linux-kernel - Re: [RFCv5 PATCH 11/46] sched: Remove blocked load and utilization contributions of dying tasks

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Message-ID: <20150722134523.GB21785@e105550-lin.cambridge.arm.com>
Date:	Wed, 22 Jul 2015 14:45:23 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Leo Yan <leo.yan@...aro.org>
Cc:	peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
	daniel.lezcano@...aro.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	yuyang.du@...el.com, mturquette@...libre.com, rjw@...ysocki.net,
	Juri Lelli <Juri.Lelli@....com>, sgurrappadi@...dia.com,
	pang.xunlei@....com.cn, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [RFCv5 PATCH 11/46] sched: Remove blocked load and utilization
 contributions of dying tasks

On Wed, Jul 22, 2015 at 02:51:01PM +0800, Leo Yan wrote:
> On Tue, Jul 07, 2015 at 07:23:54PM +0100, Morten Rasmussen wrote:
> > Tasks being dequeued for the last time (state == TASK_DEAD) are dequeued
> > with the DEQUEUE_SLEEP flag which causes their load and utilization
> > contributions to be added to the runqueue blocked load and utilization.
> > Hence they will contain load or utilization that is gone away. The issue
> > only exists for the root cfs_rq as cgroup_exit() doesn't set
> > DEQUEUE_SLEEP for task group exits.
> > 
> > If runnable+blocked load is to be used as a better estimate for cpu
> > load the dead task contributions need to be removed to prevent
> > load_balance() (idle_balance() in particular) from over-estimating the
> > cpu load.
> > 
> > cc: Ingo Molnar <mingo@...hat.com>
> > cc: Peter Zijlstra <peterz@...radead.org>
> > 
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> > ---
> >  kernel/sched/fair.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 775b0c7..fa12ce5 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3217,6 +3217,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >  	 * Update run-time statistics of the 'current'.
> >  	 */
> >  	update_curr(cfs_rq);
> > +	if (entity_is_task(se) && task_of(se)->state == TASK_DEAD)
> > +		flags &= !DEQUEUE_SLEEP;
> 
> So flags will be set to zero? Could be replaced by "flags &= ~DEQUEUE_SLEEP"?

Not could, should :)

I meant to clear the flag, but used the wrong operator. We only have
DEQUEUE_SLEEP and 0 at the moment so it doesn't matter, but it might
later.

Thanks,
Morten
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