linux-kernel - Re: [RFCv4 PATCH 11/34] sched: Remove blocked load and utilization contributions of dying tasks

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Message-ID: <20150513134957.GB26396@e105550-lin.cambridge.arm.com>
Date:	Wed, 13 May 2015 14:49:57 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Sai Gurrappadi <sgurrappadi@...dia.com>
Cc:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Peter Boonstoppel <pboonstoppel@...dia.com>
Subject: Re: [RFCv4 PATCH 11/34] sched: Remove blocked load and utilization
 contributions of dying tasks

On Wed, May 13, 2015 at 01:33:22AM +0100, Sai Gurrappadi wrote:
> On 05/12/2015 12:38 PM, Morten Rasmussen wrote:
> > Task 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 e40cd88..d045404 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3202,6 +3202,8 @@ 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;
> >  	dequeue_entity_load_avg(cfs_rq, se, flags & DEQUEUE_SLEEP);
> >  
> >  	update_stats_dequeue(cfs_rq, se);
> > 
> 
> Maybe you could use the sched_class->task_dead() callback instead? I
> remember seeing a patch from Yuyang that did that.

Now that you mention it, I remember that thread I think. I will have a
look again and see if there are any good reasons not to use task_dead().
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