linux-kernel - Re: perf: prctl(PR_TASK_PERF_EVENTS

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Message-ID: <20120130101121.GB8924@elte.hu>
Date:	Mon, 30 Jan 2012 11:11:21 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Andrew Steets <asteets@...advisors.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: perf: prctl(PR_TASK_PERF_EVENTS_DISABLE) has no effect


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Sun, 2012-01-29 at 17:32 +0100, Ingo Molnar wrote:
> > * Andrew Steets <asteets@...advisors.com> wrote:
> > 
> > > On 1/28/12 6:01 AM, Ingo Molnar wrote:
> > > 
> > > >> prctl(PR_TASK_PERF_EVENTS_DISABLE) doesn't appear to 
> > > >> disable perf event counters.  Here is a demonstration 
> > > >> program:
> > > >
> > > > btw., what's your usecase?
> > > 
> > > I'm trying to profile a small section of a long-running 
> > > program.  I ran into trouble using call-graph recording 
> > > and I thought this might be an alternative way of getting 
> > > what I was after.
> > 
> > That usecase indeed makes sense. Peter, could we allow this 
> > for privileged tasks, depending on the perf_paranoia 
> > settings or such?
> 
> I really dislike it. The sane way around this would be to 
> allow easy self-profiling instead of doing things arse about 
> face like that.

So, what workflow are you suggesting to Andrew?

I guess we are also hurting from the lack of dwarf stack 
backtrace decoding - that would allow the filtering by parent 
function without modifying the code. I think Frederic had a 
prototype working for 32-bit - any update on that?

Andrew could work that problem around right now by adding:

   -fno-omit-frame-pointer

to the build of the utility - that should activate -g and 
perf-report's --parent filter should also work fine.

Thanks,

	Ingo
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