { lat:         48, common_pid:       2301 } hitcount:          1
2529 { lat:         48, common_pid:       2302 } hitcount:          9
2530 { lat:         49, common_pid:       2302 } hitcount:          3
2531 { lat:         50, common_pid:       2302 } hitcount:          1
2532 { lat:         50, common_pid:       2301 } hitcount:          1
2533 { lat:         51, common_pid:       2302 } hitcount:          2
2534 { lat:         51, common_pid:       2301 } hitcount:          1
2535 { lat:         61, common_pid:       2302 } hitcount:          1
2536 { lat:        110, common_pid:       2302 } hitcount:          1
2537
2538 Totals:
2539     Hits: 89565
2540     Entries: 158
2541     Dropped: 0
2542
2543This doesn't tell us any information about how late cyclictest may have
2544woken up, but it does show us a nice histogram of how long it took from
2545the time that cyclictest was woken to the time it made it into user space.
2546