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

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Message-ID: <20120126022732.GH20878@somewhere>
Date:	Thu, 26 Jan 2012 03:27:35 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arun Sharma <asharma@...com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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 Wed, Jan 25, 2012 at 02:29:14PM -0800, Arun Sharma wrote:
> 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?

Well, a sched_stat_sleep event should match the sched_switch with
prev as the last targeted task.

Or am I missing something?

> 
> 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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