linux-kernel - Re: [PATCH v2 1/3] sched/fair: Clean up attach_entity_load

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Date:	Wed, 1 Jun 2016 02:37:49 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, bsegall@...gle.com,
	pjt@...gle.com, morten.rasmussen@....com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH v2 1/3] sched/fair: Clean up attach_entity_load_avg()

On Tue, May 31, 2016 at 10:34:02AM +0200, Peter Zijlstra wrote:
> On Tue, May 31, 2016 at 06:32:54AM +0800, Yuyang Du wrote:
> > +++ b/kernel/sched/fair.c
> > @@ -2961,24 +2961,6 @@ static inline void update_load_avg(struct sched_entity *se, int update_tg)
> >  
> >  static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  {
> > -	if (!sched_feat(ATTACH_AGE_LOAD))
> > -		goto skip_aging;
> > -
> > -	/*
> > -	 * If we got migrated (either between CPUs or between cgroups) we'll
> > -	 * have aged the average right before clearing @last_update_time.
> > -	 */
> > -	if (se->avg.last_update_time) {
> > -		__update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
> > -				  &se->avg, 0, 0, NULL);
> > -
> > -		/*
> > -		 * XXX: we could have just aged the entire load away if we've been
> > -		 * absent from the fair class for too long.
> > -		 */
> > -	}
> > -
> > -skip_aging:
> >  	se->avg.last_update_time = cfs_rq->avg.last_update_time;
> >  	cfs_rq->avg.load_avg += se->avg.load_avg;
> >  	cfs_rq->avg.load_sum += se->avg.load_sum;
> 
> So I'm not a big fan of this patch; the aging is a conceptual part of
> attaching the load, the fact that it only happens in one callsite is a
> mere 'accident'.

Yes, and we have to put the following to make it an "accident".

static void task_move_group_fair(struct task_struct *p)
{
	detach_task_cfs_rq(p);
	set_task_rq(p, task_cpu(p));

#ifdef CONFIG_SMP
	/* Tell se's cfs_rq has been changed -- migrated */
	p->se.avg.last_update_time = 0;
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#endif
	attach_task_cfs_rq(p);
}

So let me give it another try to keep this cleanup. Will send another version.