linux-kernel - Re: [RFC] sched/eevdf: sched feature to dismiss lag on wakeup

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Date: Fri, 22 Mar 2024 18:02:05 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Tobias Huschle <huschle@...ux.ibm.com>
Cc: Luis Machado <luis.machado@....com>, linux-kernel@...r.kernel.org, mingo@...hat.com, 
	peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, 
	vschneid@...hat.com, sshegde@...ux.vnet.ibm.com, srikar@...ux.vnet.ibm.com, 
	linuxppc-dev@...ts.ozlabs.org, nd <nd@....com>
Subject: Re: [RFC] sched/eevdf: sched feature to dismiss lag on wakeup

On Thu, 21 Mar 2024 at 13:18, Tobias Huschle <huschle@...ux.ibm.com> wrote:
>
> On Wed, Mar 20, 2024 at 02:51:00PM +0100, Vincent Guittot wrote:
> > On Wed, 20 Mar 2024 at 08:04, Tobias Huschle <huschle@...ux.ibm.com> wrote:
> > > There was no guarantee of course. place_entity was reducing the vruntime of
> > > woken up tasks though, giving it a slight boost, right?. For the scenario
> >
> > It was rather the opposite, It was ensuring that long sleeping tasks
> > will not get too much bonus because of vruntime too far in the past.
> > This is similar although not exactly the same intent as the lag. The
> > bonus was up to 24ms previously whereas it's not more than a slice now
> >
>
> I might have gotten this quite wrong then. I was looking at place_entity
> and saw that non-initial placements get their vruntime reduced via
>
>     vruntime -= thresh;

and then
    se->vruntime = max_vruntime(se->vruntime, vruntime)

>
> which would mean that the placed task would have a vruntime smaller than
> cfs_rq->min_vruntime, based on pre-EEVDF behavior, last seen at:
>
>    af4cf40470c2 sched/fair: Add cfs_rq::avg_vruntime
>
> If there was no such benefit for woken up tasks. Then the scenario I observed
> is just conincidentally worse with EEVDF, which can happen when exchanging an
> algorithm I suppose. Or EEVDF just exposes a so far hidden problem in that
> scenario.