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

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Message-ID: <ZfwlnqGsICGc1fOr@DESKTOP-2CCOB1S.>
Date: Thu, 21 Mar 2024 13:18:38 +0100
From: Tobias Huschle <huschle@...ux.ibm.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
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 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;

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.