linux-kernel - Re: [PATCH v5 4/5] sched/fair: Skip detach sched avgs for new task when changing task groups

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Date:	Tue, 14 Jun 2016 16:45:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
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,
	matt@...eblueprint.co.uk
Subject: Re: [PATCH v5 4/5] sched/fair: Skip detach sched avgs for new task
 when changing task groups

On Tue, Jun 14, 2016 at 04:36:49PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 09, 2016 at 07:15:53AM +0800, Yuyang Du wrote:
> > Newly forked task has not been enqueued, so should not be removed from
> > cfs_rq in task_move_group_fair(). To do so, we identify newly forked
> > tasks by their sched_avg's last_update_time in detach_entity_load_avg().
> 
> >  static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  {
> > +	/* Newly forked tasks are not attached yet. */
> > +	if (!se->avg.last_update_time)
> > +		return;
> 
> Urgh, so this results in two different heuristics to detect 'new' tasks
> and gives two different meanings to !last_update_time.
> 
> How about you use the existing heuristic as per vruntime_normalized()
> and do:
> 
> 	if (!se->sum_exec_runtime)
> 		return;

Hurm,. I see we already have this confusion as per
remove_entity_load_avg(). Could we fix it there too?