Re: [PATCH v2] Documentation/trace: Add clarification how histogram onmatch works

[Tom Zanussi]

Hi Steve,

On Tue, 2019-05-07 at 20:11 -0400, Steven Rostedt wrote:
> Tom,
> 
> Can you review this patch.
> 

Sure.

> Jon,
> 
> After Tom gives his review, can you take this in your tree?
> 
> Thanks!
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> 

Reviewed-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>

Thanks,

Tom

> -- Steve
> 
> 
> On Tue,  7 May 2019 17:49:46 +0300
> Tzvetomir Stoyanov <tstoyanov@xxxxxxxxxx> wrote:
> 
> > 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@xxxxxxxxxx>
> > ---
> >  Documentation/trace/histogram.txt | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/trace/histogram.txt
> > b/Documentation/trace/histogram.txt
> > index 7ffea6aa22e3..d97f0530a731 100644
> > --- a/Documentation/trace/histogram.txt
> > +++ b/Documentation/trace/histogram.txt
> > @@ -1863,7 +1863,10 @@ hist trigger specification.
> >  
> >      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
> > the case
> > +    multiple histogram keys are used, both events must have the
> > same
> > +    number of keys, and the keys must match in the same order.
> >  
> >      Finally, the number and type of variables/fields in the 'param
> >      list' must match the number and types of the fields in the
> > @@ -1920,9 +1923,10 @@ hist trigger specification.
> >  	    /sys/kernel/debug/tracing/events/sched/sched_waking/tr
> > igger
> >  
> >      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 (where the sched_waking key	
> > "saved_pid"
> > +    matches the sched_switch key "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
> > ,\
> 
>