linux-kernel - Re: [PATCH RFC] tracing: Call triggers only if event passes filter checks

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Date:   Sat, 16 Jun 2018 22:43:29 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>, tglx@...utronix.de,
        mhiramat@...nel.org, namhyung@...nel.org,
        mathieu.desnoyers@...icios.com, linux-rt-users@...r.kernel.org,
        Tom Zanussi <tom.zanussi@...ux.intel.com>, peterz@...radead.org
Subject: Re: [PATCH RFC] tracing: Call triggers only if event passes filter
 checks

On Thu, 14 Jun 2018 18:01:13 -0700
Joel Fernandes <joelaf@...gle.com> wrote:

> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> 
> Currently, trace event triggers are called regardless of if the event
> filter checks pass or fail. Thus if one were to enable event triggers
> and filters at the same time, then the triggers will always be called
> even if the filter checks didn't pass.
> 
> This is a problem for a usecase I was experimenting with: measuring the
> time preemption is disabled using synthetic events and dump the stack
> using the stacktrace trigger if the total preempt off time was greater
> than a threshold. Following are the commands for the same:
> 
> Create synthetic event:
> 
> echo 'preemptdisable u64 lat' >> \
> 	      /sys/kernel/debug/tracing/synthetic_events
> 
> echo 'hist:keys=cpu:ts0=common_timestamp.usecs:scpu=cpu' >> \
> 	      /sys/kernel/debug/tracing/events/preemptirq/preempt_disable/trigger
> 
> echo 'hist:keys=cpu:wakeup_lat=common_timestamp.usecs-$ts0:\
> onmatch(preemptirq.preempt_disable).preemptdisable($wakeup_lat)' >> \
> 	        /sys/kernel/debug/tracing/events/preemptirq/preempt_enable/trigger
> 
> Enable synthetic event:
> 
> echo stacktrace > /sys/kernel/debug/tracing/events/synthetic/preemptdisable/trigger
> echo 'lat > 400' > /sys/kernel/debug/tracing/events/synthetic/preemptdisable/filter

Have you tried if statement as below?

echo "stacktrace if lat > 400" > /sys/kernel/debug/tracing/events/synthetic/preemptdisable/trigger

As my understanding, filter is used for controlling "recording the event"
and trigger is out of its scope. Trigger itself has own filter as
"if statement", so you should use it.

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>