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Documentation/trace: Add clarification how histogram onmatch works 

The current trace documentation, the section describing histogram's "onmatch"
is not straightforward enough about how this action is applied. It is not
clear what criteria are used to "match" both events. A short note is added,
describing what exactly is compared in order to match the events.

Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Tom Zanussi <tom.zanussi@linux.intel.com>
[jc: fixed trivial conflict with docs-next]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Tzvetomir Stoyanov 2019-05-03 17:35:37 +03:00 committed by Jonathan Corbet
parent f15be33aa3
commit 39a39d5b6b
1 changed files with 7 additions and 4 deletions
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11
Documentation/trace/histogram.rst
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@ -1915,7 +1915,10 @@ The following commonly-used handler.action pairs are available:
The 'matching.event' specification is simply the fully qualified
event name of the event that matches the target event for the
onmatch() functionality, in the form 'system.event_name'.
onmatch() functionality, in the form 'system.event_name'. Histogram
keys of both events are compared to find if events match. In case
multiple histogram keys are used, they all must match in the specified
order.
Finally, the number and type of variables/fields in the 'param
list' must match the number and types of the fields in the
@ -1978,9 +1981,9 @@ The following commonly-used handler.action pairs are available:
/sys/kernel/debug/tracing/events/sched/sched_waking/trigger
Then, when the corresponding thread is actually scheduled onto the
CPU by a sched_switch event, calculate the latency and use that
along with another variable and an event field to generate a
wakeup_latency synthetic event::
CPU by a sched_switch event (saved_pid matches next_pid), calculate
the latency and use that along with another variable and an event field
to generate a wakeup_latency synthetic event::
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\