1================
2Event Histograms
3================
4
5Documentation written by Tom Zanussi
6
71. Introduction
8===============
9
10  Histogram triggers are special event triggers that can be used to
11  aggregate trace event data into histograms.  For information on
12  trace events and event triggers, see Documentation/trace/events.rst.
13
14
152. Histogram Trigger Command
16============================
17
18  A histogram trigger command is an event trigger command that
19  aggregates event hits into a hash table keyed on one or more trace
20  event format fields (or stacktrace) and a set of running totals
21  derived from one or more trace event format fields and/or event
22  counts (hitcount).
23
24  The format of a hist trigger is as follows::
25
26        hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
27          [:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
28          [:clear][:name=histname1][:<handler>.<action>] [if <filter>]
29
30  When a matching event is hit, an entry is added to a hash table
31  using the key(s) and value(s) named.  Keys and values correspond to
32  fields in the event's format description.  Values must correspond to
33  numeric fields - on an event hit, the value(s) will be added to a
34  sum kept for that field.  The special string 'hitcount' can be used
35  in place of an explicit value field - this is simply a count of
36  event hits.  If 'values' isn't specified, an implicit 'hitcount'
37  value will be automatically created and used as the only value.
38  Keys can be any field, or the special string 'stacktrace', which
39  will use the event's kernel stacktrace as the key.  The keywords
40  'keys' or 'key' can be used to specify keys, and the keywords
41  'values', 'vals', or 'val' can be used to specify values.  Compound
42  keys consisting of up to two fields can be specified by the 'keys'
43  keyword.  Hashing a compound key produces a unique entry in the
44  table for each unique combination of component keys, and can be
45  useful for providing more fine-grained summaries of event data.
46  Additionally, sort keys consisting of up to two fields can be
47  specified by the 'sort' keyword.  If more than one field is
48  specified, the result will be a 'sort within a sort': the first key
49  is taken to be the primary sort key and the second the secondary
50  key.  If a hist trigger is given a name using the 'name' parameter,
51  its histogram data will be shared with other triggers of the same
52  name, and trigger hits will update this common data.  Only triggers
53  with 'compatible' fields can be combined in this way; triggers are
54  'compatible' if the fields named in the trigger share the same
55  number and type of fields and those fields also have the same names.
56  Note that any two events always share the compatible 'hitcount' and
57  'stacktrace' fields and can therefore be combined using those
58  fields, however pointless that may be.
59
60  'hist' triggers add a 'hist' file to each event's subdirectory.
61  Reading the 'hist' file for the event will dump the hash table in
62  its entirety to stdout.  If there are multiple hist triggers
63  attached to an event, there will be a table for each trigger in the
64  output.  The table displayed for a named trigger will be the same as
65  any other instance having the same name. Each printed hash table
66  entry is a simple list of the keys and values comprising the entry;
67  keys are printed first and are delineated by curly braces, and are
68  followed by the set of value fields for the entry.  By default,
69  numeric fields are displayed as base-10 integers.  This can be
70  modified by appending any of the following modifiers to the field
71  name:
72
73	=========== ==========================================
74        .hex        display a number as a hex value
75	.sym        display an address as a symbol
76	.sym-offset display an address as a symbol and offset
77	.syscall    display a syscall id as a system call name
78	.execname   display a common_pid as a program name
79	.log2       display log2 value rather than raw number
80	.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  tracing/error_log file.  See Error Conditions in
203  :file:`Documentation/trace/ftrace.rst` for details.
204
2056.2 'hist' trigger examples
206---------------------------
207
208  The first set of examples creates aggregations using the kmalloc
209  event.  The fields that can be used for the hist trigger are listed
210  in the kmalloc event's format file::
211
212    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
213    name: kmalloc
214    ID: 374
215    format:
216	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
217	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
218	field:unsigned char common_preempt_count;		offset:3;	size:1;	signed:0;
219	field:int common_pid;					offset:4;	size:4;	signed:1;
220
221	field:unsigned long call_site;				offset:8;	size:8;	signed:0;
222	field:const void * ptr;					offset:16;	size:8;	signed:0;
223	field:size_t bytes_req;					offset:24;	size:8;	signed:0;
224	field:size_t bytes_alloc;				offset:32;	size:8;	signed:0;
225	field:gfp_t gfp_flags;					offset:40;	size:4;	signed:0;
226
227  We'll start by creating a hist trigger that generates a simple table
228  that lists the total number of bytes requested for each function in
229  the kernel that made one or more calls to kmalloc::
230
231    # echo 'hist:key=call_site:val=bytes_req' > \
232            /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
233
234  This tells the tracing system to create a 'hist' trigger using the
235  call_site field of the kmalloc event as the key for the table, which
236  just means that each unique call_site address will have an entry
237  created for it in the table.  The 'val=bytes_req' parameter tells
238  the hist trigger that for each unique entry (call_site) in the
239  table, it should keep a running total of the number of bytes
240  requested by that call_site.
241
242  We'll let it run for awhile and then dump the contents of the 'hist'
243  file in the kmalloc event's subdirectory (for readability, a number
244  of entries have been omitted)::
245
246    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
247    # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
248
249    { call_site: 18446744072106379007 } hitcount:          1  bytes_req:        176
250    { call_site: 18446744071579557049 } hitcount:          1  bytes_req:       1024
251    { call_site: 18446744071580608289 } hitcount:          1  bytes_req:      16384
252    { call_site: 18446744071581827654 } hitcount:          1  bytes_req:         24
253    { call_site: 18446744071580700980 } hitcount:          1  bytes_req:          8
254    { call_site: 18446744071579359876 } hitcount:          1  bytes_req:        152
255    { call_site: 18446744071580795365 } hitcount:          3  bytes_req:        144
256    { call_site: 18446744071581303129 } hitcount:          3  bytes_req:        144
257    { call_site: 18446744071580713234 } hitcount:          4  bytes_req:       2560
258    { call_site: 18446744071580933750 } hitcount:          4  bytes_req:        736
259    .
260    .
261    .
262    { call_site: 18446744072106047046 } hitcount:         69  bytes_req:       5576
263    { call_site: 18446744071582116407 } hitcount:         73  bytes_req:       2336
264    { call_site: 18446744072106054684 } hitcount:        136  bytes_req:     140504
265    { call_site: 18446744072106224230 } hitcount:        136  bytes_req:      19584
266    { call_site: 18446744072106078074 } hitcount:        153  bytes_req:       2448
267    { call_site: 18446744072106062406 } hitcount:        153  bytes_req:      36720
268    { call_site: 18446744071582507929 } hitcount:        153  bytes_req:      37088
269    { call_site: 18446744072102520590 } hitcount:        273  bytes_req:      10920
270    { call_site: 18446744071582143559 } hitcount:        358  bytes_req:        716
271    { call_site: 18446744072106465852 } hitcount:        417  bytes_req:      56712
272    { call_site: 18446744072102523378 } hitcount:        485  bytes_req:      27160
273    { call_site: 18446744072099568646 } hitcount:       1676  bytes_req:      33520
274
275    Totals:
276        Hits: 4610
277        Entries: 45
278        Dropped: 0
279
280  The output displays a line for each entry, beginning with the key
281  specified in the trigger, followed by the value(s) also specified in
282  the trigger.  At the beginning of the output is a line that displays
283  the trigger info, which can also be displayed by reading the
284  'trigger' file::
285
286    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
287    hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
288
289  At the end of the output are a few lines that display the overall
290  totals for the run.  The 'Hits' field shows the total number of
291  times the event trigger was hit, the 'Entries' field shows the total
292  number of used entries in the hash table, and the 'Dropped' field
293  shows the number of hits that were dropped because the number of
294  used entries for the run exceeded the maximum number of entries
295  allowed for the table (normally 0, but if not a hint that you may
296  want to increase the size of the table using the 'size' parameter).
297
298  Notice in the above output that there's an extra field, 'hitcount',
299  which wasn't specified in the trigger.  Also notice that in the
300  trigger info output, there's a parameter, 'sort=hitcount', which
301  wasn't specified in the trigger either.  The reason for that is that
302  every trigger implicitly keeps a count of the total number of hits
303  attributed to a given entry, called the 'hitcount'.  That hitcount
304  information is explicitly displayed in the output, and in the
305  absence of a user-specified sort parameter, is used as the default
306  sort field.
307
308  The value 'hitcount' can be used in place of an explicit value in
309  the 'values' parameter if you don't really need to have any
310  particular field summed and are mainly interested in hit
311  frequencies.
312
313  To turn the hist trigger off, simply call up the trigger in the
314  command history and re-execute it with a '!' prepended::
315
316    # echo '!hist:key=call_site:val=bytes_req' > \
317           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
318
319  Finally, notice that the call_site as displayed in the output above
320  isn't really very useful.  It's an address, but normally addresses
321  are displayed in hex.  To have a numeric field displayed as a hex
322  value, simply append '.hex' to the field name in the trigger::
323
324    # echo 'hist:key=call_site.hex:val=bytes_req' > \
325           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
326
327    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
328    # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]
329
330    { call_site: ffffffffa026b291 } hitcount:          1  bytes_req:        433
331    { call_site: ffffffffa07186ff } hitcount:          1  bytes_req:        176
332    { call_site: ffffffff811ae721 } hitcount:          1  bytes_req:      16384
333    { call_site: ffffffff811c5134 } hitcount:          1  bytes_req:          8
334    { call_site: ffffffffa04a9ebb } hitcount:          1  bytes_req:        511
335    { call_site: ffffffff8122e0a6 } hitcount:          1  bytes_req:         12
336    { call_site: ffffffff8107da84 } hitcount:          1  bytes_req:        152
337    { call_site: ffffffff812d8246 } hitcount:          1  bytes_req:         24
338    { call_site: ffffffff811dc1e5 } hitcount:          3  bytes_req:        144
339    { call_site: ffffffffa02515e8 } hitcount:          3  bytes_req:        648
340    { call_site: ffffffff81258159 } hitcount:          3  bytes_req:        144
341    { call_site: ffffffff811c80f4 } hitcount:          4  bytes_req:        544
342    .
343    .
344    .
345    { call_site: ffffffffa06c7646 } hitcount:        106  bytes_req:       8024
346    { call_site: ffffffffa06cb246 } hitcount:        132  bytes_req:      31680
347    { call_site: ffffffffa06cef7a } hitcount:        132  bytes_req:       2112
348    { call_site: ffffffff8137e399 } hitcount:        132  bytes_req:      23232
349    { call_site: ffffffffa06c941c } hitcount:        185  bytes_req:     171360
350    { call_site: ffffffffa06f2a66 } hitcount:        185  bytes_req:      26640
351    { call_site: ffffffffa036a70e } hitcount:        265  bytes_req:      10600
352    { call_site: ffffffff81325447 } hitcount:        292  bytes_req:        584
353    { call_site: ffffffffa072da3c } hitcount:        446  bytes_req:      60656
354    { call_site: ffffffffa036b1f2 } hitcount:        526  bytes_req:      29456
355    { call_site: ffffffffa0099c06 } hitcount:       1780  bytes_req:      35600
356
357    Totals:
358        Hits: 4775
359        Entries: 46
360        Dropped: 0
361
362  Even that's only marginally more useful - while hex values do look
363  more like addresses, what users are typically more interested in
364  when looking at text addresses are the corresponding symbols
365  instead.  To have an address displayed as symbolic value instead,
366  simply append '.sym' or '.sym-offset' to the field name in the
367  trigger::
368
369    # echo 'hist:key=call_site.sym:val=bytes_req' > \
370           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
371
372    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
373    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]
374
375    { call_site: [ffffffff810adcb9] syslog_print_all                              } hitcount:          1  bytes_req:       1024
376    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
377    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
378    { call_site: [ffffffff8154acbe] usb_alloc_urb                                 } hitcount:          1  bytes_req:        192
379    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
380    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
381    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
382    { call_site: [ffffffff811febd5] fsnotify_alloc_group                          } hitcount:          2  bytes_req:        528
383    { call_site: [ffffffff81440f58] __tty_buffer_request_room                     } hitcount:          2  bytes_req:       2624
384    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          2  bytes_req:         96
385    { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211]      } hitcount:          2  bytes_req:        464
386    { call_site: [ffffffff81672406] tcp_get_metrics                               } hitcount:          2  bytes_req:        304
387    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
388    { call_site: [ffffffff81089b05] sched_create_group                            } hitcount:          2  bytes_req:       1424
389    .
390    .
391    .
392    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1185  bytes_req:     123240
393    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:       1185  bytes_req:     104280
394    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       1402  bytes_req:     190672
395    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       1518  bytes_req:     146208
396    { call_site: [ffffffffa029070e] drm_vma_node_allow [drm]                      } hitcount:       1746  bytes_req:      69840
397    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       2021  bytes_req:     792312
398    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       2592  bytes_req:     145152
399    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2629  bytes_req:     378576
400    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2629  bytes_req:    3783248
401    { call_site: [ffffffff81325607] apparmor_file_alloc_security                  } hitcount:       5192  bytes_req:      10384
402    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       5529  bytes_req:     110584
403    { call_site: [ffffffff8131ebf7] aa_alloc_task_context                         } hitcount:      21943  bytes_req:     702176
404    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:      55759  bytes_req:    5074265
405
406    Totals:
407        Hits: 109928
408        Entries: 71
409        Dropped: 0
410
411  Because the default sort key above is 'hitcount', the above shows a
412  the list of call_sites by increasing hitcount, so that at the bottom
413  we see the functions that made the most kmalloc calls during the
414  run.  If instead we we wanted to see the top kmalloc callers in
415  terms of the number of bytes requested rather than the number of
416  calls, and we wanted the top caller to appear at the top, we can use
417  the 'sort' parameter, along with the 'descending' modifier::
418
419    # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
420           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
421
422    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
423    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
424
425    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2186  bytes_req:    3397464
426    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1790  bytes_req:     712176
427    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       8132  bytes_req:     513135
428    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        106  bytes_req:     440128
429    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2186  bytes_req:     314784
430    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       2174  bytes_req:     208992
431    { call_site: [ffffffff811ae8e1] __kmalloc                                     } hitcount:          8  bytes_req:     131072
432    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:        859  bytes_req:     116824
433    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       1834  bytes_req:     102704
434    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:        972  bytes_req:     101088
435    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:        972  bytes_req:      85536
436    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       3333  bytes_req:      66664
437    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        209  bytes_req:      61632
438    .
439    .
440    .
441    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
442    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
443    { call_site: [ffffffff812d8406] copy_semundo                                  } hitcount:          2  bytes_req:         48
444    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          1  bytes_req:         48
445    { call_site: [ffffffffa027121a] drm_getmagic [drm]                            } hitcount:          1  bytes_req:         48
446    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
447    { call_site: [ffffffff811c52f4] bprm_change_interp                            } hitcount:          2  bytes_req:         16
448    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
449    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
450    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
451
452    Totals:
453        Hits: 32133
454        Entries: 81
455        Dropped: 0
456
457  To display the offset and size information in addition to the symbol
458  name, just use 'sym-offset' instead::
459
460    # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
461           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
462
463    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
464    # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
465
466    { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915]                  } hitcount:       4569  bytes_req:    3163720
467    { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915]                      } hitcount:       4569  bytes_req:     657936
468    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915]      } hitcount:       1519  bytes_req:     472936
469    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915]      } hitcount:       3050  bytes_req:     211832
470    { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50                                 } hitcount:         34  bytes_req:     148384
471    { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915]                  } hitcount:       1385  bytes_req:     144040
472    { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0                                   } hitcount:          8  bytes_req:     131072
473    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm]              } hitcount:       1385  bytes_req:     121880
474    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm]                  } hitcount:       1848  bytes_req:     103488
475    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915]            } hitcount:        461  bytes_req:      62696
476    { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm]                      } hitcount:       1541  bytes_req:      61640
477    { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0                                } hitcount:         57  bytes_req:      57456
478    .
479    .
480    .
481    { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0                       } hitcount:          2  bytes_req:        128
482    { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm]                      } hitcount:          3  bytes_req:         96
483    { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0                         } hitcount:          8  bytes_req:         96
484    { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650                            } hitcount:          3  bytes_req:         84
485    { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110                              } hitcount:          1  bytes_req:          8
486    { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid]                     } hitcount:          1  bytes_req:          7
487    { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid]                    } hitcount:          1  bytes_req:          7
488
489    Totals:
490        Hits: 26098
491        Entries: 64
492        Dropped: 0
493
494  We can also add multiple fields to the 'values' parameter.  For
495  example, we might want to see the total number of bytes allocated
496  alongside bytes requested, and display the result sorted by bytes
497  allocated in a descending order::
498
499    # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
500           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
501
502    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
503    # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]
504
505    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       7403  bytes_req:    4084360  bytes_alloc:    5958016
506    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        541  bytes_req:    2213968  bytes_alloc:    2228224
507    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       7404  bytes_req:    1066176  bytes_alloc:    1421568
508    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1565  bytes_req:     557368  bytes_alloc:    1037760
509    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       9557  bytes_req:     595778  bytes_alloc:     695744
510    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       5839  bytes_req:     430680  bytes_alloc:     470400
511    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       2388  bytes_req:     324768  bytes_alloc:     458496
512    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       3911  bytes_req:     219016  bytes_alloc:     250304
513    { call_site: [ffffffff815f8d7b] sk_prot_alloc                                 } hitcount:        235  bytes_req:     236880  bytes_alloc:     240640
514    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        557  bytes_req:     169024  bytes_alloc:     221760
515    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       9378  bytes_req:     187548  bytes_alloc:     206312
516    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1519  bytes_req:     157976  bytes_alloc:     194432
517    .
518    .
519    .
520    { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach                 } hitcount:          2  bytes_req:        144  bytes_alloc:        192
521    { call_site: [ffffffff81097ee8] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
522    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
523    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
524    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
525    { call_site: [ffffffff81213e80] load_elf_binary                               } hitcount:          3  bytes_req:         84  bytes_alloc:         96
526    { call_site: [ffffffff81079a2e] kthread_create_on_node                        } hitcount:          1  bytes_req:         56  bytes_alloc:         64
527    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7  bytes_alloc:          8
528    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8  bytes_alloc:          8
529    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7  bytes_alloc:          8
530
531    Totals:
532        Hits: 66598
533        Entries: 65
534        Dropped: 0
535
536  Finally, to finish off our kmalloc example, instead of simply having
537  the hist trigger display symbolic call_sites, we can have the hist
538  trigger additionally display the complete set of kernel stack traces
539  that led to each call_site.  To do that, we simply use the special
540  value 'stacktrace' for the key parameter::
541
542    # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
543           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
544
545  The above trigger will use the kernel stack trace in effect when an
546  event is triggered as the key for the hash table.  This allows the
547  enumeration of every kernel callpath that led up to a particular
548  event, along with a running total of any of the event fields for
549  that event.  Here we tally bytes requested and bytes allocated for
550  every callpath in the system that led up to a kmalloc (in this case
551  every callpath to a kmalloc for a kernel compile)::
552
553    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
554    # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]
555
556    { stacktrace:
557         __kmalloc_track_caller+0x10b/0x1a0
558         kmemdup+0x20/0x50
559         hidraw_report_event+0x8a/0x120 [hid]
560         hid_report_raw_event+0x3ea/0x440 [hid]
561         hid_input_report+0x112/0x190 [hid]
562         hid_irq_in+0xc2/0x260 [usbhid]
563         __usb_hcd_giveback_urb+0x72/0x120
564         usb_giveback_urb_bh+0x9e/0xe0
565         tasklet_hi_action+0xf8/0x100
566         __do_softirq+0x114/0x2c0
567         irq_exit+0xa5/0xb0
568         do_IRQ+0x5a/0xf0
569         ret_from_intr+0x0/0x30
570         cpuidle_enter+0x17/0x20
571         cpu_startup_entry+0x315/0x3e0
572         rest_init+0x7c/0x80
573    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
574    { stacktrace:
575         __kmalloc_track_caller+0x10b/0x1a0
576         kmemdup+0x20/0x50
577         hidraw_report_event+0x8a/0x120 [hid]
578         hid_report_raw_event+0x3ea/0x440 [hid]
579         hid_input_report+0x112/0x190 [hid]
580         hid_irq_in+0xc2/0x260 [usbhid]
581         __usb_hcd_giveback_urb+0x72/0x120
582         usb_giveback_urb_bh+0x9e/0xe0
583         tasklet_hi_action+0xf8/0x100
584         __do_softirq+0x114/0x2c0
585         irq_exit+0xa5/0xb0
586         do_IRQ+0x5a/0xf0
587         ret_from_intr+0x0/0x30
588    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
589    { stacktrace:
590         kmem_cache_alloc_trace+0xeb/0x150
591         aa_alloc_task_context+0x27/0x40
592         apparmor_cred_prepare+0x1f/0x50
593         security_prepare_creds+0x16/0x20
594         prepare_creds+0xdf/0x1a0
595         SyS_capset+0xb5/0x200
596         system_call_fastpath+0x12/0x6a
597    } hitcount:          1  bytes_req:         32  bytes_alloc:         32
598    .
599    .
600    .
601    { stacktrace:
602         __kmalloc+0x11b/0x1b0
603         i915_gem_execbuffer2+0x6c/0x2c0 [i915]
604         drm_ioctl+0x349/0x670 [drm]
605         do_vfs_ioctl+0x2f0/0x4f0
606         SyS_ioctl+0x81/0xa0
607         system_call_fastpath+0x12/0x6a
608    } hitcount:      17726  bytes_req:   13944120  bytes_alloc:   19593808
609    { stacktrace:
610         __kmalloc+0x11b/0x1b0
611         load_elf_phdrs+0x76/0xa0
612         load_elf_binary+0x102/0x1650
613         search_binary_handler+0x97/0x1d0
614         do_execveat_common.isra.34+0x551/0x6e0
615         SyS_execve+0x3a/0x50
616         return_from_execve+0x0/0x23
617    } hitcount:      33348  bytes_req:   17152128  bytes_alloc:   20226048
618    { stacktrace:
619         kmem_cache_alloc_trace+0xeb/0x150
620         apparmor_file_alloc_security+0x27/0x40
621         security_file_alloc+0x16/0x20
622         get_empty_filp+0x93/0x1c0
623         path_openat+0x31/0x5f0
624         do_filp_open+0x3a/0x90
625         do_sys_open+0x128/0x220
626         SyS_open+0x1e/0x20
627         system_call_fastpath+0x12/0x6a
628    } hitcount:    4766422  bytes_req:    9532844  bytes_alloc:   38131376
629    { stacktrace:
630         __kmalloc+0x11b/0x1b0
631         seq_buf_alloc+0x1b/0x50
632         seq_read+0x2cc/0x370
633         proc_reg_read+0x3d/0x80
634         __vfs_read+0x28/0xe0
635         vfs_read+0x86/0x140
636         SyS_read+0x46/0xb0
637         system_call_fastpath+0x12/0x6a
638    } hitcount:      19133  bytes_req:   78368768  bytes_alloc:   78368768
639
640    Totals:
641        Hits: 6085872
642        Entries: 253
643        Dropped: 0
644
645  If you key a hist trigger on common_pid, in order for example to
646  gather and display sorted totals for each process, you can use the
647  special .execname modifier to display the executable names for the
648  processes in the table rather than raw pids.  The example below
649  keeps a per-process sum of total bytes read::
650
651    # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
652           /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger
653
654    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
655    # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]
656
657    { common_pid: gnome-terminal  [      3196] } hitcount:        280  count:    1093512
658    { common_pid: Xorg            [      1309] } hitcount:        525  count:     256640
659    { common_pid: compiz          [      2889] } hitcount:         59  count:     254400
660    { common_pid: bash            [      8710] } hitcount:          3  count:      66369
661    { common_pid: dbus-daemon-lau [      8703] } hitcount:         49  count:      47739
662    { common_pid: irqbalance      [      1252] } hitcount:         27  count:      27648
663    { common_pid: 01ifupdown      [      8705] } hitcount:          3  count:      17216
664    { common_pid: dbus-daemon     [       772] } hitcount:         10  count:      12396
665    { common_pid: Socket Thread   [      8342] } hitcount:         11  count:      11264
666    { common_pid: nm-dhcp-client. [      8701] } hitcount:          6  count:       7424
667    { common_pid: gmain           [      1315] } hitcount:         18  count:       6336
668    .
669    .
670    .
671    { common_pid: postgres        [      1892] } hitcount:          2  count:         32
672    { common_pid: postgres        [      1891] } hitcount:          2  count:         32
673    { common_pid: gmain           [      8704] } hitcount:          2  count:         32
674    { common_pid: upstart-dbus-br [      2740] } hitcount:         21  count:         21
675    { common_pid: nm-dispatcher.a [      8696] } hitcount:          1  count:         16
676    { common_pid: indicator-datet [      2904] } hitcount:          1  count:         16
677    { common_pid: gdbus           [      2998] } hitcount:          1  count:         16
678    { common_pid: rtkit-daemon    [      2052] } hitcount:          1  count:          8
679    { common_pid: init            [         1] } hitcount:          2  count:          2
680
681    Totals:
682        Hits: 2116
683        Entries: 51
684        Dropped: 0
685
686  Similarly, if you key a hist trigger on syscall id, for example to
687  gather and display a list of systemwide syscall hits, you can use
688  the special .syscall modifier to display the syscall names rather
689  than raw ids.  The example below keeps a running total of syscall
690  counts for the system during the run::
691
692    # echo 'hist:key=id.syscall:val=hitcount' > \
693           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
694
695    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
696    # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]
697
698    { id: sys_fsync                     [ 74] } hitcount:          1
699    { id: sys_newuname                  [ 63] } hitcount:          1
700    { id: sys_prctl                     [157] } hitcount:          1
701    { id: sys_statfs                    [137] } hitcount:          1
702    { id: sys_symlink                   [ 88] } hitcount:          1
703    { id: sys_sendmmsg                  [307] } hitcount:          1
704    { id: sys_semctl                    [ 66] } hitcount:          1
705    { id: sys_readlink                  [ 89] } hitcount:          3
706    { id: sys_bind                      [ 49] } hitcount:          3
707    { id: sys_getsockname               [ 51] } hitcount:          3
708    { id: sys_unlink                    [ 87] } hitcount:          3
709    { id: sys_rename                    [ 82] } hitcount:          4
710    { id: unknown_syscall               [ 58] } hitcount:          4
711    { id: sys_connect                   [ 42] } hitcount:          4
712    { id: sys_getpid                    [ 39] } hitcount:          4
713    .
714    .
715    .
716    { id: sys_rt_sigprocmask            [ 14] } hitcount:        952
717    { id: sys_futex                     [202] } hitcount:       1534
718    { id: sys_write                     [  1] } hitcount:       2689
719    { id: sys_setitimer                 [ 38] } hitcount:       2797
720    { id: sys_read                      [  0] } hitcount:       3202
721    { id: sys_select                    [ 23] } hitcount:       3773
722    { id: sys_writev                    [ 20] } hitcount:       4531
723    { id: sys_poll                      [  7] } hitcount:       8314
724    { id: sys_recvmsg                   [ 47] } hitcount:      13738
725    { id: sys_ioctl                     [ 16] } hitcount:      21843
726
727    Totals:
728        Hits: 67612
729        Entries: 72
730        Dropped: 0
731
732  The syscall counts above provide a rough overall picture of system
733  call activity on the system; we can see for example that the most
734  popular system call on this system was the 'sys_ioctl' system call.
735
736  We can use 'compound' keys to refine that number and provide some
737  further insight as to which processes exactly contribute to the
738  overall ioctl count.
739
740  The command below keeps a hitcount for every unique combination of
741  system call id and pid - the end result is essentially a table
742  that keeps a per-pid sum of system call hits.  The results are
743  sorted using the system call id as the primary key, and the
744  hitcount sum as the secondary key::
745
746    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
747           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
748
749    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
750    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]
751
752    { id: sys_read                      [  0], common_pid: rtkit-daemon    [      1877] } hitcount:          1
753    { id: sys_read                      [  0], common_pid: gdbus           [      2976] } hitcount:          1
754    { id: sys_read                      [  0], common_pid: console-kit-dae [      3400] } hitcount:          1
755    { id: sys_read                      [  0], common_pid: postgres        [      1865] } hitcount:          1
756    { id: sys_read                      [  0], common_pid: deja-dup-monito [      3543] } hitcount:          2
757    { id: sys_read                      [  0], common_pid: NetworkManager  [       890] } hitcount:          2
758    { id: sys_read                      [  0], common_pid: evolution-calen [      3048] } hitcount:          2
759    { id: sys_read                      [  0], common_pid: postgres        [      1864] } hitcount:          2
760    { id: sys_read                      [  0], common_pid: nm-applet       [      3022] } hitcount:          2
761    { id: sys_read                      [  0], common_pid: whoopsie        [      1212] } hitcount:          2
762    .
763    .
764    .
765    { id: sys_ioctl                     [ 16], common_pid: bash            [      8479] } hitcount:          1
766    { id: sys_ioctl                     [ 16], common_pid: bash            [      3472] } hitcount:         12
767    { id: sys_ioctl                     [ 16], common_pid: gnome-terminal  [      3199] } hitcount:         16
768    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:       1808
769    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:       5580
770    .
771    .
772    .
773    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2690] } hitcount:          3
774    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2688] } hitcount:         16
775    { id: sys_inotify_add_watch         [254], common_pid: gmain           [       975] } hitcount:          2
776    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3204] } hitcount:          4
777    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2888] } hitcount:          4
778    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3003] } hitcount:          4
779    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2873] } hitcount:          4
780    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3196] } hitcount:          6
781    { id: sys_openat                    [257], common_pid: java            [      2623] } hitcount:          2
782    { id: sys_eventfd2                  [290], common_pid: ibus-ui-gtk3    [      2760] } hitcount:          4
783    { id: sys_eventfd2                  [290], common_pid: compiz          [      2994] } hitcount:          6
784
785    Totals:
786        Hits: 31536
787        Entries: 323
788        Dropped: 0
789
790  The above list does give us a breakdown of the ioctl syscall by
791  pid, but it also gives us quite a bit more than that, which we
792  don't really care about at the moment.  Since we know the syscall
793  id for sys_ioctl (16, displayed next to the sys_ioctl name), we
794  can use that to filter out all the other syscalls::
795
796    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
797           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
798
799    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
800    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]
801
802    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2769] } hitcount:          1
803    { id: sys_ioctl                     [ 16], common_pid: evolution-addre [      8571] } hitcount:          1
804    { id: sys_ioctl                     [ 16], common_pid: gmain           [      3003] } hitcount:          1
805    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2781] } hitcount:          1
806    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2829] } hitcount:          1
807    { id: sys_ioctl                     [ 16], common_pid: bash            [      8726] } hitcount:          1
808    { id: sys_ioctl                     [ 16], common_pid: bash            [      8508] } hitcount:          1
809    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2970] } hitcount:          1
810    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2768] } hitcount:          1
811    .
812    .
813    .
814    { id: sys_ioctl                     [ 16], common_pid: pool            [      8559] } hitcount:         45
815    { id: sys_ioctl                     [ 16], common_pid: pool            [      8555] } hitcount:         48
816    { id: sys_ioctl                     [ 16], common_pid: pool            [      8551] } hitcount:         48
817    { id: sys_ioctl                     [ 16], common_pid: avahi-daemon    [       896] } hitcount:         66
818    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:      26674
819    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:      73443
820
821    Totals:
822        Hits: 101162
823        Entries: 103
824        Dropped: 0
825
826  The above output shows that 'compiz' and 'Xorg' are far and away
827  the heaviest ioctl callers (which might lead to questions about
828  whether they really need to be making all those calls and to
829  possible avenues for further investigation.)
830
831  The compound key examples used a key and a sum value (hitcount) to
832  sort the output, but we can just as easily use two keys instead.
833  Here's an example where we use a compound key composed of the the
834  common_pid and size event fields.  Sorting with pid as the primary
835  key and 'size' as the secondary key allows us to display an
836  ordered summary of the recvfrom sizes, with counts, received by
837  each process::
838
839    # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
840           /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger
841
842    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
843    # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]
844
845    { common_pid: smbd            [       784], size:          4 } hitcount:          1
846    { common_pid: dnsmasq         [      1412], size:       4096 } hitcount:        672
847    { common_pid: postgres        [      1796], size:       1000 } hitcount:          6
848    { common_pid: postgres        [      1867], size:       1000 } hitcount:         10
849    { common_pid: bamfdaemon      [      2787], size:         28 } hitcount:          2
850    { common_pid: bamfdaemon      [      2787], size:      14360 } hitcount:          1
851    { common_pid: compiz          [      2994], size:          8 } hitcount:          1
852    { common_pid: compiz          [      2994], size:         20 } hitcount:         11
853    { common_pid: gnome-terminal  [      3199], size:          4 } hitcount:          2
854    { common_pid: firefox         [      8817], size:          4 } hitcount:          1
855    { common_pid: firefox         [      8817], size:          8 } hitcount:          5
856    { common_pid: firefox         [      8817], size:        588 } hitcount:          2
857    { common_pid: firefox         [      8817], size:        628 } hitcount:          1
858    { common_pid: firefox         [      8817], size:       6944 } hitcount:          1
859    { common_pid: firefox         [      8817], size:     408880 } hitcount:          2
860    { common_pid: firefox         [      8822], size:          8 } hitcount:          2
861    { common_pid: firefox         [      8822], size:        160 } hitcount:          2
862    { common_pid: firefox         [      8822], size:        320 } hitcount:          2
863    { common_pid: firefox         [      8822], size:        352 } hitcount:          1
864    .
865    .
866    .
867    { common_pid: pool            [      8923], size:       1960 } hitcount:         10
868    { common_pid: pool            [      8923], size:       2048 } hitcount:         10
869    { common_pid: pool            [      8924], size:       1960 } hitcount:         10
870    { common_pid: pool            [      8924], size:       2048 } hitcount:         10
871    { common_pid: pool            [      8928], size:       1964 } hitcount:          4
872    { common_pid: pool            [      8928], size:       1965 } hitcount:          2
873    { common_pid: pool            [      8928], size:       2048 } hitcount:          6
874    { common_pid: pool            [      8929], size:       1982 } hitcount:          1
875    { common_pid: pool            [      8929], size:       2048 } hitcount:          1
876
877    Totals:
878        Hits: 2016
879        Entries: 224
880        Dropped: 0
881
882  The above example also illustrates the fact that although a compound
883  key is treated as a single entity for hashing purposes, the sub-keys
884  it's composed of can be accessed independently.
885
886  The next example uses a string field as the hash key and
887  demonstrates how you can manually pause and continue a hist trigger.
888  In this example, we'll aggregate fork counts and don't expect a
889  large number of entries in the hash table, so we'll drop it to a
890  much smaller number, say 256::
891
892    # echo 'hist:key=child_comm:val=hitcount:size=256' > \
893           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
894
895    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
896    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
897
898    { child_comm: dconf worker                        } hitcount:          1
899    { child_comm: ibus-daemon                         } hitcount:          1
900    { child_comm: whoopsie                            } hitcount:          1
901    { child_comm: smbd                                } hitcount:          1
902    { child_comm: gdbus                               } hitcount:          1
903    { child_comm: kthreadd                            } hitcount:          1
904    { child_comm: dconf worker                        } hitcount:          1
905    { child_comm: evolution-alarm                     } hitcount:          2
906    { child_comm: Socket Thread                       } hitcount:          2
907    { child_comm: postgres                            } hitcount:          2
908    { child_comm: bash                                } hitcount:          3
909    { child_comm: compiz                              } hitcount:          3
910    { child_comm: evolution-sourc                     } hitcount:          4
911    { child_comm: dhclient                            } hitcount:          4
912    { child_comm: pool                                } hitcount:          5
913    { child_comm: nm-dispatcher.a                     } hitcount:          8
914    { child_comm: firefox                             } hitcount:          8
915    { child_comm: dbus-daemon                         } hitcount:          8
916    { child_comm: glib-pacrunner                      } hitcount:         10
917    { child_comm: evolution                           } hitcount:         23
918
919    Totals:
920        Hits: 89
921        Entries: 20
922        Dropped: 0
923
924  If we want to pause the hist trigger, we can simply append :pause to
925  the command that started the trigger.  Notice that the trigger info
926  displays as [paused]::
927
928    # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
929           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
930
931    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
932    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]
933
934    { child_comm: dconf worker                        } hitcount:          1
935    { child_comm: kthreadd                            } hitcount:          1
936    { child_comm: dconf worker                        } hitcount:          1
937    { child_comm: gdbus                               } hitcount:          1
938    { child_comm: ibus-daemon                         } hitcount:          1
939    { child_comm: Socket Thread                       } hitcount:          2
940    { child_comm: evolution-alarm                     } hitcount:          2
941    { child_comm: smbd                                } hitcount:          2
942    { child_comm: bash                                } hitcount:          3
943    { child_comm: whoopsie                            } hitcount:          3
944    { child_comm: compiz                              } hitcount:          3
945    { child_comm: evolution-sourc                     } hitcount:          4
946    { child_comm: pool                                } hitcount:          5
947    { child_comm: postgres                            } hitcount:          6
948    { child_comm: firefox                             } hitcount:          8
949    { child_comm: dhclient                            } hitcount:         10
950    { child_comm: emacs                               } hitcount:         12
951    { child_comm: dbus-daemon                         } hitcount:         20
952    { child_comm: nm-dispatcher.a                     } hitcount:         20
953    { child_comm: evolution                           } hitcount:         35
954    { child_comm: glib-pacrunner                      } hitcount:         59
955
956    Totals:
957        Hits: 199
958        Entries: 21
959        Dropped: 0
960
961  To manually continue having the trigger aggregate events, append
962  :cont instead.  Notice that the trigger info displays as [active]
963  again, and the data has changed::
964
965    # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
966           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
967
968    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
969    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
970
971    { child_comm: dconf worker                        } hitcount:          1
972    { child_comm: dconf worker                        } hitcount:          1
973    { child_comm: kthreadd                            } hitcount:          1
974    { child_comm: gdbus                               } hitcount:          1
975    { child_comm: ibus-daemon                         } hitcount:          1
976    { child_comm: Socket Thread                       } hitcount:          2
977    { child_comm: evolution-alarm                     } hitcount:          2
978    { child_comm: smbd                                } hitcount:          2
979    { child_comm: whoopsie                            } hitcount:          3
980    { child_comm: compiz                              } hitcount:          3
981    { child_comm: evolution-sourc                     } hitcount:          4
982    { child_comm: bash                                } hitcount:          5
983    { child_comm: pool                                } hitcount:          5
984    { child_comm: postgres                            } hitcount:          6
985    { child_comm: firefox                             } hitcount:          8
986    { child_comm: dhclient                            } hitcount:         11
987    { child_comm: emacs                               } hitcount:         12
988    { child_comm: dbus-daemon                         } hitcount:         22
989    { child_comm: nm-dispatcher.a                     } hitcount:         22
990    { child_comm: evolution                           } hitcount:         35
991    { child_comm: glib-pacrunner                      } hitcount:         59
992
993    Totals:
994        Hits: 206
995        Entries: 21
996        Dropped: 0
997
998  The previous example showed how to start and stop a hist trigger by
999  appending 'pause' and 'continue' to the hist trigger command.  A
1000  hist trigger can also be started in a paused state by initially
1001  starting the trigger with ':pause' appended.  This allows you to
1002  start the trigger only when you're ready to start collecting data
1003  and not before.  For example, you could start the trigger in a
1004  paused state, then unpause it and do something you want to measure,
1005  then pause the trigger again when done.
1006
1007  Of course, doing this manually can be difficult and error-prone, but
1008  it is possible to automatically start and stop a hist trigger based
1009  on some condition, via the enable_hist and disable_hist triggers.
1010
1011  For example, suppose we wanted to take a look at the relative
1012  weights in terms of skb length for each callpath that leads to a
1013  netif_receive_skb event when downloading a decent-sized file using
1014  wget.
1015
1016  First we set up an initially paused stacktrace trigger on the
1017  netif_receive_skb event::
1018
1019    # echo 'hist:key=stacktrace:vals=len:pause' > \
1020           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1021
1022  Next, we set up an 'enable_hist' trigger on the sched_process_exec
1023  event, with an 'if filename==/usr/bin/wget' filter.  The effect of
1024  this new trigger is that it will 'unpause' the hist trigger we just
1025  set up on netif_receive_skb if and only if it sees a
1026  sched_process_exec event with a filename of '/usr/bin/wget'.  When
1027  that happens, all netif_receive_skb events are aggregated into a
1028  hash table keyed on stacktrace::
1029
1030    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
1031           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1032
1033  The aggregation continues until the netif_receive_skb is paused
1034  again, which is what the following disable_hist event does by
1035  creating a similar setup on the sched_process_exit event, using the
1036  filter 'comm==wget'::
1037
1038    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
1039           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1040
1041  Whenever a process exits and the comm field of the disable_hist
1042  trigger filter matches 'comm==wget', the netif_receive_skb hist
1043  trigger is disabled.
1044
1045  The overall effect is that netif_receive_skb events are aggregated
1046  into the hash table for only the duration of the wget.  Executing a
1047  wget command and then listing the 'hist' file will display the
1048  output generated by the wget command::
1049
1050    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
1051
1052    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1053    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
1054
1055    { stacktrace:
1056         __netif_receive_skb_core+0x46d/0x990
1057         __netif_receive_skb+0x18/0x60
1058         netif_receive_skb_internal+0x23/0x90
1059         napi_gro_receive+0xc8/0x100
1060         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
1061         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
1062         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
1063         ieee80211_rx+0x31d/0x900 [mac80211]
1064         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
1065         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
1066         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
1067         irq_thread_fn+0x20/0x50
1068         irq_thread+0x11f/0x150
1069         kthread+0xd2/0xf0
1070         ret_from_fork+0x42/0x70
1071    } hitcount:         85  len:      28884
1072    { stacktrace:
1073         __netif_receive_skb_core+0x46d/0x990
1074         __netif_receive_skb+0x18/0x60
1075         netif_receive_skb_internal+0x23/0x90
1076         napi_gro_complete+0xa4/0xe0
1077         dev_gro_receive+0x23a/0x360
1078         napi_gro_receive+0x30/0x100
1079         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
1080         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
1081         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
1082         ieee80211_rx+0x31d/0x900 [mac80211]
1083         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
1084         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
1085         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
1086         irq_thread_fn+0x20/0x50
1087         irq_thread+0x11f/0x150
1088         kthread+0xd2/0xf0
1089    } hitcount:         98  len:     664329
1090    { stacktrace:
1091         __netif_receive_skb_core+0x46d/0x990
1092         __netif_receive_skb+0x18/0x60
1093         process_backlog+0xa8/0x150
1094         net_rx_action+0x15d/0x340
1095         __do_softirq+0x114/0x2c0
1096         do_softirq_own_stack+0x1c/0x30
1097         do_softirq+0x65/0x70
1098         __local_bh_enable_ip+0xb5/0xc0
1099         ip_finish_output+0x1f4/0x840
1100         ip_output+0x6b/0xc0
1101         ip_local_out_sk+0x31/0x40
1102         ip_send_skb+0x1a/0x50
1103         udp_send_skb+0x173/0x2a0
1104         udp_sendmsg+0x2bf/0x9f0
1105         inet_sendmsg+0x64/0xa0
1106         sock_sendmsg+0x3d/0x50
1107    } hitcount:        115  len:      13030
1108    { stacktrace:
1109         __netif_receive_skb_core+0x46d/0x990
1110         __netif_receive_skb+0x18/0x60
1111         netif_receive_skb_internal+0x23/0x90
1112         napi_gro_complete+0xa4/0xe0
1113         napi_gro_flush+0x6d/0x90
1114         iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
1115         irq_thread_fn+0x20/0x50
1116         irq_thread+0x11f/0x150
1117         kthread+0xd2/0xf0
1118         ret_from_fork+0x42/0x70
1119    } hitcount:        934  len:    5512212
1120
1121    Totals:
1122        Hits: 1232
1123        Entries: 4
1124        Dropped: 0
1125
1126  The above shows all the netif_receive_skb callpaths and their total
1127  lengths for the duration of the wget command.
1128
1129  The 'clear' hist trigger param can be used to clear the hash table.
1130  Suppose we wanted to try another run of the previous example but
1131  this time also wanted to see the complete list of events that went
1132  into the histogram.  In order to avoid having to set everything up
1133  again, we can just clear the histogram first::
1134
1135    # echo 'hist:key=stacktrace:vals=len:clear' >> \
1136           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1137
1138  Just to verify that it is in fact cleared, here's what we now see in
1139  the hist file::
1140
1141    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1142    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
1143
1144    Totals:
1145        Hits: 0
1146        Entries: 0
1147        Dropped: 0
1148
1149  Since we want to see the detailed list of every netif_receive_skb
1150  event occurring during the new run, which are in fact the same
1151  events being aggregated into the hash table, we add some additional
1152  'enable_event' events to the triggering sched_process_exec and
1153  sched_process_exit events as such::
1154
1155    # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
1156           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1157
1158    # echo 'disable_event:net:netif_receive_skb if comm==wget' > \
1159           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1160
1161  If you read the trigger files for the sched_process_exec and
1162  sched_process_exit triggers, you should see two triggers for each:
1163  one enabling/disabling the hist aggregation and the other
1164  enabling/disabling the logging of events::
1165
1166    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
1167    enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
1168    enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
1169
1170    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
1171    enable_event:net:netif_receive_skb:unlimited if comm==wget
1172    disable_hist:net:netif_receive_skb:unlimited if comm==wget
1173
1174  In other words, whenever either of the sched_process_exec or
1175  sched_process_exit events is hit and matches 'wget', it enables or
1176  disables both the histogram and the event log, and what you end up
1177  with is a hash table and set of events just covering the specified
1178  duration.  Run the wget command again::
1179
1180    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
1181
1182  Displaying the 'hist' file should show something similar to what you
1183  saw in the last run, but this time you should also see the
1184  individual events in the trace file::
1185
1186    # cat /sys/kernel/debug/tracing/trace
1187
1188    # tracer: nop
1189    #
1190    # entries-in-buffer/entries-written: 183/1426   #P:4
1191    #
1192    #                              _-----=> irqs-off
1193    #                             / _----=> need-resched
1194    #                            | / _---=> hardirq/softirq
1195    #                            || / _--=> preempt-depth
1196    #                            ||| /     delay
1197    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
1198    #              | |       |   ||||       |         |
1199                wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
1200                wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
1201             dnsmasq-1382  [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
1202             dnsmasq-1382  [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
1203    ##### CPU 2 buffer started ####
1204      irq/29-iwlwifi-559   [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
1205      irq/29-iwlwifi-559   [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
1206      irq/29-iwlwifi-559   [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
1207      irq/29-iwlwifi-559   [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
1208      irq/29-iwlwifi-559   [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
1209    .
1210    .
1211    .
1212
1213  The following example demonstrates how multiple hist triggers can be
1214  attached to a given event.  This capability can be useful for
1215  creating a set of different summaries derived from the same set of
1216  events, or for comparing the effects of different filters, among
1217  other things::
1218
1219    # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
1220           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1221    # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
1222           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1223    # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
1224           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1225    # echo 'hist:keys=skbaddr.hex:vals=len' >> \
1226           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1227    # echo 'hist:keys=len:vals=common_preempt_count' >> \
1228           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1229
1230  The above set of commands create four triggers differing only in
1231  their filters, along with a completely different though fairly
1232  nonsensical trigger.  Note that in order to append multiple hist
1233  triggers to the same file, you should use the '>>' operator to
1234  append them ('>' will also add the new hist trigger, but will remove
1235  any existing hist triggers beforehand).
1236
1237  Displaying the contents of the 'hist' file for the event shows the
1238  contents of all five histograms::
1239
1240    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
1241
1242    # event histogram
1243    #
1244    # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
1245    #
1246
1247    { len:        176 } hitcount:          1  common_preempt_count:          0
1248    { len:        223 } hitcount:          1  common_preempt_count:          0
1249    { len:       4854 } hitcount:          1  common_preempt_count:          0
1250    { len:        395 } hitcount:          1  common_preempt_count:          0
1251    { len:        177 } hitcount:          1  common_preempt_count:          0
1252    { len:        446 } hitcount:          1  common_preempt_count:          0
1253    { len:       1601 } hitcount:          1  common_preempt_count:          0
1254    .
1255    .
1256    .
1257    { len:       1280 } hitcount:         66  common_preempt_count:          0
1258    { len:        116 } hitcount:         81  common_preempt_count:         40
1259    { len:        708 } hitcount:        112  common_preempt_count:          0
1260    { len:         46 } hitcount:        221  common_preempt_count:          0
1261    { len:       1264 } hitcount:        458  common_preempt_count:          0
1262
1263    Totals:
1264        Hits: 1428
1265        Entries: 147
1266        Dropped: 0
1267
1268
1269    # event histogram
1270    #
1271    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1272    #
1273
1274    { skbaddr: ffff8800baee5e00 } hitcount:          1  len:        130
1275    { skbaddr: ffff88005f3d5600 } hitcount:          1  len:       1280
1276    { skbaddr: ffff88005f3d4900 } hitcount:          1  len:       1280
1277    { skbaddr: ffff88009fed6300 } hitcount:          1  len:        115
1278    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:        115
1279    { skbaddr: ffff88008cdb1900 } hitcount:          1  len:         46
1280    { skbaddr: ffff880064b5ef00 } hitcount:          1  len:        118
1281    { skbaddr: ffff880044e3c700 } hitcount:          1  len:         60
1282    { skbaddr: ffff880100065900 } hitcount:          1  len:         46
1283    { skbaddr: ffff8800d46bd500 } hitcount:          1  len:        116
1284    { skbaddr: ffff88005f3d5f00 } hitcount:          1  len:       1280
1285    { skbaddr: ffff880100064700 } hitcount:          1  len:        365
1286    { skbaddr: ffff8800badb6f00 } hitcount:          1  len:         60
1287    .
1288    .
1289    .
1290    { skbaddr: ffff88009fe0be00 } hitcount:         27  len:      24677
1291    { skbaddr: ffff88009fe0a400 } hitcount:         27  len:      23052
1292    { skbaddr: ffff88009fe0b700 } hitcount:         31  len:      25589
1293    { skbaddr: ffff88009fe0b600 } hitcount:         32  len:      27326
1294    { skbaddr: ffff88006a462800 } hitcount:         68  len:      71678
1295    { skbaddr: ffff88006a463700 } hitcount:         70  len:      72678
1296    { skbaddr: ffff88006a462b00 } hitcount:         71  len:      77589
1297    { skbaddr: ffff88006a463600 } hitcount:         73  len:      71307
1298    { skbaddr: ffff88006a462200 } hitcount:         81  len:      81032
1299
1300    Totals:
1301        Hits: 1451
1302        Entries: 318
1303        Dropped: 0
1304
1305
1306    # event histogram
1307    #
1308    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
1309    #
1310
1311
1312    Totals:
1313        Hits: 0
1314        Entries: 0
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 > 4096 [active]
1321    #
1322
1323    { skbaddr: ffff88009fd2c300 } hitcount:          1  len:       7212
1324    { skbaddr: ffff8800d2bcce00 } hitcount:          1  len:       7212
1325    { skbaddr: ffff8800d2bcd700 } hitcount:          1  len:       7212
1326    { skbaddr: ffff8800d2bcda00 } hitcount:          1  len:      21492
1327    { skbaddr: ffff8800ae2e2d00 } hitcount:          1  len:       7212
1328    { skbaddr: ffff8800d2bcdb00 } hitcount:          1  len:       7212
1329    { skbaddr: ffff88006a4df500 } hitcount:          1  len:       4854
1330    { skbaddr: ffff88008ce47b00 } hitcount:          1  len:      18636
1331    { skbaddr: ffff8800ae2e2200 } hitcount:          1  len:      12924
1332    { skbaddr: ffff88005f3e1000 } hitcount:          1  len:       4356
1333    { skbaddr: ffff8800d2bcdc00 } hitcount:          2  len:      24420
1334    { skbaddr: ffff8800d2bcc200 } hitcount:          2  len:      12996
1335
1336    Totals:
1337        Hits: 14
1338        Entries: 12
1339        Dropped: 0
1340
1341
1342    # event histogram
1343    #
1344    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
1345    #
1346
1347
1348    Totals:
1349        Hits: 0
1350        Entries: 0
1351        Dropped: 0
1352
1353  Named triggers can be used to have triggers share a common set of
1354  histogram data.  This capability is mostly useful for combining the
1355  output of events generated by tracepoints contained inside inline
1356  functions, but names can be used in a hist trigger on any event.
1357  For example, these two triggers when hit will update the same 'len'
1358  field in the shared 'foo' histogram data::
1359
1360    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
1361           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
1362    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
1363           /sys/kernel/debug/tracing/events/net/netif_rx/trigger
1364
1365  You can see that they're updating common histogram data by reading
1366  each event's hist files at the same time::
1367
1368    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
1369      cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
1370
1371    # event histogram
1372    #
1373    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1374    #
1375
1376    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
1377    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
1378    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
1379    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
1380    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
1381    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
1382    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
1383    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
1384    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
1385    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
1386    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
1387    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
1388    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
1389    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
1390    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
1391    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
1392    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
1393    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
1394    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
1395    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
1396    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
1397    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
1398    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
1399    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
1400    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
1401    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
1402    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
1403    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
1404    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
1405    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
1406    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
1407    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
1408    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
1409    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
1410    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
1411    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
1412    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
1413    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
1414    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
1415    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
1416    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
1417    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276
1418
1419    Totals:
1420        Hits: 81
1421        Entries: 42
1422        Dropped: 0
1423    # event histogram
1424    #
1425    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
1426    #
1427
1428    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
1429    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
1430    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
1431    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
1432    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
1433    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
1434    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
1435    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
1436    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
1437    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
1438    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
1439    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
1440    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
1441    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
1442    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
1443    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
1444    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
1445    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
1446    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
1447    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
1448    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
1449    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
1450    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
1451    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
1452    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
1453    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
1454    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
1455    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
1456    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
1457    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
1458    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
1459    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
1460    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
1461    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
1462    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
1463    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
1464    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
1465    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
1466    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
1467    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
1468    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
1469    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276
1470
1471    Totals:
1472        Hits: 81
1473        Entries: 42
1474        Dropped: 0
1475
1476  And here's an example that shows how to combine histogram data from
1477  any two events even if they don't share any 'compatible' fields
1478  other than 'hitcount' and 'stacktrace'.  These commands create a
1479  couple of triggers named 'bar' using those fields::
1480
1481    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
1482           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
1483    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
1484          /sys/kernel/debug/tracing/events/net/netif_rx/trigger
1485
1486  And displaying the output of either shows some interesting if
1487  somewhat confusing output::
1488
1489    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
1490    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
1491
1492    # event histogram
1493    #
1494    # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
1495    #
1496
1497    { stacktrace:
1498             kernel_clone+0x18e/0x330
1499             kernel_thread+0x29/0x30
1500             kthreadd+0x154/0x1b0
1501             ret_from_fork+0x3f/0x70
1502    } hitcount:          1
1503    { stacktrace:
1504             netif_rx_internal+0xb2/0xd0
1505             netif_rx_ni+0x20/0x70
1506             dev_loopback_xmit+0xaa/0xd0
1507             ip_mc_output+0x126/0x240
1508             ip_local_out_sk+0x31/0x40
1509             igmp_send_report+0x1e9/0x230
1510             igmp_timer_expire+0xe9/0x120
1511             call_timer_fn+0x39/0xf0
1512             run_timer_softirq+0x1e1/0x290
1513             __do_softirq+0xfd/0x290
1514             irq_exit+0x98/0xb0
1515             smp_apic_timer_interrupt+0x4a/0x60
1516             apic_timer_interrupt+0x6d/0x80
1517             cpuidle_enter+0x17/0x20
1518             call_cpuidle+0x3b/0x60
1519             cpu_startup_entry+0x22d/0x310
1520    } hitcount:          1
1521    { stacktrace:
1522             netif_rx_internal+0xb2/0xd0
1523             netif_rx_ni+0x20/0x70
1524             dev_loopback_xmit+0xaa/0xd0
1525             ip_mc_output+0x17f/0x240
1526             ip_local_out_sk+0x31/0x40
1527             ip_send_skb+0x1a/0x50
1528             udp_send_skb+0x13e/0x270
1529             udp_sendmsg+0x2bf/0x980
1530             inet_sendmsg+0x67/0xa0
1531             sock_sendmsg+0x38/0x50
1532             SYSC_sendto+0xef/0x170
1533             SyS_sendto+0xe/0x10
1534             entry_SYSCALL_64_fastpath+0x12/0x6a
1535    } hitcount:          2
1536    { stacktrace:
1537             netif_rx_internal+0xb2/0xd0
1538             netif_rx+0x1c/0x60
1539             loopback_xmit+0x6c/0xb0
1540             dev_hard_start_xmit+0x219/0x3a0
1541             __dev_queue_xmit+0x415/0x4f0
1542             dev_queue_xmit_sk+0x13/0x20
1543             ip_finish_output2+0x237/0x340
1544             ip_finish_output+0x113/0x1d0
1545             ip_output+0x66/0xc0
1546             ip_local_out_sk+0x31/0x40
1547             ip_send_skb+0x1a/0x50
1548             udp_send_skb+0x16d/0x270
1549             udp_sendmsg+0x2bf/0x980
1550             inet_sendmsg+0x67/0xa0
1551             sock_sendmsg+0x38/0x50
1552             ___sys_sendmsg+0x14e/0x270
1553    } hitcount:         76
1554    { stacktrace:
1555             netif_rx_internal+0xb2/0xd0
1556             netif_rx+0x1c/0x60
1557             loopback_xmit+0x6c/0xb0
1558             dev_hard_start_xmit+0x219/0x3a0
1559             __dev_queue_xmit+0x415/0x4f0
1560             dev_queue_xmit_sk+0x13/0x20
1561             ip_finish_output2+0x237/0x340
1562             ip_finish_output+0x113/0x1d0
1563             ip_output+0x66/0xc0
1564             ip_local_out_sk+0x31/0x40
1565             ip_send_skb+0x1a/0x50
1566             udp_send_skb+0x16d/0x270
1567             udp_sendmsg+0x2bf/0x980
1568             inet_sendmsg+0x67/0xa0
1569             sock_sendmsg+0x38/0x50
1570             ___sys_sendmsg+0x269/0x270
1571    } hitcount:         77
1572    { stacktrace:
1573             netif_rx_internal+0xb2/0xd0
1574             netif_rx+0x1c/0x60
1575             loopback_xmit+0x6c/0xb0
1576             dev_hard_start_xmit+0x219/0x3a0
1577             __dev_queue_xmit+0x415/0x4f0
1578             dev_queue_xmit_sk+0x13/0x20
1579             ip_finish_output2+0x237/0x340
1580             ip_finish_output+0x113/0x1d0
1581             ip_output+0x66/0xc0
1582             ip_local_out_sk+0x31/0x40
1583             ip_send_skb+0x1a/0x50
1584             udp_send_skb+0x16d/0x270
1585             udp_sendmsg+0x2bf/0x980
1586             inet_sendmsg+0x67/0xa0
1587             sock_sendmsg+0x38/0x50
1588             SYSC_sendto+0xef/0x170
1589    } hitcount:         88
1590    { stacktrace:
1591             kernel_clone+0x18e/0x330
1592             SyS_clone+0x19/0x20
1593             entry_SYSCALL_64_fastpath+0x12/0x6a
1594    } hitcount:        244
1595
1596    Totals:
1597        Hits: 489
1598        Entries: 7
1599        Dropped: 0
1600
16012.2 Inter-event hist triggers
1602-----------------------------
1603
1604Inter-event hist triggers are hist triggers that combine values from
1605one or more other events and create a histogram using that data.  Data
1606from an inter-event histogram can in turn become the source for
1607further combined histograms, thus providing a chain of related
1608histograms, which is important for some applications.
1609
1610The most important example of an inter-event quantity that can be used
1611in this manner is latency, which is simply a difference in timestamps
1612between two events.  Although latency is the most important
1613inter-event quantity, note that because the support is completely
1614general across the trace event subsystem, any event field can be used
1615in an inter-event quantity.
1616
1617An example of a histogram that combines data from other histograms
1618into a useful chain would be a 'wakeupswitch latency' histogram that
1619combines a 'wakeup latency' histogram and a 'switch latency'
1620histogram.
1621
1622Normally, a hist trigger specification consists of a (possibly
1623compound) key along with one or more numeric values, which are
1624continually updated sums associated with that key.  A histogram
1625specification in this case consists of individual key and value
1626specifications that refer to trace event fields associated with a
1627single event type.
1628
1629The inter-event hist trigger extension allows fields from multiple
1630events to be referenced and combined into a multi-event histogram
1631specification.  In support of this overall goal, a few enabling
1632features have been added to the hist trigger support:
1633
1634  - In order to compute an inter-event quantity, a value from one
1635    event needs to saved and then referenced from another event.  This
1636    requires the introduction of support for histogram 'variables'.
1637
1638  - The computation of inter-event quantities and their combination
1639    require some minimal amount of support for applying simple
1640    expressions to variables (+ and -).
1641
1642  - A histogram consisting of inter-event quantities isn't logically a
1643    histogram on either event (so having the 'hist' file for either
1644    event host the histogram output doesn't really make sense).  To
1645    address the idea that the histogram is associated with a
1646    combination of events, support is added allowing the creation of
1647    'synthetic' events that are events derived from other events.
1648    These synthetic events are full-fledged events just like any other
1649    and can be used as such, as for instance to create the
1650    'combination' histograms mentioned previously.
1651
1652  - A set of 'actions' can be associated with histogram entries -
1653    these can be used to generate the previously mentioned synthetic
1654    events, but can also be used for other purposes, such as for
1655    example saving context when a 'max' latency has been hit.
1656
1657  - Trace events don't have a 'timestamp' associated with them, but
1658    there is an implicit timestamp saved along with an event in the
1659    underlying ftrace ring buffer.  This timestamp is now exposed as a
1660    a synthetic field named 'common_timestamp' which can be used in
1661    histograms as if it were any other event field; it isn't an actual
1662    field in the trace format but rather is a synthesized value that
1663    nonetheless can be used as if it were an actual field.  By default
1664    it is in units of nanoseconds; appending '.usecs' to a
1665    common_timestamp field changes the units to microseconds.
1666
1667A note on inter-event timestamps: If common_timestamp is used in a
1668histogram, the trace buffer is automatically switched over to using
1669absolute timestamps and the "global" trace clock, in order to avoid
1670bogus timestamp differences with other clocks that aren't coherent
1671across CPUs.  This can be overridden by specifying one of the other
1672trace clocks instead, using the "clock=XXX" hist trigger attribute,
1673where XXX is any of the clocks listed in the tracing/trace_clock
1674pseudo-file.
1675
1676These features are described in more detail in the following sections.
1677
16782.2.1 Histogram Variables
1679-------------------------
1680
1681Variables are simply named locations used for saving and retrieving
1682values between matching events.  A 'matching' event is defined as an
1683event that has a matching key - if a variable is saved for a histogram
1684entry corresponding to that key, any subsequent event with a matching
1685key can access that variable.
1686
1687A variable's value is normally available to any subsequent event until
1688it is set to something else by a subsequent event.  The one exception
1689to that rule is that any variable used in an expression is essentially
1690'read-once' - once it's used by an expression in a subsequent event,
1691it's reset to its 'unset' state, which means it can't be used again
1692unless it's set again.  This ensures not only that an event doesn't
1693use an uninitialized variable in a calculation, but that that variable
1694is used only once and not for any unrelated subsequent match.
1695
1696The basic syntax for saving a variable is to simply prefix a unique
1697variable name not corresponding to any keyword along with an '=' sign
1698to any event field.
1699
1700Either keys or values can be saved and retrieved in this way.  This
1701creates a variable named 'ts0' for a histogram entry with the key
1702'next_pid'::
1703
1704  # echo 'hist:keys=next_pid:vals=$ts0:ts0=common_timestamp ... >> \
1705	event/trigger
1706
1707The ts0 variable can be accessed by any subsequent event having the
1708same pid as 'next_pid'.
1709
1710Variable references are formed by prepending the variable name with
1711the '$' sign.  Thus for example, the ts0 variable above would be
1712referenced as '$ts0' in expressions.
1713
1714Because 'vals=' is used, the common_timestamp variable value above
1715will also be summed as a normal histogram value would (though for a
1716timestamp it makes little sense).
1717
1718The below shows that a key value can also be saved in the same way::
1719
1720  # echo 'hist:timer_pid=common_pid:key=timer_pid ...' >> event/trigger
1721
1722If a variable isn't a key variable or prefixed with 'vals=', the
1723associated event field will be saved in a variable but won't be summed
1724as a value::
1725
1726  # echo 'hist:keys=next_pid:ts1=common_timestamp ...' >> event/trigger
1727
1728Multiple variables can be assigned at the same time.  The below would
1729result in both ts0 and b being created as variables, with both
1730common_timestamp and field1 additionally being summed as values::
1731
1732  # echo 'hist:keys=pid:vals=$ts0,$b:ts0=common_timestamp,b=field1 ...' >> \
1733	event/trigger
1734
1735Note that variable assignments can appear either preceding or
1736following their use.  The command below behaves identically to the
1737command above::
1738
1739  # echo 'hist:keys=pid:ts0=common_timestamp,b=field1:vals=$ts0,$b ...' >> \
1740	event/trigger
1741
1742Any number of variables not bound to a 'vals=' prefix can also be
1743assigned by simply separating them with colons.  Below is the same
1744thing but without the values being summed in the histogram::
1745
1746  # echo 'hist:keys=pid:ts0=common_timestamp:b=field1 ...' >> event/trigger
1747
1748Variables set as above can be referenced and used in expressions on
1749another event.
1750
1751For example, here's how a latency can be calculated::
1752
1753  # echo 'hist:keys=pid,prio:ts0=common_timestamp ...' >> event1/trigger
1754  # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...' >> event2/trigger
1755
1756In the first line above, the event's timestamp is saved into the
1757variable ts0.  In the next line, ts0 is subtracted from the second
1758event's timestamp to produce the latency, which is then assigned into
1759yet another variable, 'wakeup_lat'.  The hist trigger below in turn
1760makes use of the wakeup_lat variable to compute a combined latency
1761using the same key and variable from yet another event::
1762
1763  # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
1764
17652.2.2 Synthetic Events
1766----------------------
1767
1768Synthetic events are user-defined events generated from hist trigger
1769variables or fields associated with one or more other events.  Their
1770purpose is to provide a mechanism for displaying data spanning
1771multiple events consistent with the existing and already familiar
1772usage for normal events.
1773
1774To define a synthetic event, the user writes a simple specification
1775consisting of the name of the new event along with one or more
1776variables and their types, which can be any valid field type,
1777separated by semicolons, to the tracing/synthetic_events file.
1778
1779See synth_field_size() for available types.
1780
1781If field_name contains [n], the field is considered to be a static array.
1782
1783If field_names contains[] (no subscript), the field is considered to
1784be a dynamic array, which will only take as much space in the event as
1785is required to hold the array.
1786
1787A string field can be specified using either the static notation:
1788
1789  char name[32];
1790
1791Or the dynamic:
1792
1793  char name[];
1794
1795The size limit for either is 256.
1796
1797For instance, the following creates a new event named 'wakeup_latency'
1798with 3 fields: lat, pid, and prio.  Each of those fields is simply a
1799variable reference to a variable on another event::
1800
1801  # echo 'wakeup_latency \
1802          u64 lat; \
1803          pid_t pid; \
1804	  int prio' >> \
1805	  /sys/kernel/debug/tracing/synthetic_events
1806
1807Reading the tracing/synthetic_events file lists all the currently
1808defined synthetic events, in this case the event defined above::
1809
1810  # cat /sys/kernel/debug/tracing/synthetic_events
1811    wakeup_latency u64 lat; pid_t pid; int prio
1812
1813An existing synthetic event definition can be removed by prepending
1814the command that defined it with a '!'::
1815
1816  # echo '!wakeup_latency u64 lat pid_t pid int prio' >> \
1817    /sys/kernel/debug/tracing/synthetic_events
1818
1819At this point, there isn't yet an actual 'wakeup_latency' event
1820instantiated in the event subsystem - for this to happen, a 'hist
1821trigger action' needs to be instantiated and bound to actual fields
1822and variables defined on other events (see Section 2.2.3 below on
1823how that is done using hist trigger 'onmatch' action). Once that is
1824done, the 'wakeup_latency' synthetic event instance is created.
1825
1826A histogram can now be defined for the new synthetic event::
1827
1828  # echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
1829        /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
1830
1831The new event is created under the tracing/events/synthetic/ directory
1832and looks and behaves just like any other event::
1833
1834  # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
1835        enable  filter  format  hist  id  trigger
1836
1837Like any other event, once a histogram is enabled for the event, the
1838output can be displayed by reading the event's 'hist' file.
1839
18402.2.3 Hist trigger 'handlers' and 'actions'
1841-------------------------------------------
1842
1843A hist trigger 'action' is a function that's executed (in most cases
1844conditionally) whenever a histogram entry is added or updated.
1845
1846When a histogram entry is added or updated, a hist trigger 'handler'
1847is what decides whether the corresponding action is actually invoked
1848or not.
1849
1850Hist trigger handlers and actions are paired together in the general
1851form:
1852
1853  <handler>.<action>
1854
1855To specify a handler.action pair for a given event, simply specify
1856that handler.action pair between colons in the hist trigger
1857specification.
1858
1859In theory, any handler can be combined with any action, but in
1860practice, not every handler.action combination is currently supported;
1861if a given handler.action combination isn't supported, the hist
1862trigger will fail with -EINVAL;
1863
1864The default 'handler.action' if none is explicitly specified is as it
1865always has been, to simply update the set of values associated with an
1866entry.  Some applications, however, may want to perform additional
1867actions at that point, such as generate another event, or compare and
1868save a maximum.
1869
1870The supported handlers and actions are listed below, and each is
1871described in more detail in the following paragraphs, in the context
1872of descriptions of some common and useful handler.action combinations.
1873
1874The available handlers are:
1875
1876  - onmatch(matching.event)    - invoke action on any addition or update
1877  - onmax(var)                 - invoke action if var exceeds current max
1878  - onchange(var)              - invoke action if var changes
1879
1880The available actions are:
1881
1882  - trace(<synthetic_event_name>,param list)   - generate synthetic event
1883  - save(field,...)                            - save current event fields
1884  - snapshot()                                 - snapshot the trace buffer
1885
1886The following commonly-used handler.action pairs are available:
1887
1888  - onmatch(matching.event).trace(<synthetic_event_name>,param list)
1889
1890    The 'onmatch(matching.event).trace(<synthetic_event_name>,param
1891    list)' hist trigger action is invoked whenever an event matches
1892    and the histogram entry would be added or updated.  It causes the
1893    named synthetic event to be generated with the values given in the
1894    'param list'.  The result is the generation of a synthetic event
1895    that consists of the values contained in those variables at the
1896    time the invoking event was hit.  For example, if the synthetic
1897    event name is 'wakeup_latency', a wakeup_latency event is
1898    generated using onmatch(event).trace(wakeup_latency,arg1,arg2).
1899
1900    There is also an equivalent alternative form available for
1901    generating synthetic events.  In this form, the synthetic event
1902    name is used as if it were a function name.  For example, using
1903    the 'wakeup_latency' synthetic event name again, the
1904    wakeup_latency event would be generated by invoking it as if it
1905    were a function call, with the event field values passed in as
1906    arguments: onmatch(event).wakeup_latency(arg1,arg2).  The syntax
1907    for this form is:
1908
1909      onmatch(matching.event).<synthetic_event_name>(param list)
1910
1911    In either case, the 'param list' consists of one or more
1912    parameters which may be either variables or fields defined on
1913    either the 'matching.event' or the target event.  The variables or
1914    fields specified in the param list may be either fully-qualified
1915    or unqualified.  If a variable is specified as unqualified, it
1916    must be unique between the two events.  A field name used as a
1917    param can be unqualified if it refers to the target event, but
1918    must be fully qualified if it refers to the matching event.  A
1919    fully-qualified name is of the form 'system.event_name.$var_name'
1920    or 'system.event_name.field'.
1921
1922    The 'matching.event' specification is simply the fully qualified
1923    event name of the event that matches the target event for the
1924    onmatch() functionality, in the form 'system.event_name'. Histogram
1925    keys of both events are compared to find if events match. In case
1926    multiple histogram keys are used, they all must match in the specified
1927    order.
1928
1929    Finally, the number and type of variables/fields in the 'param
1930    list' must match the number and types of the fields in the
1931    synthetic event being generated.
1932
1933    As an example the below defines a simple synthetic event and uses
1934    a variable defined on the sched_wakeup_new event as a parameter
1935    when invoking the synthetic event.  Here we define the synthetic
1936    event::
1937
1938      # echo 'wakeup_new_test pid_t pid' >> \
1939             /sys/kernel/debug/tracing/synthetic_events
1940
1941      # cat /sys/kernel/debug/tracing/synthetic_events
1942            wakeup_new_test pid_t pid
1943
1944    The following hist trigger both defines the missing testpid
1945    variable and specifies an onmatch() action that generates a
1946    wakeup_new_test synthetic event whenever a sched_wakeup_new event
1947    occurs, which because of the 'if comm == "cyclictest"' filter only
1948    happens when the executable is cyclictest::
1949
1950      # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
1951              wakeup_new_test($testpid) if comm=="cyclictest"' >> \
1952              /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
1953
1954    Or, equivalently, using the 'trace' keyword syntax:
1955
1956    # echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
1957            trace(wakeup_new_test,$testpid) if comm=="cyclictest"' >> \
1958            /sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
1959
1960    Creating and displaying a histogram based on those events is now
1961    just a matter of using the fields and new synthetic event in the
1962    tracing/events/synthetic directory, as usual::
1963
1964      # echo 'hist:keys=pid:sort=pid' >> \
1965             /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/trigger
1966
1967    Running 'cyclictest' should cause wakeup_new events to generate
1968    wakeup_new_test synthetic events which should result in histogram
1969    output in the wakeup_new_test event's hist file::
1970
1971      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/hist
1972
1973    A more typical usage would be to use two events to calculate a
1974    latency.  The following example uses a set of hist triggers to
1975    produce a 'wakeup_latency' histogram.
1976
1977    First, we define a 'wakeup_latency' synthetic event::
1978
1979      # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
1980              /sys/kernel/debug/tracing/synthetic_events
1981
1982    Next, we specify that whenever we see a sched_waking event for a
1983    cyclictest thread, save the timestamp in a 'ts0' variable::
1984
1985      # echo 'hist:keys=$saved_pid:saved_pid=pid:ts0=common_timestamp.usecs \
1986              if comm=="cyclictest"' >> \
1987	      /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
1988
1989    Then, when the corresponding thread is actually scheduled onto the
1990    CPU by a sched_switch event (saved_pid matches next_pid), calculate
1991    the latency and use that along with another variable and an event field
1992    to generate a wakeup_latency synthetic event::
1993
1994      # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
1995              onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\
1996	              $saved_pid,next_prio) if next_comm=="cyclictest"' >> \
1997	      /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
1998
1999    We also need to create a histogram on the wakeup_latency synthetic
2000    event in order to aggregate the generated synthetic event data::
2001
2002      # echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
2003              /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
2004
2005    Finally, once we've run cyclictest to actually generate some
2006    events, we can see the output by looking at the wakeup_latency
2007    synthetic event's hist file::
2008
2009      # cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist
2010
2011  - onmax(var).save(field,..	.)
2012
2013    The 'onmax(var).save(field,...)' hist trigger action is invoked
2014    whenever the value of 'var' associated with a histogram entry
2015    exceeds the current maximum contained in that variable.
2016
2017    The end result is that the trace event fields specified as the
2018    onmax.save() params will be saved if 'var' exceeds the current
2019    maximum for that hist trigger entry.  This allows context from the
2020    event that exhibited the new maximum to be saved for later
2021    reference.  When the histogram is displayed, additional fields
2022    displaying the saved values will be printed.
2023
2024    As an example the below defines a couple of hist triggers, one for
2025    sched_waking and another for sched_switch, keyed on pid.  Whenever
2026    a sched_waking occurs, the timestamp is saved in the entry
2027    corresponding to the current pid, and when the scheduler switches
2028    back to that pid, the timestamp difference is calculated.  If the
2029    resulting latency, stored in wakeup_lat, exceeds the current
2030    maximum latency, the values specified in the save() fields are
2031    recorded::
2032
2033      # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
2034              if comm=="cyclictest"' >> \
2035              /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
2036
2037      # echo 'hist:keys=next_pid:\
2038              wakeup_lat=common_timestamp.usecs-$ts0:\
2039              onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) \
2040              if next_comm=="cyclictest"' >> \
2041              /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
2042
2043    When the histogram is displayed, the max value and the saved
2044    values corresponding to the max are displayed following the rest
2045    of the fields::
2046
2047      # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
2048        { next_pid:       2255 } hitcount:        239
2049          common_timestamp-ts0:          0
2050          max:         27
2051	  next_comm: cyclictest
2052          prev_pid:          0  prev_prio:        120  prev_comm: swapper/1
2053
2054        { next_pid:       2256 } hitcount:       2355
2055          common_timestamp-ts0: 0
2056          max:         49  next_comm: cyclictest
2057          prev_pid:          0  prev_prio:        120  prev_comm: swapper/0
2058
2059        Totals:
2060            Hits: 12970
2061            Entries: 2
2062            Dropped: 0
2063
2064  - onmax(var).snapshot()
2065
2066    The 'onmax(var).snapshot()' hist trigger action is invoked
2067    whenever the value of 'var' associated with a histogram entry
2068    exceeds the current maximum contained in that variable.
2069
2070    The end result is that a global snapshot of the trace buffer will
2071    be saved in the tracing/snapshot file if 'var' exceeds the current
2072    maximum for any hist trigger entry.
2073
2074    Note that in this case the maximum is a global maximum for the
2075    current trace instance, which is the maximum across all buckets of
2076    the histogram.  The key of the specific trace event that caused
2077    the global maximum and the global maximum itself are displayed,
2078    along with a message stating that a snapshot has been taken and
2079    where to find it.  The user can use the key information displayed
2080    to locate the corresponding bucket in the histogram for even more
2081    detail.
2082
2083    As an example the below defines a couple of hist triggers, one for
2084    sched_waking and another for sched_switch, keyed on pid.  Whenever
2085    a sched_waking event occurs, the timestamp is saved in the entry
2086    corresponding to the current pid, and when the scheduler switches
2087    back to that pid, the timestamp difference is calculated.  If the
2088    resulting latency, stored in wakeup_lat, exceeds the current
2089    maximum latency, a snapshot is taken.  As part of the setup, all
2090    the scheduler events are also enabled, which are the events that
2091    will show up in the snapshot when it is taken at some point:
2092
2093    # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
2094
2095    # echo 'hist:keys=pid:ts0=common_timestamp.usecs \
2096            if comm=="cyclictest"' >> \
2097            /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
2098
2099    # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
2100            onmax($wakeup_lat).save(next_prio,next_comm,prev_pid,prev_prio, \
2101	    prev_comm):onmax($wakeup_lat).snapshot() \
2102	    if next_comm=="cyclictest"' >> \
2103	    /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
2104
2105    When the histogram is displayed, for each bucket the max value
2106    and the saved values corresponding to the max are displayed
2107    following the rest of the fields.
2108
2109    If a snapshot was taken, there is also a message indicating that,
2110    along with the value and event that triggered the global maximum:
2111
2112    # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
2113      { next_pid:       2101 } hitcount:        200
2114	max:         52  next_prio:        120  next_comm: cyclictest \
2115        prev_pid:          0  prev_prio:        120  prev_comm: swapper/6
2116
2117      { next_pid:       2103 } hitcount:       1326
2118	max:        572  next_prio:         19  next_comm: cyclictest \
2119        prev_pid:          0  prev_prio:        120  prev_comm: swapper/1
2120
2121      { next_pid:       2102 } hitcount:       1982 \
2122	max:         74  next_prio:         19  next_comm: cyclictest \
2123        prev_pid:          0  prev_prio:        120  prev_comm: swapper/5
2124
2125    Snapshot taken (see tracing/snapshot).  Details:
2126	triggering value { onmax($wakeup_lat) }:        572	\
2127	triggered by event with key: { next_pid:       2103 }
2128
2129    Totals:
2130        Hits: 3508
2131        Entries: 3
2132        Dropped: 0
2133
2134    In the above case, the event that triggered the global maximum has
2135    the key with next_pid == 2103.  If you look at the bucket that has
2136    2103 as the key, you'll find the additional values save()'d along
2137    with the local maximum for that bucket, which should be the same
2138    as the global maximum (since that was the same value that
2139    triggered the global snapshot).
2140
2141    And finally, looking at the snapshot data should show at or near
2142    the end the event that triggered the snapshot (in this case you
2143    can verify the timestamps between the sched_waking and
2144    sched_switch events, which should match the time displayed in the
2145    global maximum)::
2146
2147     # cat /sys/kernel/debug/tracing/snapshot
2148
2149         <...>-2103  [005] d..3   309.873125: sched_switch: prev_comm=cyclictest prev_pid=2103 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
2150         <idle>-0     [005] d.h3   309.873611: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
2151         <idle>-0     [005] dNh4   309.873613: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
2152         <idle>-0     [005] d..3   309.873616: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
2153         <...>-2102  [005] d..3   309.873625: sched_switch: prev_comm=cyclictest prev_pid=2102 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
2154         <idle>-0     [005] d.h3   309.874624: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
2155         <idle>-0     [005] dNh4   309.874626: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
2156         <idle>-0     [005] dNh3   309.874628: sched_waking: comm=cyclictest pid=2103 prio=19 target_cpu=005
2157         <idle>-0     [005] dNh4   309.874630: sched_wakeup: comm=cyclictest pid=2103 prio=19 target_cpu=005
2158         <idle>-0     [005] d..3   309.874633: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
2159         <idle>-0     [004] d.h3   309.874757: sched_waking: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
2160         <idle>-0     [004] dNh4   309.874762: sched_wakeup: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
2161         <idle>-0     [004] d..3   309.874766: sched_switch: prev_comm=swapper/4 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=gnome-terminal- next_pid=1699 next_prio=120
2162     gnome-terminal--1699  [004] d.h2   309.874941: sched_stat_runtime: comm=gnome-terminal- pid=1699 runtime=180706 [ns] vruntime=1126870572 [ns]
2163         <idle>-0     [003] d.s4   309.874956: sched_waking: comm=rcu_sched pid=9 prio=120 target_cpu=007
2164         <idle>-0     [003] d.s5   309.874960: sched_wake_idle_without_ipi: cpu=7
2165         <idle>-0     [003] d.s5   309.874961: sched_wakeup: comm=rcu_sched pid=9 prio=120 target_cpu=007
2166         <idle>-0     [007] d..3   309.874963: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=rcu_sched next_pid=9 next_prio=120
2167      rcu_sched-9     [007] d..3   309.874973: sched_stat_runtime: comm=rcu_sched pid=9 runtime=13646 [ns] vruntime=22531430286 [ns]
2168      rcu_sched-9     [007] d..3   309.874978: sched_switch: prev_comm=rcu_sched prev_pid=9 prev_prio=120 prev_state=R+ ==> next_comm=swapper/7 next_pid=0 next_prio=120
2169          <...>-2102  [005] d..4   309.874994: sched_migrate_task: comm=cyclictest pid=2103 prio=19 orig_cpu=5 dest_cpu=1
2170          <...>-2102  [005] d..4   309.875185: sched_wake_idle_without_ipi: cpu=1
2171         <idle>-0     [001] d..3   309.875200: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2103 next_prio=19
2172
2173  - onchange(var).save(field,..	.)
2174
2175    The 'onchange(var).save(field,...)' hist trigger action is invoked
2176    whenever the value of 'var' associated with a histogram entry
2177    changes.
2178
2179    The end result is that the trace event fields specified as the
2180    onchange.save() params will be saved if 'var' changes for that
2181    hist trigger entry.  This allows context from the event that
2182    changed the value to be saved for later reference.  When the
2183    histogram is displayed, additional fields displaying the saved
2184    values will be printed.
2185
2186  - onchange(var).snapshot()
2187
2188    The 'onchange(var).snapshot()' hist trigger action is invoked
2189    whenever the value of 'var' associated with a histogram entry
2190    changes.
2191
2192    The end result is that a global snapshot of the trace buffer will
2193    be saved in the tracing/snapshot file if 'var' changes for any
2194    hist trigger entry.
2195
2196    Note that in this case the changed value is a global variable
2197    associated with current trace instance.  The key of the specific
2198    trace event that caused the value to change and the global value
2199    itself are displayed, along with a message stating that a snapshot
2200    has been taken and where to find it.  The user can use the key
2201    information displayed to locate the corresponding bucket in the
2202    histogram for even more detail.
2203
2204    As an example the below defines a hist trigger on the tcp_probe
2205    event, keyed on dport.  Whenever a tcp_probe event occurs, the
2206    cwnd field is checked against the current value stored in the
2207    $cwnd variable.  If the value has changed, a snapshot is taken.
2208    As part of the setup, all the scheduler and tcp events are also
2209    enabled, which are the events that will show up in the snapshot
2210    when it is taken at some point:
2211
2212    # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
2213    # echo 1 > /sys/kernel/debug/tracing/events/tcp/enable
2214
2215    # echo 'hist:keys=dport:cwnd=snd_cwnd: \
2216            onchange($cwnd).save(snd_wnd,srtt,rcv_wnd): \
2217	    onchange($cwnd).snapshot()' >> \
2218	    /sys/kernel/debug/tracing/events/tcp/tcp_probe/trigger
2219
2220    When the histogram is displayed, for each bucket the tracked value
2221    and the saved values corresponding to that value are displayed
2222    following the rest of the fields.
2223
2224    If a snapshot was taken, there is also a message indicating that,
2225    along with the value and event that triggered the snapshot::
2226
2227      # cat /sys/kernel/debug/tracing/events/tcp/tcp_probe/hist
2228
2229      { dport:       1521 } hitcount:          8
2230	changed:         10  snd_wnd:      35456  srtt:     154262  rcv_wnd:      42112
2231
2232      { dport:         80 } hitcount:         23
2233	changed:         10  snd_wnd:      28960  srtt:      19604  rcv_wnd:      29312
2234
2235      { dport:       9001 } hitcount:        172
2236	changed:         10  snd_wnd:      48384  srtt:     260444  rcv_wnd:      55168
2237
2238      { dport:        443 } hitcount:        211
2239	changed:         10  snd_wnd:      26960  srtt:      17379  rcv_wnd:      28800
2240
2241    Snapshot taken (see tracing/snapshot).  Details::
2242
2243        triggering value { onchange($cwnd) }:         10
2244        triggered by event with key: { dport:         80 }
2245
2246      Totals:
2247          Hits: 414
2248          Entries: 4
2249          Dropped: 0
2250
2251    In the above case, the event that triggered the snapshot has the
2252    key with dport == 80.  If you look at the bucket that has 80 as
2253    the key, you'll find the additional values save()'d along with the
2254    changed value for that bucket, which should be the same as the
2255    global changed value (since that was the same value that triggered
2256    the global snapshot).
2257
2258    And finally, looking at the snapshot data should show at or near
2259    the end the event that triggered the snapshot::
2260
2261      # cat /sys/kernel/debug/tracing/snapshot
2262
2263         gnome-shell-1261  [006] dN.3    49.823113: sched_stat_runtime: comm=gnome-shell pid=1261 runtime=49347 [ns] vruntime=1835730389 [ns]
2264       kworker/u16:4-773   [003] d..3    49.823114: sched_switch: prev_comm=kworker/u16:4 prev_pid=773 prev_prio=120 prev_state=R+ ==> next_comm=kworker/3:2 next_pid=135 next_prio=120
2265         gnome-shell-1261  [006] d..3    49.823114: sched_switch: prev_comm=gnome-shell prev_pid=1261 prev_prio=120 prev_state=R+ ==> next_comm=kworker/6:2 next_pid=387 next_prio=120
2266         kworker/3:2-135   [003] d..3    49.823118: sched_stat_runtime: comm=kworker/3:2 pid=135 runtime=5339 [ns] vruntime=17815800388 [ns]
2267         kworker/6:2-387   [006] d..3    49.823120: sched_stat_runtime: comm=kworker/6:2 pid=387 runtime=9594 [ns] vruntime=14589605367 [ns]
2268         kworker/6:2-387   [006] d..3    49.823122: sched_switch: prev_comm=kworker/6:2 prev_pid=387 prev_prio=120 prev_state=R+ ==> next_comm=gnome-shell next_pid=1261 next_prio=120
2269         kworker/3:2-135   [003] d..3    49.823123: sched_switch: prev_comm=kworker/3:2 prev_pid=135 prev_prio=120 prev_state=T ==> next_comm=swapper/3 next_pid=0 next_prio=120
2270              <idle>-0     [004] ..s7    49.823798: tcp_probe: src=10.0.0.10:54326 dest=23.215.104.193:80 mark=0x0 length=32 snd_nxt=0xe3ae2ff5 snd_una=0xe3ae2ecd snd_cwnd=10 ssthresh=2147483647 snd_wnd=28960 srtt=19604 rcv_wnd=29312
2271
22723. User space creating a trigger
2273--------------------------------
2274
2275Writing into /sys/kernel/tracing/trace_marker writes into the ftrace
2276ring buffer. This can also act like an event, by writing into the trigger
2277file located in /sys/kernel/tracing/events/ftrace/print/
2278
2279Modifying cyclictest to write into the trace_marker file before it sleeps
2280and after it wakes up, something like this::
2281
2282  static void traceputs(char *str)
2283  {
2284	/* tracemark_fd is the trace_marker file descriptor */
2285	if (tracemark_fd < 0)
2286		return;
2287	/* write the tracemark message */
2288	write(tracemark_fd, str, strlen(str));
2289  }
2290
2291And later add something like::
2292
2293	traceputs("start");
2294	clock_nanosleep(...);
2295	traceputs("end");
2296
2297We can make a histogram from this::
2298
2299 # cd /sys/kernel/tracing
2300 # echo 'latency u64 lat' > synthetic_events
2301 # echo 'hist:keys=common_pid:ts0=common_timestamp.usecs if buf == "start"' > events/ftrace/print/trigger
2302 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(ftrace.print).latency($lat) if buf == "end"' >> events/ftrace/print/trigger
2303 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
2304
2305The above created a synthetic event called "latency" and two histograms
2306against the trace_marker, one gets triggered when "start" is written into the
2307trace_marker file and the other when "end" is written. If the pids match, then
2308it will call the "latency" synthetic event with the calculated latency as its
2309parameter. Finally, a histogram is added to the latency synthetic event to
2310record the calculated latency along with the pid.
2311
2312Now running cyclictest with::
2313
2314 # ./cyclictest -p80 -d0 -i250 -n -a -t --tracemark -b 1000
2315
2316 -p80  : run threads at priority 80
2317 -d0   : have all threads run at the same interval
2318 -i250 : start the interval at 250 microseconds (all threads will do this)
2319 -n    : sleep with nanosleep
2320 -a    : affine all threads to a separate CPU
2321 -t    : one thread per available CPU
2322 --tracemark : enable trace mark writing
2323 -b 1000 : stop if any latency is greater than 1000 microseconds
2324
2325Note, the -b 1000 is used just to make --tracemark available.
2326
2327Then we can see the histogram created by this with::
2328
2329 # cat events/synthetic/latency/hist
2330 # event histogram
2331 #
2332 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
2333 #
2334
2335 { lat:        107, common_pid:       2039 } hitcount:          1
2336 { lat:        122, common_pid:       2041 } hitcount:          1
2337 { lat:        166, common_pid:       2039 } hitcount:          1
2338 { lat:        174, common_pid:       2039 } hitcount:          1
2339 { lat:        194, common_pid:       2041 } hitcount:          1
2340 { lat:        196, common_pid:       2036 } hitcount:          1
2341 { lat:        197, common_pid:       2038 } hitcount:          1
2342 { lat:        198, common_pid:       2039 } hitcount:          1
2343 { lat:        199, common_pid:       2039 } hitcount:          1
2344 { lat:        200, common_pid:       2041 } hitcount:          1
2345 { lat:        201, common_pid:       2039 } hitcount:          2
2346 { lat:        202, common_pid:       2038 } hitcount:          1
2347 { lat:        202, common_pid:       2043 } hitcount:          1
2348 { lat:        203, common_pid:       2039 } hitcount:          1
2349 { lat:        203, common_pid:       2036 } hitcount:          1
2350 { lat:        203, common_pid:       2041 } hitcount:          1
2351 { lat:        206, common_pid:       2038 } hitcount:          2
2352 { lat:        207, common_pid:       2039 } hitcount:          1
2353 { lat:        207, common_pid:       2036 } hitcount:          1
2354 { lat:        208, common_pid:       2040 } hitcount:          1
2355 { lat:        209, common_pid:       2043 } hitcount:          1
2356 { lat:        210, common_pid:       2039 } hitcount:          1
2357 { lat:        211, common_pid:       2039 } hitcount:          4
2358 { lat:        212, common_pid:       2043 } hitcount:          1
2359 { lat:        212, common_pid:       2039 } hitcount:          2
2360 { lat:        213, common_pid:       2039 } hitcount:          1
2361 { lat:        214, common_pid:       2038 } hitcount:          1
2362 { lat:        214, common_pid:       2039 } hitcount:          2
2363 { lat:        214, common_pid:       2042 } hitcount:          1
2364 { lat:        215, common_pid:       2039 } hitcount:          1
2365 { lat:        217, common_pid:       2036 } hitcount:          1
2366 { lat:        217, common_pid:       2040 } hitcount:          1
2367 { lat:        217, common_pid:       2039 } hitcount:          1
2368 { lat:        218, common_pid:       2039 } hitcount:          6
2369 { lat:        219, common_pid:       2039 } hitcount:          9
2370 { lat:        220, common_pid:       2039 } hitcount:         11
2371 { lat:        221, common_pid:       2039 } hitcount:          5
2372 { lat:        221, common_pid:       2042 } hitcount:          1
2373 { lat:        222, common_pid:       2039 } hitcount:          7
2374 { lat:        223, common_pid:       2036 } hitcount:          1
2375 { lat:        223, common_pid:       2039 } hitcount:          3
2376 { lat:        224, common_pid:       2039 } hitcount:          4
2377 { lat:        224, common_pid:       2037 } hitcount:          1
2378 { lat:        224, common_pid:       2036 } hitcount:          2
2379 { lat:        225, common_pid:       2039 } hitcount:          5
2380 { lat:        225, common_pid:       2042 } hitcount:          1
2381 { lat:        226, common_pid:       2039 } hitcount:          7
2382 { lat:        226, common_pid:       2036 } hitcount:          4
2383 { lat:        227, common_pid:       2039 } hitcount:          6
2384 { lat:        227, common_pid:       2036 } hitcount:         12
2385 { lat:        227, common_pid:       2043 } hitcount:          1
2386 { lat:        228, common_pid:       2039 } hitcount:          7
2387 { lat:        228, common_pid:       2036 } hitcount:         14
2388 { lat:        229, common_pid:       2039 } hitcount:          9
2389 { lat:        229, common_pid:       2036 } hitcount:          8
2390 { lat:        229, common_pid:       2038 } hitcount:          1
2391 { lat:        230, common_pid:       2039 } hitcount:         11
2392 { lat:        230, common_pid:       2036 } hitcount:          6
2393 { lat:        230, common_pid:       2043 } hitcount:          1
2394 { lat:        230, common_pid:       2042 } hitcount:          2
2395 { lat:        231, common_pid:       2041 } hitcount:          1
2396 { lat:        231, common_pid:       2036 } hitcount:          6
2397 { lat:        231, common_pid:       2043 } hitcount:          1
2398 { lat:        231, common_pid:       2039 } hitcount:          8
2399 { lat:        232, common_pid:       2037 } hitcount:          1
2400 { lat:        232, common_pid:       2039 } hitcount:          6
2401 { lat:        232, common_pid:       2040 } hitcount:          2
2402 { lat:        232, common_pid:       2036 } hitcount:          5
2403 { lat:        232, common_pid:       2043 } hitcount:          1
2404 { lat:        233, common_pid:       2036 } hitcount:          5
2405 { lat:        233, common_pid:       2039 } hitcount:         11
2406 { lat:        234, common_pid:       2039 } hitcount:          4
2407 { lat:        234, common_pid:       2038 } hitcount:          2
2408 { lat:        234, common_pid:       2043 } hitcount:          2
2409 { lat:        234, common_pid:       2036 } hitcount:         11
2410 { lat:        234, common_pid:       2040 } hitcount:          1
2411 { lat:        235, common_pid:       2037 } hitcount:          2
2412 { lat:        235, common_pid:       2036 } hitcount:          8
2413 { lat:        235, common_pid:       2043 } hitcount:          2
2414 { lat:        235, common_pid:       2039 } hitcount:          5
2415 { lat:        235, common_pid:       2042 } hitcount:          2
2416 { lat:        235, common_pid:       2040 } hitcount:          4
2417 { lat:        235, common_pid:       2041 } hitcount:          1
2418 { lat:        236, common_pid:       2036 } hitcount:          7
2419 { lat:        236, common_pid:       2037 } hitcount:          1
2420 { lat:        236, common_pid:       2041 } hitcount:          5
2421 { lat:        236, common_pid:       2039 } hitcount:          3
2422 { lat:        236, common_pid:       2043 } hitcount:          9
2423 { lat:        236, common_pid:       2040 } hitcount:          7
2424 { lat:        237, common_pid:       2037 } hitcount:          1
2425 { lat:        237, common_pid:       2040 } hitcount:          1
2426 { lat:        237, common_pid:       2036 } hitcount:          9
2427 { lat:        237, common_pid:       2039 } hitcount:          3
2428 { lat:        237, common_pid:       2043 } hitcount:          8
2429 { lat:        237, common_pid:       2042 } hitcount:          2
2430 { lat:        237, common_pid:       2041 } hitcount:          2
2431 { lat:        238, common_pid:       2043 } hitcount:         10
2432 { lat:        238, common_pid:       2040 } hitcount:          1
2433 { lat:        238, common_pid:       2037 } hitcount:          9
2434 { lat:        238, common_pid:       2038 } hitcount:          1
2435 { lat:        238, common_pid:       2039 } hitcount:          1
2436 { lat:        238, common_pid:       2042 } hitcount:          3
2437 { lat:        238, common_pid:       2036 } hitcount:          7
2438 { lat:        239, common_pid:       2041 } hitcount:          1
2439 { lat:        239, common_pid:       2043 } hitcount:         11
2440 { lat:        239, common_pid:       2037 } hitcount:         11
2441 { lat:        239, common_pid:       2038 } hitcount:          6
2442 { lat:        239, common_pid:       2036 } hitcount:          7
2443 { lat:        239, common_pid:       2040 } hitcount:          1
2444 { lat:        239, common_pid:       2042 } hitcount:          9
2445 { lat:        240, common_pid:       2037 } hitcount:         29
2446 { lat:        240, common_pid:       2043 } hitcount:         15
2447 { lat:        240, common_pid:       2040 } hitcount:         44
2448 { lat:        240, common_pid:       2039 } hitcount:          1
2449 { lat:        240, common_pid:       2041 } hitcount:          2
2450 { lat:        240, common_pid:       2038 } hitcount:          1
2451 { lat:        240, common_pid:       2036 } hitcount:         10
2452 { lat:        240, common_pid:       2042 } hitcount:         13
2453 { lat:        241, common_pid:       2036 } hitcount:         21
2454 { lat:        241, common_pid:       2041 } hitcount:         36
2455 { lat:        241, common_pid:       2037 } hitcount:         34
2456 { lat:        241, common_pid:       2042 } hitcount:         14
2457 { lat:        241, common_pid:       2040 } hitcount:         94
2458 { lat:        241, common_pid:       2039 } hitcount:         12
2459 { lat:        241, common_pid:       2038 } hitcount:          2
2460 { lat:        241, common_pid:       2043 } hitcount:         28
2461 { lat:        242, common_pid:       2040 } hitcount:        109
2462 { lat:        242, common_pid:       2041 } hitcount:        506
2463 { lat:        242, common_pid:       2039 } hitcount:        155
2464 { lat:        242, common_pid:       2042 } hitcount:         21
2465 { lat:        242, common_pid:       2037 } hitcount:         52
2466 { lat:        242, common_pid:       2043 } hitcount:         21
2467 { lat:        242, common_pid:       2036 } hitcount:         16
2468 { lat:        242, common_pid:       2038 } hitcount:        156
2469 { lat:        243, common_pid:       2037 } hitcount:         46
2470 { lat:        243, common_pid:       2039 } hitcount:         40
2471 { lat:        243, common_pid:       2042 } hitcount:        119
2472 { lat:        243, common_pid:       2041 } hitcount:        611
2473 { lat:        243, common_pid:       2036 } hitcount:         69
2474 { lat:        243, common_pid:       2038 } hitcount:        784
2475 { lat:        243, common_pid:       2040 } hitcount:        323
2476 { lat:        243, common_pid:       2043 } hitcount:         14
2477 { lat:        244, common_pid:       2043 } hitcount:         35
2478 { lat:        244, common_pid:       2042 } hitcount:        305
2479 { lat:        244, common_pid:       2039 } hitcount:          8
2480 { lat:        244, common_pid:       2040 } hitcount:       4515
2481 { lat:        244, common_pid:       2038 } hitcount:        371
2482 { lat:        244, common_pid:       2037 } hitcount:         31
2483 { lat:        244, common_pid:       2036 } hitcount:        114
2484 { lat:        244, common_pid:       2041 } hitcount:       3396
2485 { lat:        245, common_pid:       2036 } hitcount:        700
2486 { lat:        245, common_pid:       2041 } hitcount:       2772
2487 { lat:        245, common_pid:       2037 } hitcount:        268
2488 { lat:        245, common_pid:       2039 } hitcount:        472
2489 { lat:        245, common_pid:       2038 } hitcount:       2758
2490 { lat:        245, common_pid:       2042 } hitcount:       3833
2491 { lat:        245, common_pid:       2040 } hitcount:       3105
2492 { lat:        245, common_pid:       2043 } hitcount:        645
2493 { lat:        246, common_pid:       2038 } hitcount:       3451
2494 { lat:        246, common_pid:       2041 } hitcount:        142
2495 { lat:        246, common_pid:       2037 } hitcount:       5101
2496 { lat:        246, common_pid:       2040 } hitcount:         68
2497 { lat:        246, common_pid:       2043 } hitcount:       5099
2498 { lat:        246, common_pid:       2039 } hitcount:       5608
2499 { lat:        246, common_pid:       2042 } hitcount:       3723
2500 { lat:        246, common_pid:       2036 } hitcount:       4738
2501 { lat:        247, common_pid:       2042 } hitcount:        312
2502 { lat:        247, common_pid:       2043 } hitcount:       2385
2503 { lat:        247, common_pid:       2041 } hitcount:        452
2504 { lat:        247, common_pid:       2038 } hitcount:        792
2505 { lat:        247, common_pid:       2040 } hitcount:         78
2506 { lat:        247, common_pid:       2036 } hitcount:       2375
2507 { lat:        247, common_pid:       2039 } hitcount:       1834
2508 { lat:        247, common_pid:       2037 } hitcount:       2655
2509 { lat:        248, common_pid:       2037 } hitcount:         36
2510 { lat:        248, common_pid:       2042 } hitcount:         11
2511 { lat:        248, common_pid:       2038 } hitcount:        122
2512 { lat:        248, common_pid:       2036 } hitcount:        135
2513 { lat:        248, common_pid:       2039 } hitcount:         26
2514 { lat:        248, common_pid:       2041 } hitcount:        503
2515 { lat:        248, common_pid:       2043 } hitcount:         66
2516 { lat:        248, common_pid:       2040 } hitcount:         46
2517 { lat:        249, common_pid:       2037 } hitcount:         29
2518 { lat:        249, common_pid:       2038 } hitcount:          1
2519 { lat:        249, common_pid:       2043 } hitcount:         29
2520 { lat:        249, common_pid:       2039 } hitcount:          8
2521 { lat:        249, common_pid:       2042 } hitcount:         56
2522 { lat:        249, common_pid:       2040 } hitcount:         27
2523 { lat:        249, common_pid:       2041 } hitcount:         11
2524 { lat:        249, common_pid:       2036 } hitcount:         27
2525 { lat:        250, common_pid:       2038 } hitcount:          1
2526 { lat:        250, common_pid:       2036 } hitcount:         30
2527 { lat:        250, common_pid:       2040 } hitcount:         19
2528 { lat:        250, common_pid:       2043 } hitcount:         22
2529 { lat:        250, common_pid:       2042 } hitcount:         20
2530 { lat:        250, common_pid:       2041 } hitcount:          1
2531 { lat:        250, common_pid:       2039 } hitcount:          6
2532 { lat:        250, common_pid:       2037 } hitcount:         48
2533 { lat:        251, common_pid:       2037 } hitcount:         43
2534 { lat:        251, common_pid:       2039 } hitcount:          1
2535 { lat:        251, common_pid:       2036 } hitcount:         12
2536 { lat:        251, common_pid:       2042 } hitcount:          2
2537 { lat:        251, common_pid:       2041 } hitcount:          1
2538 { lat:        251, common_pid:       2043 } hitcount:         15
2539 { lat:        251, common_pid:       2040 } hitcount:          3
2540 { lat:        252, common_pid:       2040 } hitcount:          1
2541 { lat:        252, common_pid:       2036 } hitcount:         12
2542 { lat:        252, common_pid:       2037 } hitcount:         21
2543 { lat:        252, common_pid:       2043 } hitcount:         14
2544 { lat:        253, common_pid:       2037 } hitcount:         21
2545 { lat:        253, common_pid:       2039 } hitcount:          2
2546 { lat:        253, common_pid:       2036 } hitcount:          9
2547 { lat:        253, common_pid:       2043 } hitcount:          6
2548 { lat:        253, common_pid:       2040 } hitcount:          1
2549 { lat:        254, common_pid:       2036 } hitcount:          8
2550 { lat:        254, common_pid:       2043 } hitcount:          3
2551 { lat:        254, common_pid:       2041 } hitcount:          1
2552 { lat:        254, common_pid:       2042 } hitcount:          1
2553 { lat:        254, common_pid:       2039 } hitcount:          1
2554 { lat:        254, common_pid:       2037 } hitcount:         12
2555 { lat:        255, common_pid:       2043 } hitcount:          1
2556 { lat:        255, common_pid:       2037 } hitcount:          2
2557 { lat:        255, common_pid:       2036 } hitcount:          2
2558 { lat:        255, common_pid:       2039 } hitcount:          8
2559 { lat:        256, common_pid:       2043 } hitcount:          1
2560 { lat:        256, common_pid:       2036 } hitcount:          4
2561 { lat:        256, common_pid:       2039 } hitcount:          6
2562 { lat:        257, common_pid:       2039 } hitcount:          5
2563 { lat:        257, common_pid:       2036 } hitcount:          4
2564 { lat:        258, common_pid:       2039 } hitcount:          5
2565 { lat:        258, common_pid:       2036 } hitcount:          2
2566 { lat:        259, common_pid:       2036 } hitcount:          7
2567 { lat:        259, common_pid:       2039 } hitcount:          7
2568 { lat:        260, common_pid:       2036 } hitcount:          8
2569 { lat:        260, common_pid:       2039 } hitcount:          6
2570 { lat:        261, common_pid:       2036 } hitcount:          5
2571 { lat:        261, common_pid:       2039 } hitcount:          7
2572 { lat:        262, common_pid:       2039 } hitcount:          5
2573 { lat:        262, common_pid:       2036 } hitcount:          5
2574 { lat:        263, common_pid:       2039 } hitcount:          7
2575 { lat:        263, common_pid:       2036 } hitcount:          7
2576 { lat:        264, common_pid:       2039 } hitcount:          9
2577 { lat:        264, common_pid:       2036 } hitcount:          9
2578 { lat:        265, common_pid:       2036 } hitcount:          5
2579 { lat:        265, common_pid:       2039 } hitcount:          1
2580 { lat:        266, common_pid:       2036 } hitcount:          1
2581 { lat:        266, common_pid:       2039 } hitcount:          3
2582 { lat:        267, common_pid:       2036 } hitcount:          1
2583 { lat:        267, common_pid:       2039 } hitcount:          3
2584 { lat:        268, common_pid:       2036 } hitcount:          1
2585 { lat:        268, common_pid:       2039 } hitcount:          6
2586 { lat:        269, common_pid:       2036 } hitcount:          1
2587 { lat:        269, common_pid:       2043 } hitcount:          1
2588 { lat:        269, common_pid:       2039 } hitcount:          2
2589 { lat:        270, common_pid:       2040 } hitcount:          1
2590 { lat:        270, common_pid:       2039 } hitcount:          6
2591 { lat:        271, common_pid:       2041 } hitcount:          1
2592 { lat:        271, common_pid:       2039 } hitcount:          5
2593 { lat:        272, common_pid:       2039 } hitcount:         10
2594 { lat:        273, common_pid:       2039 } hitcount:          8
2595 { lat:        274, common_pid:       2039 } hitcount:          2
2596 { lat:        275, common_pid:       2039 } hitcount:          1
2597 { lat:        276, common_pid:       2039 } hitcount:          2
2598 { lat:        276, common_pid:       2037 } hitcount:          1
2599 { lat:        276, common_pid:       2038 } hitcount:          1
2600 { lat:        277, common_pid:       2039 } hitcount:          1
2601 { lat:        277, common_pid:       2042 } hitcount:          1
2602 { lat:        278, common_pid:       2039 } hitcount:          1
2603 { lat:        279, common_pid:       2039 } hitcount:          4
2604 { lat:        279, common_pid:       2043 } hitcount:          1
2605 { lat:        280, common_pid:       2039 } hitcount:          3
2606 { lat:        283, common_pid:       2036 } hitcount:          2
2607 { lat:        284, common_pid:       2039 } hitcount:          1
2608 { lat:        284, common_pid:       2043 } hitcount:          1
2609 { lat:        288, common_pid:       2039 } hitcount:          1
2610 { lat:        289, common_pid:       2039 } hitcount:          1
2611 { lat:        300, common_pid:       2039 } hitcount:          1
2612 { lat:        384, common_pid:       2039 } hitcount:          1
2613
2614 Totals:
2615     Hits: 67625
2616     Entries: 278
2617     Dropped: 0
2618
2619Note, the writes are around the sleep, so ideally they will all be of 250
2620microseconds. If you are wondering how there are several that are under
2621250 microseconds, that is because the way cyclictest works, is if one
2622iteration comes in late, the next one will set the timer to wake up less that
2623250. That is, if an iteration came in 50 microseconds late, the next wake up
2624will be at 200 microseconds.
2625
2626But this could easily be done in userspace. To make this even more
2627interesting, we can mix the histogram between events that happened in the
2628kernel with trace_marker::
2629
2630 # cd /sys/kernel/tracing
2631 # echo 'latency u64 lat' > synthetic_events
2632 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' > events/sched/sched_waking/trigger
2633 # echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).latency($lat) if buf == "end"' > events/ftrace/print/trigger
2634 # echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
2635
2636The difference this time is that instead of using the trace_marker to start
2637the latency, the sched_waking event is used, matching the common_pid for the
2638trace_marker write with the pid that is being woken by sched_waking.
2639
2640After running cyclictest again with the same parameters, we now have::
2641
2642 # cat events/synthetic/latency/hist
2643 # event histogram
2644 #
2645 # trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
2646 #
2647
2648 { lat:          7, common_pid:       2302 } hitcount:        640
2649 { lat:          7, common_pid:       2299 } hitcount:         42
2650 { lat:          7, common_pid:       2303 } hitcount:         18
2651 { lat:          7, common_pid:       2305 } hitcount:        166
2652 { lat:          7, common_pid:       2306 } hitcount:          1
2653 { lat:          7, common_pid:       2301 } hitcount:         91
2654 { lat:          7, common_pid:       2300 } hitcount:         17
2655 { lat:          8, common_pid:       2303 } hitcount:       8296
2656 { lat:          8, common_pid:       2304 } hitcount:       6864
2657 { lat:          8, common_pid:       2305 } hitcount:       9464
2658 { lat:          8, common_pid:       2301 } hitcount:       9213
2659 { lat:          8, common_pid:       2306 } hitcount:       6246
2660 { lat:          8, common_pid:       2302 } hitcount:       8797
2661 { lat:          8, common_pid:       2299 } hitcount:       8771
2662 { lat:          8, common_pid:       2300 } hitcount:       8119
2663 { lat:          9, common_pid:       2305 } hitcount:       1519
2664 { lat:          9, common_pid:       2299 } hitcount:       2346
2665 { lat:          9, common_pid:       2303 } hitcount:       2841
2666 { lat:          9, common_pid:       2301 } hitcount:       1846
2667 { lat:          9, common_pid:       2304 } hitcount:       3861
2668 { lat:          9, common_pid:       2302 } hitcount:       1210
2669 { lat:          9, common_pid:       2300 } hitcount:       2762
2670 { lat:          9, common_pid:       2306 } hitcount:       4247
2671 { lat:         10, common_pid:       2299 } hitcount:         16
2672 { lat:         10, common_pid:       2306 } hitcount:        333
2673 { lat:         10, common_pid:       2303 } hitcount:         16
2674 { lat:         10, common_pid:       2304 } hitcount:        168
2675 { lat:         10, common_pid:       2302 } hitcount:        240
2676 { lat:         10, common_pid:       2301 } hitcount:         28
2677 { lat:         10, common_pid:       2300 } hitcount:         95
2678 { lat:         10, common_pid:       2305 } hitcount:         18
2679 { lat:         11, common_pid:       2303 } hitcount:          5
2680 { lat:         11, common_pid:       2305 } hitcount:          8
2681 { lat:         11, common_pid:       2306 } hitcount:        221
2682 { lat:         11, common_pid:       2302 } hitcount:         76
2683 { lat:         11, common_pid:       2304 } hitcount:         26
2684 { lat:         11, common_pid:       2300 } hitcount:        125
2685 { lat:         11, common_pid:       2299 } hitcount:          2
2686 { lat:         12, common_pid:       2305 } hitcount:          3
2687 { lat:         12, common_pid:       2300 } hitcount:          6
2688 { lat:         12, common_pid:       2306 } hitcount:         90
2689 { lat:         12, common_pid:       2302 } hitcount:          4
2690 { lat:         12, common_pid:       2303 } hitcount:          1
2691 { lat:         12, common_pid:       2304 } hitcount:        122
2692 { lat:         13, common_pid:       2300 } hitcount:         12
2693 { lat:         13, common_pid:       2301 } hitcount:          1
2694 { lat:         13, common_pid:       2306 } hitcount:         32
2695 { lat:         13, common_pid:       2302 } hitcount:          5
2696 { lat:         13, common_pid:       2305 } hitcount:          1
2697 { lat:         13, common_pid:       2303 } hitcount:          1
2698 { lat:         13, common_pid:       2304 } hitcount:         61
2699 { lat:         14, common_pid:       2303 } hitcount:          4
2700 { lat:         14, common_pid:       2306 } hitcount:          5
2701 { lat:         14, common_pid:       2305 } hitcount:          4
2702 { lat:         14, common_pid:       2304 } hitcount:         62
2703 { lat:         14, common_pid:       2302 } hitcount:         19
2704 { lat:         14, common_pid:       2300 } hitcount:         33
2705 { lat:         14, common_pid:       2299 } hitcount:          1
2706 { lat:         14, common_pid:       2301 } hitcount:          4
2707 { lat:         15, common_pid:       2305 } hitcount:          1
2708 { lat:         15, common_pid:       2302 } hitcount:         25
2709 { lat:         15, common_pid:       2300 } hitcount:         11
2710 { lat:         15, common_pid:       2299 } hitcount:          5
2711 { lat:         15, common_pid:       2301 } hitcount:          1
2712 { lat:         15, common_pid:       2304 } hitcount:          8
2713 { lat:         15, common_pid:       2303 } hitcount:          1
2714 { lat:         15, common_pid:       2306 } hitcount:          6
2715 { lat:         16, common_pid:       2302 } hitcount:         31
2716 { lat:         16, common_pid:       2306 } hitcount:          3
2717 { lat:         16, common_pid:       2300 } hitcount:          5
2718 { lat:         17, common_pid:       2302 } hitcount:          6
2719 { lat:         17, common_pid:       2303 } hitcount:          1
2720 { lat:         18, common_pid:       2304 } hitcount:          1
2721 { lat:         18, common_pid:       2302 } hitcount:          8
2722 { lat:         18, common_pid:       2299 } hitcount:          1
2723 { lat:         18, common_pid:       2301 } hitcount:          1
2724 { lat:         19, common_pid:       2303 } hitcount:          4
2725 { lat:         19, common_pid:       2304 } hitcount:          5
2726 { lat:         19, common_pid:       2302 } hitcount:          4
2727 { lat:         19, common_pid:       2299 } hitcount:          3
2728 { lat:         19, common_pid:       2306 } hitcount:          1
2729 { lat:         19, common_pid:       2300 } hitcount:          4
2730 { lat:         19, common_pid:       2305 } hitcount:          5
2731 { lat:         20, common_pid:       2299 } hitcount:          2
2732 { lat:         20, common_pid:       2302 } hitcount:          3
2733 { lat:         20, common_pid:       2305 } hitcount:          1
2734 { lat:         20, common_pid:       2300 } hitcount:          2
2735 { lat:         20, common_pid:       2301 } hitcount:          2
2736 { lat:         20, common_pid:       2303 } hitcount:          3
2737 { lat:         21, common_pid:       2305 } hitcount:          1
2738 { lat:         21, common_pid:       2299 } hitcount:          5
2739 { lat:         21, common_pid:       2303 } hitcount:          4
2740 { lat:         21, common_pid:       2302 } hitcount:          7
2741 { lat:         21, common_pid:       2300 } hitcount:          1
2742 { lat:         21, common_pid:       2301 } hitcount:          5
2743 { lat:         21, common_pid:       2304 } hitcount:          2
2744 { lat:         22, common_pid:       2302 } hitcount:          5
2745 { lat:         22, common_pid:       2303 } hitcount:          1
2746 { lat:         22, common_pid:       2306 } hitcount:          3
2747 { lat:         22, common_pid:       2301 } hitcount:          2
2748 { lat:         22, common_pid:       2300 } hitcount:          1
2749 { lat:         22, common_pid:       2299 } hitcount:          1
2750 { lat:         22, common_pid:       2305 } hitcount:          1
2751 { lat:         22, common_pid:       2304 } hitcount:          1
2752 { lat:         23, common_pid:       2299 } hitcount:          1
2753 { lat:         23, common_pid:       2306 } hitcount:          2
2754 { lat:         23, common_pid:       2302 } hitcount:          6
2755 { lat:         24, common_pid:       2302 } hitcount:          3
2756 { lat:         24, common_pid:       2300 } hitcount:          1
2757 { lat:         24, common_pid:       2306 } hitcount:          2
2758 { lat:         24, common_pid:       2305 } hitcount:          1
2759 { lat:         24, common_pid:       2299 } hitcount:          1
2760 { lat:         25, common_pid:       2300 } hitcount:          1
2761 { lat:         25, common_pid:       2302 } hitcount:          4
2762 { lat:         26, common_pid:       2302 } hitcount:          2
2763 { lat:         27, common_pid:       2305 } hitcount:          1
2764 { lat:         27, common_pid:       2300 } hitcount:          1
2765 { lat:         27, common_pid:       2302 } hitcount:          3
2766 { lat:         28, common_pid:       2306 } hitcount:          1
2767 { lat:         28, common_pid:       2302 } hitcount:          4
2768 { lat:         29, common_pid:       2302 } hitcount:          1
2769 { lat:         29, common_pid:       2300 } hitcount:          2
2770 { lat:         29, common_pid:       2306 } hitcount:          1
2771 { lat:         29, common_pid:       2304 } hitcount:          1
2772 { lat:         30, common_pid:       2302 } hitcount:          4
2773 { lat:         31, common_pid:       2302 } hitcount:          6
2774 { lat:         32, common_pid:       2302 } hitcount:          1
2775 { lat:         33, common_pid:       2299 } hitcount:          1
2776 { lat:         33, common_pid:       2302 } hitcount:          3
2777 { lat:         34, common_pid:       2302 } hitcount:          2
2778 { lat:         35, common_pid:       2302 } hitcount:          1
2779 { lat:         35, common_pid:       2304 } hitcount:          1
2780 { lat:         36, common_pid:       2302 } hitcount:          4
2781 { lat:         37, common_pid:       2302 } hitcount:          6
2782 { lat:         38, common_pid:       2302 } hitcount:          2
2783 { lat:         39, common_pid:       2302 } hitcount:          2
2784 { lat:         39, common_pid:       2304 } hitcount:          1
2785 { lat:         40, common_pid:       2304 } hitcount:          2
2786 { lat:         40, common_pid:       2302 } hitcount:          5
2787 { lat:         41, common_pid:       2304 } hitcount:          1
2788 { lat:         41, common_pid:       2302 } hitcount:          8
2789 { lat:         42, common_pid:       2302 } hitcount:          6
2790 { lat:         42, common_pid:       2304 } hitcount:          1
2791 { lat:         43, common_pid:       2302 } hitcount:          3
2792 { lat:         43, common_pid:       2304 } hitcount:          4
2793 { lat:         44, common_pid:       2302 } hitcount:          6
2794 { lat:         45, common_pid:       2302 } hitcount:          5
2795 { lat:         46, common_pid:       2302 } hitcount:          5
2796 { lat:         47, common_pid:       2302 } hitcount:          7
2797 { lat:         48, common_pid:       2301 } hitcount:          1
2798 { lat:         48, common_pid:       2302 } hitcount:          9
2799 { lat:         49, common_pid:       2302 } hitcount:          3
2800 { lat:         50, common_pid:       2302 } hitcount:          1
2801 { lat:         50, common_pid:       2301 } hitcount:          1
2802 { lat:         51, common_pid:       2302 } hitcount:          2
2803 { lat:         51, common_pid:       2301 } hitcount:          1
2804 { lat:         61, common_pid:       2302 } hitcount:          1
2805 { lat:        110, common_pid:       2302 } hitcount:          1
2806
2807 Totals:
2808     Hits: 89565
2809     Entries: 158
2810     Dropped: 0
2811
2812This doesn't tell us any information about how late cyclictest may have
2813woken up, but it does show us a nice histogram of how long it took from
2814the time that cyclictest was woken to the time it made it into user space.
2815