linux-kernel - Re: [PATCH v3 1/3] posix_timers: do not account group_exec

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Date:	Sat, 11 May 2013 02:33:18 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Olivier Langlois <olivier@...llion01.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, schwidefsky@...ibm.com,
	Steven Rostedt <rostedt@...dmis.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] posix_timers: do not account group_exec_runtime
 for dying autoreaped tasks

On Mon, May 06, 2013 at 11:35:50PM -0400, Olivier Langlois wrote:
> Frederic,
> 
> On Tue, 2013-05-07 at 01:18 +0200, Frederic Weisbecker wrote:
> > On Mon, Apr 29, 2013 at 02:04:09PM -0400, Olivier Langlois wrote:
> > > 
> > > 
> > > Forbids the cputimer to drift ahead of its process clock by
> > > blocking its update when a tick occurs while a autoreaping task
> > > is currently in do_exit() between the call to release_task() and
> > > its final call to schedule().
> > > 
> > > Any task stats update after having called release_task() will
> > > be lost because they are added to the global process stats located
> > > in the signal struct from release_task().
> > 
> > I wonder if this is real problem that the clock is ahead of the timer.
> > Have you seen any issue in practice with this?
> 
> note that it is the timer that will be ahead because the process clock
> is the sum of past group tasks exec time plus current group tasks exec
> time. Few updates are missing in the past group tasks exec time counter
> in struct signal.
> 
> The effect of this is failure of the unittest testing POSIX compliance
> in glibc rt/tst-cputimer1.c
> 
> pseudo code of the test is:
> 
> 1. call clock_gettime() to get now
> 2. Compute timer expiring time now+timer interval
> 3. call clock_gettime() when timer cb is called
> 4. Compare that clock_gettime() result is >= than computed expiring time
> 
> The error is adding up as more thread exits and usually the test fails
> when testing periodic timers where threads are created to handle the
> timer timeouts.

I see. Ok if it breaks a glibc test, it seems like a good reason to fix it :)

Thanks!
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