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

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Date:	Fri, 17 Jun 2016 18:02:39 +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 04:28:14PM +0200, Peter Zijlstra wrote:
> 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 ?

Right, an attach + detach, which basically ends up being:

> > We just want to update
> > last_update_time of RT task to something different from 0

this. That's the same as starting as fair and then doing
switched_from_fair().

So yes, ho-humm, how to go about doing that bestest. Lemme have a play.