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

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Date:	Thu, 30 Jul 2009 00:17:03 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	eranian@...il.com, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>,
	Michael Kerrisk <mtk.manpages@...glemail.com>
Subject: Re: perf_counters issue with self-sampling threads

(add Roland)

On 07/29, Peter Zijlstra wrote:
>
> On Mon, 2009-07-27 at 18:51 +0200, stephane eranian wrote:
> >
> > POSIX does not mandate that asynchronous signals be delivered
> > to the thread in which they originated. Any thread in the process
> > may process the signal, assuming it does not have the signal
> > blocked.

Yes. I now nothing about POSIX, but this is what Linux does at least.
I don't think we can/should change this behaviour.

> fcntl(2) for F_SETOWN says:
>
> If a non-zero value is given to F_SETSIG  in  a  multi‐ threaded
> process running with a threading library that supports thread groups
> (e.g., NPTL),  then  a  positive value  given  to  F_SETOWN  has  a
> different  meaning: instead of being a process ID identifying a whole
> pro‐ cess,  it  is a thread ID identifying a specific thread within a
> process.

Heh. Definitely this is not what Linux does ;)

> Which seems to imply that when we feed fcntl(F_SETOWN) a TID instead of
> a PID it should deliver SIGIO to the thread instead of the whole process
> -- which, to me, seems a sane semantic.

I am not sure I understand the man above... But to me it looks like we
should always send a private signal when fown->signum != 0 ?

The change should be simple, but as you pointed out we can break things.

Oleg.

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