linux-kernel - Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

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Date:	Fri, 17 Jun 2016 16:28:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Yuyang Du <yuyang.du@...el.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

On Fri, Jun 17, 2016 at 04:09:01PM +0200, Vincent Guittot wrote:
> >
> > Scenario 1: switch to fair class
> >
> >   p->sched_class = fair_class;
> >   if (queued)
> >     enqueue_task(p);
> >       ...
> >         enqueue_entity()
> >           enqueue_entity_load_avg()
> >             migrated = !sa->last_update_time (true)
> >             if (migrated)
> >               attach_entity_load_avg()
> >   check_class_changed()
> >     switched_from() (!fair)
> >     switched_to()   (fair)
> >       switched_to_fair()
> >         attach_entity_load_avg()

> > @@ -733,18 +737,21 @@ void post_init_entity_util_avg(struct sc
> >                 }
> >                 sa->util_sum = sa->util_avg * LOAD_AVG_MAX;
> >         }
> > +
> > +       update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, false);
> > +       attach_entity_load_avg(cfs_rq, se);
> 
> A new RT task will be attached and will contribute to the load until
> it decays to 0
> Should we detach it for non cfs task ? We just want to update
> last_update_time of RT task to something different from 0

Won't we open up that (scenario 1) hole again? Maybe we can initialize
!fair tasks to 0 load and have them ramp up once they get demoted back
to fair?

Blergh, I'll go ponder moar.