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SubjectRe: [RFC][PATCH 00/21] tracing: Inter-event (e.g. latency) support
On Fri, 10 Feb 2017 13:16:17 +0900
Namhyung Kim <namhyung@kernel.org> wrote:

> > ====
> > Example - wakeup latency
> > ====
> >
> > This basically implements the -RT latency_hist 'wakeup_latency'
> > histogram using the synthetic events, variables, and actions
> > described. The output below is from a run of cyclictest using the
> > following command:
> >
> > # rt-tests/cyclictest -p 80 -n -s -t 2
> >
> > What we're measuring the latency of is the time between when a
> > thread (of cyclictest) is awakened and when it's scheduled in. To
> > do that we add triggers to sched_wakeup and sched_switch with the
> > appropriate variables, and on a matching sched_switch event,
> > generate a synthetic 'wakeup_latency' event. Since it's just
> > another trace event like any other, we can also define a histogram
> > on that event, the output of which is what we see displayed when
> > reading the wakeup_latency 'hist' file.
> >
> > First, we create a synthetic event called wakeup_latency, that
> > references 3 variables from other events:
> >
> > # echo 'wakeup_latency lat=sched_switch:wakeup_lat \
> > pid=sched_switch:woken_pid \
> > prio=sched_switch:woken_prio' >> \
> > /sys/kernel/debug/tracing/synthetic_events
> >
> > Next we add a trigger to sched_wakeup, which saves the value of the
> > 'common_timestamp' when that event is hit in a variable, ts0. Note
> > that this happens only when 'comm==cyclictest'.
> >
> > Also, 'common_timestamp' is a new field defined on every event (if
> > needed - if there are no users of timestamps in a trace, timestamps
> > won't be saved and there's no additional overhead from that).
> >
> > # echo 'hist:keys=pid:ts0=common_timestamp.usecs if \
> > comm=="cyclictest"' >> \
> > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
> >
> > Next, we add a trigger to sched_switch. When the pid being switched
> > to matches the pid woken up by a previous sched_wakeup event, this
> > event grabs the ts0 saved on that event, takes the difference
> > between it and the current sched_switch's common_timestamp, and
> > assigns it to a new 'wakeup_lat' variable. It also saves a couple
> > other variables and then invokes the onmatch().trace() action which
> > generates a new wakeup_latency event using those variables.
> >
> > # echo 'hist:keys=woken_pid=next_pid:woken_prio=next_prio:\
> > wakeup_lat=common_timestamp.usecs-ts0:onmatch().trace(wakeup_latency) \
> > if next_comm=="cyclictest"' >> \
> > /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
>
> As Masami said, I think the syntax is a bit hard to understand. Also
> it'd be nice to access an event field directly (i.e. not by adding a
> field in a hist). Maybe we can use a prefix like '$' to identify hist
> fields..

Ah that's a nice idea!

>
> How about below?
>
> # echo 'wakeup_latency \
> lat=sched_switch.$wakeup_lat \
> pid=sched_switch.next_pid \
> prio=sched_switch.next_prio' >> \
> /sys/kernel/debug/tracing/synthetic_events

Should we define these parameter assignment at this.point?

I think this syntax binds wakeup_latency event to sched_switch too tight. I
mean, if someone kicks this event from some other event, it may easily lose
values.
So, at this point, we will define event name and what parameters it has,
until binding this event to onmatch().

> # echo 'hist: \
> keys=pid: \
> ts0=common_timestamp.usec \
> if comm=="cyclictest"' >> \
> /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
>
> # echo 'hist: \
> keys=next_pid: \
> wakeup_lat=common_timestamp.usec-$ts0: \
> onmatch(sched_wakeup).trace(wakeup_latency) \

This one seems much better for me, but I would like to ask you call event
directly from onmatch, like as

"onmatch(sched_wakeup).wakeup_latency(wakeup_lat,next_pid,next_prio)"

At this point, kernel will finalize the wakeup_latency event with wakeup_lat,
next_pid and next_prio.

> if next_comm=="cyclictest"' >> \
> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
>
> By passing an event name to 'onmatch', we can know where to find $ts0
> easily IMHO.

Agree. That's easiler to understand :)

Thank you,

--
Masami Hiramatsu <mhiramat@kernel.org>

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