linux-kernel - Re: [PATCH 1/1] sched/fair: improve yield

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Message-ID: <YQAKSO759lvZurgw@hirez.programming.kicks-ass.net>
Date:   Tue, 27 Jul 2021 15:29:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>, bristot@...hat.com,
        bsegall@...gle.com, dietmar.eggemann@....com, joshdon@...gle.com,
        juri.lelli@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        linux@...musvillemoes.dk, mgorman@...e.de, mingo@...nel.org,
        rostedt@...dmis.org, valentin.schneider@....com,
        vincent.guittot@...aro.org
Subject: Re: [PATCH 1/1] sched/fair: improve yield_to vs fairness

On Fri, Jul 23, 2021 at 05:21:37PM +0100, Mel Gorman wrote:

> I'm still not a fan because vruntime gets distorted. From the docs
> 
>    Small detail: on "ideal" hardware, at any time all tasks would have the same
>    p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
>    would ever get "out of balance" from the "ideal" share of CPU time
> 
> If yield_to impacts this "ideal share" then it could have other
> consequences.

Note that we already violate this ideal both subtly and explicitly.

For an example of the latter consider pretty much the entirety of
place_entity() with GENTLE_FAIR_SLEEPERS being the most egregious
example.

That said; adding to vruntime will only penalize the task itself, while
subtracting from vruntime will penalize everyone else. And in that sense
addition to vruntime is a safe option.

I've not fully considered the case at hand; just wanted to give some
context.