linux-kernel - Re: [PATCH] trace: add ability to set a target task for events (v2)

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Message-ID: <20120711143121.GA17991@somewhere>
Date:	Wed, 11 Jul 2012 16:31:24 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andrew Vagin <avagin@...nvz.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Arun Sharma <asharma@...com>
Subject: Re: [PATCH] trace: add ability to set a target task for events (v2)

On Wed, Jul 11, 2012 at 06:14:58PM +0400, Andrew Vagin wrote:
> A few events are interesting not only for a current task.
> For example, sched_stat_* are interesting for a task, which
> wake up. For this reason, it will be good, if such events will
> be delivered to a target task too.
> 
> Now a target task can be set by using __perf_task().
> 
> The original idea and a draft patch belongs to Peter Zijlstra.
> 
> I need this events for profiling sleep times.  sched_switch is used for
> getting callchains and sched_stat_* is used for getting time periods.
> This events are combined in user space, then it can be analized by
> perf tools.

We've talked about that numerous times. But I still don't really
understand why you're not using sched switch events and compute
the difference between schedule in and schedule out.

I think you said that's because you got too much events with sched
switch. Are you loosing events? Otherwise I don't see why it's
a problem.

Also the sched_stat_sleep event produce an event which period equals the
time slept. Internally, perf split this into as many events as that period
because the requested period for trace events is 1 by default. We probably
should allow to send events with a higher number than the one requested. This
this produce sometimes a huge pile of events, and that even often result in
tons of lost events. We definetly need to fix that.

In the meantime you'll certainly get saner results by just recording
sched switch events.
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