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

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Date:	Fri, 17 Jul 2015 15:19:49 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	bsegall@...gle.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

On Thu, Jul 16, 2015 at 10:00:00AM -0700, bsegall@...gle.com wrote:

hello,

> 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.

as you said, it should done here. :)

> update_entity_load_avg() accounts that (approximation of) time blocked

i mean the entity was already accounted the blocked time in
__synchronize_entity_decay().

> 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).

the thing to keep in mind is that, currently load tracking is done by 
per-entity. that is, the entity already has its own whole load_avg_contrib
with considering the entity's blocked time, after __synchronize_entity_decay().
and cfs_rq can account the se's load by adding se->avg.load_avg_contrib to 
cfs_rq->runnable_load_avg, like enqueue_entity_load_avg() code.

wrong?

thanks,
byungchul

> 
> >
> > --------------->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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