linux-kernel - Re: [patch v8 4/9] sched: fix slept time double counting in enqueue entity

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Message-ID: <CALZhoSSweOzTgjykRMxv_kha9TH-8EAKvuhinf8ADFyFDzDTuQ@mail.gmail.com>
Date:	Thu, 20 Jun 2013 09:43:45 +0800
From:	Lei Wen <adrian.wenl@...il.com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Changlong Xie <changlongx.xie@...el.com>, sgruszka@...hat.com,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [patch v8 4/9] sched: fix slept time double counting in enqueue entity

Hi Alex,

On Mon, Jun 17, 2013 at 11:41 PM, Alex Shi <alex.shi@...el.com> wrote:
> On 06/17/2013 07:51 PM, Paul Turner wrote:
>> Can you add something like:
>>
>> +               /*
>> +                * Task re-woke on same cpu (or else
>> migrate_task_rq_fair()
>> +                * would have made count negative); we must be careful
>> to avoid
>> +                * double-accounting blocked time after synchronizing
>> decays.
>> +                */
>>
>>
>> Reviewed-by: Paul Turner <pjt@...gle.com>
>
> thanks for review!
> ---
>
> From 24d9b43e7a269e6ffee5b874d39812b83812a809 Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@...el.com>
> Date: Thu, 9 May 2013 10:54:13 +0800
> Subject: [PATCH] sched: fix slept time double counting in enqueue entity
>
> The wakeuped migrated task will __synchronize_entity_decay(se); in
> migrate_task_fair, then it needs to set

Should be migrate_task_rq_fair, right? :)

> `se->avg.last_runnable_update -= (-se->avg.decay_count) << 20'
> before update_entity_load_avg, in order to avoid slept time is updated
> twice for se.avg.load_avg_contrib in both __syncchronize and
> update_entity_load_avg.
>
> but if the slept task is waked up from self cpu, it miss the
> last_runnable_update before update_entity_load_avg(se, 0, 1), then the
> slept time was used twice in both functions.
> So we need to remove the double slept time counting.
>
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> Reviewed-by: Paul Turner <pjt@...gle.com>
> ---
>  kernel/sched/fair.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index df5b8a9..1e5a5e6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1571,7 +1571,13 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
>                 }
>                 wakeup = 0;
>         } else {
> -               __synchronize_entity_decay(se);
> +               /*
> +                * Task re-woke on same cpu (or else migrate_task_rq_fair()
> +                * would have made count negative); we must be careful to avoid
> +                * double-accounting blocked time after synchronizing decays.
> +                */
> +               se->avg.last_runnable_update += __synchronize_entity_decay(se)
> +                                                       << 20;

I'm kind of getting confused at here, since migrate_task_rq_fair use
below equation to
avoid sleep time be accounted for twice.
 `se->avg.last_runnable_update -= (-se->avg.decay_count) << 20'

Why here we need to use "+=", which the reversed version comparing
with previous?

Thanks,
Lei

>         }
>
>         /* migrated tasks did not contribute to our blocked load */
> --
> 1.7.5.4
>
>
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