linux-kernel - Re: perf_counters issue with self-sampling threads

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Date:	Tue, 4 Aug 2009 18:09:33 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Robert Richter <robert.richter@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Mackerras <paulus@...ba.org>, Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>, eranian@...il.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: perf_counters issue with self-sampling threads

On Tue, Jul 28, 2009 at 7:13 AM, stephane eranian<eranian@...glemail.com> wrote:
> Andi,
>
> Looks like SIGPROF is calling _group_send_sig_info(), so I think it is
> subject to the same problem.
>
I did not look into SIGPROF a bit more.

First, SIGPROF is generated from ITIMER_PROF. My understanding is that this is
a global timer for the process. It may therefore fire in any thread.
Then SIGPROF
is pended to the shared signal queue as per the group_send_sig_info() code path.
That means the thread receiving the signal may not be the one in which the timer
expired. But typically things even out. But things change if the
monitored process
is using signal, and in particular signals pended to the private
signal queue which is
what happens with pthread_kill() I would think. Then, yes there may be
an imbalance.

But in the case of SIGPROF, it is not clear to me if you want to
change this behavior
as well. It all depends on the definition for ITIMER_PROF.
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