linux-kernel - Re: perf: prctl(PR_TASK_PERF_EVENTS

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Message-ID: <20120201190327.GI6731@somewhere.redhat.com>
Date:	Wed, 1 Feb 2012 20:03:29 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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

On Mon, Jan 30, 2012 at 11:11:21AM +0100, Ingo Molnar wrote:
> 
> * 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?

I haven't touched that for a while. I would be glad if
somebody else could relay me on this work.

I think Jiri Olsa has been working on something about Dwarf cfi
unwinding with a different approach. Mine was about dumping chunks
of stack and regs and do the unwinding on post processing. I think
Jiri is doing the unwinding from the event overflow fast path.

> 
> 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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