linux-kernel - Re: [PATCH] trace: reset sleep/block start time on task switch

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Message-ID: <4F20823A.6010808@fb.com>
Date:	Wed, 25 Jan 2012 14:29:14 -0800
From:	Arun Sharma <asharma@...com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	<linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andrew Vagin <avagin@...nvz.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] trace: reset sleep/block start time on task switch

On 1/25/12 12:15 PM, Steven Rostedt wrote:

> trace-cmd record -e sched_switch -f 'prev_state == 1 || prev_state == 2' -e sched_stat_sleep sleep 3
>
> I'm not sure the filter is even needed, and it should still keep up
> fine.

Better - I didn't see any overruns with trace-cmd.

Even if we resolve the sampling rate related problems, there is the 
issue of: can we trust that a sampled sched_switch event and a sampled 
sched_stat_sleep event actually match each other?

There is consensus here that touching an extra cacheline in the context 
switch path is a good trade-off given the usefulness of sleep profiling 
in troubleshooting latency problems.

Since it may not be a good trade-off for everyone, we just need to 
figure out where to store some per-task state. To recap two potential 
paths to investigate:

* store sleep_start someplace else that no one other than 
sched_stat_sleeptime() knows about.

* store state in task struct that remembers whether the last context 
switch was a preemption or not.

  -Arun
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