linux-kernel - Re: pref record question

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Date:	Tue, 20 Oct 2009 14:46:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Tim Blechmann <tim@...ngt.org>, linux-kernel@...r.kernel.org
Subject: Re: pref record question


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2009-10-20 at 08:00 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Mon, 2009-10-19 at 19:04 +0200, Tim Blechmann wrote:
> > >
> > > > i am trying to do some profiling with perf (2.6.31.4). while 
> > > > thread-level profiling works fine, i haven't been able to record 
> > > > profiling data of a whole process (i.e. main thread + child 
> > > > threads). from my understanding, this should be enabled by running 
> > > > 'perf record -i' on the main thread. this does only collect the data 
> > > > from the main thread, though.
> > > > 
> > > > is this an issue with perf or with my understanding of perf?
> > > 
> > > -i will only inherit the counters on new fork()/clone() calls, so an 
> > > existing process/task tree will not automagically get the counters.
> > 
> > Looks like something very much worth fixing.
> 
> Well, maybe.
> 
> There's two ways to go about this, either iterate the tasks in 
> userspace and attach a counter to each one (repeat until there's no 
> new ones left).
> 
> Or add a new flag in perf_event_attr to iterate the tasks on attach in 
> an atomic manner.
> 
> We cannot simply extend the current attach behaviour as that would 
> make it impossible to attach to a single thread in a thread group.

Correct.

> Also, if you extend the interface, it would make sense to allow 
> automatically attaching to a process group, not only a thread group, 
> etc.

Indeed. This more automated approach sounds like the better solution to 
me.

	Ingo
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