linux-kernel - Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock

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Date:	Mon, 15 Aug 2016 17:13:30 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Giovanni Gherdovich <ggherdovich@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <mgalbraith@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock_gettime()

2016-08-12 20:10 GMT+08:00 Stanislaw Gruszka <sgruszka@...hat.com>:
> Hi
>
> On Wed, Aug 10, 2016 at 01:26:41PM +0200, Ingo Molnar wrote:
>> Nice detective work! I'm wondering, where do we stand if compared with a
>> pre-6e998916dfe3 kernel?
>>
>> I admit this is a difficult question: 6e998916dfe3 does not revert cleanly and I
>> suspect v3.17 does not run easily on a recent distro. Could you attempt to revert
>> the bad effects of 6e998916dfe3 perhaps, just to get numbers - i.e. don't try to
>> make the result correct, just see what the performance gap is, roughly.
>>
>> If there's still a significant gap then it might make sense to optimize this some
>> more.
>
> I measured (partial) revert performance on 4.7 using mmtest instructions
> from Giovanni and also tested some other possible fix (draft version):
>
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 75f98c5..54fdf6d 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -294,6 +294,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>         unsigned int seq, nextseq;
>         unsigned long flags;
>
> +       (void) task_sched_runtime(tsk);
> +
>         rcu_read_lock();
>         /* Attempt a lockless read on the first round. */
>         nextseq = 0;
> @@ -308,7 +310,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>                         task_cputime(t, &utime, &stime);
>                         times->utime += utime;
>                         times->stime += stime;
> -                       times->sum_exec_runtime += task_sched_runtime(t);
> +                       times->sum_exec_runtime += t->se.sum_exec_runtime;

If this will not have updated stats for other threads?

Regards,
Wanpeng Li