linux-kernel - Re: [PATCH 1/2] sched: reset blocked load decay

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Message-ID: <xm26d2l5q51d.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Mon, 09 Dec 2013 09:59:26 -0800
From:	bsegall@...gle.com
To:	Chris Redpath <chris.redpath@....com>
Cc:	pjt@...gle.com, mingo@...hat.com, peterz@...radead.org,
	alex.shi@...aro.org, morten.rasmussen@....com,
	dietmar.eggemann@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: reset blocked load decay_count during synchronization

Chris Redpath <chris.redpath@....com> writes:

> If an entity happens to sleep for less than one tick duration
> the tracked load associated with that entity can be decayed by an
> unexpectedly large amount if it is later migrated to a different
> CPU. This can interfere with correct scheduling when entity load
> is used for decision making.
>
> The reason for this is that when an entity is dequeued and enqueued
> quickly, such that se.avg.decay_count and cfs_rq.decay_counter
> do not differ when that entity is enqueued again,
> __synchronize_entity_decay skips the calculation step and also skips
> clearing the decay_count. At a later time that entity may be
> migrated and its load will be decayed incorrectly.
>
> All users of this function expect decay_count to be zero'ed after
> use.
>
> Signed-off-by: Chris Redpath <chris.redpath@....com>
Reviewed-by: Ben Segall <bsegall@...gle.com>

> ---
>  kernel/sched/fair.c |    8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e8b652e..b7e5945 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2142,12 +2142,10 @@ static inline u64 __synchronize_entity_decay(struct sched_entity *se)
>  	u64 decays = atomic64_read(&cfs_rq->decay_counter);
>  
>  	decays -= se->avg.decay_count;
> -	if (!decays)
> -		return 0;
> -
> -	se->avg.load_avg_contrib = decay_load(se->avg.load_avg_contrib, decays);
> +	if (decays)
> +		se->avg.load_avg_contrib =
> +			decay_load(se->avg.load_avg_contrib, decays);
>  	se->avg.decay_count = 0;
> -
>  	return decays;
>  }
--
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