linux-kernel - Re: [PATCH v2 1/3] process cputimer is moving faster than its corresponding clock

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Date:	Mon, 29 Apr 2013 01:06:49 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] process cputimer is moving faster than its corresponding
 clock

(4/27/13 12:40 AM), 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().
> 
> Ideally, you should postpone the release_task() call after the
> final context switch to get all the stats added but this is
> more complex to achieve.
> 
> In other words, this is slowing down the cputimer so it keep the same
> pace than the process clock but in fact, what should be done is to
> speed up the process clock by adding the missing stats to it.
> 
> Signed-off-by: Olivier Langlois <olivier@...llion01.com>
> ---
>  kernel/sched/fair.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7a33e59..52d7b10 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -708,7 +708,15 @@ static void update_curr(struct cfs_rq *cfs_rq)
>  
>                 trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
>                 cpuacct_charge(curtask, delta_exec);
> -               account_group_exec_runtime(curtask, delta_exec);
> +               /*
> +                * Do not update the cputimer if the task is already released by
> +                * release_task().
> +                *
> +                * it would preferable to defer the autoreap release_task
> +                * after the last context switch but harder to do.
> +                */
> +               if (likely(curtask->sighand))
> +                       account_group_exec_runtime(curtask, delta_exec);
>         }

I'm confused. glibc's rt/tst-cputimer1 doesn't have thread exiting code. I have
no seen any issue in this accounting.



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