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Message-ID: <1342018508.3462.163.camel@twins>
Subject: Re: [PATCH] trace: add ability to set a target task for events (v2)
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Vagin <avagin@openvz.org>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Paul Mackerras <paulus@samba.org>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Arun Sharma <asharma@fb.com>
Date: Wed, 11 Jul 2012 16:55:08 +0200
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On Wed, 2012-07-11 at 16:48 +0200, Frederic Weisbecker wrote:
> On Wed, Jul 11, 2012 at 04:38:19PM +0200, Peter Zijlstra wrote:
> > On Wed, 2012-07-11 at 16:36 +0200, Frederic Weisbecker wrote:
> > > 
> > > In this case he can just record sched wakeup as well. With sched_switch
> > > + sched_wakeup, he'll unlikely lose events.
> > > 
> > > With sched_stat_sleep he will lose events, unless we fix this period
> > > demux thing. 
> > 
> > But without this patch, the sched_wakeup will belong to another task, so
> > if you trace task A, and B wakes you, you'll never see the wakeup.
> 
> Ah so the goal is to minimize the amount of events by only tracing task A?

Right, or just not having sufficient privs to trace the world. And a
wakeup of A is very much also part of A, not only the task doing the
wakeup.

Hence the proposed mechanism.

> Ok then. Still we need to fix these events that use __perf_count() because
> wide tracing of sched_switch/wake_up still generate less events than
> sched stat sleep.
> 
> I believe:
> 
> 	perf record -e sched:sched_stat_sleep sleep 1
> 
> produces 1 billion events because we sleep 1 billion nanosecs. Or
> something like that.

Right.. back when I did that the plan was to make PERF_SAMPLE_PERIOD fix
that, of course that never seemed to have happened.

With PERF_SAMPLE_PERIOD you can simply write the 1b into the period of 1
event and be done with it.