linux-kernel - Re: pref record question

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Message-Id: <1256030390.17774.35.camel@laptop>
Date:	Tue, 20 Oct 2009 11:19:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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.

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



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