linux-kernel - Re: [PATCH] sched: prevent sched entity from being decayed twice when both waking and migrating it

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Message-ID: <xm26r3o8jawf.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Thu, 16 Jul 2015 10:00:00 -0700
From:	bsegall@...gle.com
To:	byungchul.park@....com
Cc:	pjt@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: prevent sched entity from being decayed twice when both waking and migrating it

byungchul.park@....com writes:

> From: Byungchul Park <byungchul.park@....com>
>
> hello paul,
>
> can i ask you something?
>
> when a sched entity is both waken and migrated, it looks being decayed twice.
> did you do it on purpose?
> or am i missing something? :(
>
> thanks,
> byungchul

__synchronize_entity_decay() updates only se->avg.load_avg_contrib so
that removing from blocked_load is done correctly.
update_entity_load_avg() accounts that (approximation of) time blocked
against runnable_avg/running_avg (and then recomputes load_avg_contrib
to match while load_avg_contrib isn't part of any cfs_rq's sum).

>
> --------------->8---------------
> From 793c963d0b29977a0f6f9330291a9ea469cc54f0 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@....com>
> Date: Thu, 16 Jul 2015 16:49:48 +0900
> Subject: [PATCH] sched: prevent sched entity from being decayed twice when
>  both waking and migrating it
>
> current code is decaying load average variables with a sleep time twice,
> when both waking and migrating it. the first decaying happens in a call path
> "migrate_task_rq_fair() -> __synchronize_entity_decay()". the second
> decaying happens in a call path "enqueue_entity_load_avg() ->
> update_entity_load_avg()". so make it happen once.
>
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/sched/fair.c |   29 +++--------------------------
>  1 file changed, 3 insertions(+), 26 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 09456fc..c86cca0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2873,32 +2873,9 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
>  						  struct sched_entity *se,
>  						  int wakeup)
>  {
> -	/*
> -	 * We track migrations using entity decay_count <= 0, on a wake-up
> -	 * migration we use a negative decay count to track the remote decays
> -	 * accumulated while sleeping.
> -	 *
> -	 * Newly forked tasks are enqueued with se->avg.decay_count == 0, they
> -	 * are seen by enqueue_entity_load_avg() as a migration with an already
> -	 * constructed load_avg_contrib.
> -	 */
> -	if (unlikely(se->avg.decay_count <= 0)) {
> +	/* we track migrations using entity decay_count == 0 */
> +	if (unlikely(!se->avg.decay_count)) {
>  		se->avg.last_runnable_update = rq_clock_task(rq_of(cfs_rq));
> -		if (se->avg.decay_count) {
> -			/*
> -			 * In a wake-up migration we have to approximate the
> -			 * time sleeping.  This is because we can't synchronize
> -			 * clock_task between the two cpus, and it is not
> -			 * guaranteed to be read-safe.  Instead, we can
> -			 * approximate this using our carried decays, which are
> -			 * explicitly atomically readable.
> -			 */
> -			se->avg.last_runnable_update -= (-se->avg.decay_count)
> -							<< 20;
> -			update_entity_load_avg(se, 0);
> -			/* Indicate that we're now synchronized and on-rq */
> -			se->avg.decay_count = 0;
> -		}
>  		wakeup = 0;
>  	} else {
>  		__synchronize_entity_decay(se);
> @@ -5114,7 +5091,7 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
>  	 * be negative here since on-rq tasks have decay-count == 0.
>  	 */
>  	if (se->avg.decay_count) {
> -		se->avg.decay_count = -__synchronize_entity_decay(se);
> +		__synchronize_entity_decay(se);
>  		atomic_long_add(se->avg.load_avg_contrib,
>  						&cfs_rq->removed_load);
>  	}
--
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