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From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
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>
Subject: Re: [PATCH] trace: add ability to set a target task for events (v2)
Date: Wed, 11 Jul 2012 17:07:36 +0200	[thread overview]
Message-ID: <20120711150733.GD17991@somewhere> (raw)
In-Reply-To: <1342018508.3462.163.camel@twins>

On Wed, Jul 11, 2012 at 04:55:08PM +0200, Peter Zijlstra wrote:
> 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.

Yeah that's fair.

> 
> > 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.

I believe the perf tools handle pretty well variable periods of an event
on top of PERF_SAMPLE_PERIOD. We just need to tweak the maths in
perf_swevent_overflow() I think...

  reply	other threads:[~2012-07-11 15:07 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-07-11 14:14 Andrew Vagin
2012-07-11 14:31 ` Frederic Weisbecker
2012-07-11 14:33   ` Peter Zijlstra
2012-07-11 14:36     ` Frederic Weisbecker
2012-07-11 14:38       ` Peter Zijlstra
2012-07-11 14:48         ` Frederic Weisbecker
2012-07-11 14:55           ` Peter Zijlstra
2012-07-11 15:07             ` Frederic Weisbecker [this message]
2012-07-11 15:12             ` Peter Zijlstra
2012-07-11 15:17               ` Frederic Weisbecker
2012-07-11 15:09 ` Peter Zijlstra
2012-07-31 17:58 ` [tip:perf/urgent] perf/trace: Add ability to set a target task for events tip-bot for Andrew Vagin

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