Commit 39a39d5b authored by Tzvetomir Stoyanov's avatar Tzvetomir Stoyanov Committed by Jonathan Corbet
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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: default avatarTzvetomir Stoyanov <tstoyanov@vmware.com>
Reviewed-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: default avatarTom Zanussi <tom.zanussi@linux.intel.com>
[jc: fixed trivial conflict with docs-next]
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent f15be33a

Documentation/trace/histogram.rst

+7 −4
Original line number Diff line number Diff line
@@ -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,\