linux-kernel - Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

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Message-ID: <20231107095231.GS8262@noisy.programming.kicks-ass.net>
Date:   Tue, 7 Nov 2023 10:52:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Barry Song <21cnbao@...il.com>,
        Benjamin Segall <bsegall@...gle.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Mike Galbraith <efault@....de>,
        Qais Yousef <qyousef@...alina.io>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Yicong Yang <yangyicong@...wei.com>,
        Youssef Esmat <youssefesmat@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

On Tue, Nov 07, 2023 at 05:05:07PM +0800, Abel Wu wrote:
> vruntime of the (on_rq && !0-lag) entity needs to be adjusted when
> it gets re-weighted, and the calculations can be simplified based
> on the fact that re-weight won't change the w-average of all the
> entities. Please check the proofs in comments.
> 
> But adjusting vruntime can also cause position change in RB-tree
> hence require re-queue to fix up which might be costly. This might
> be avoided by deferring adjustment to the time the entity actually
> leaves tree (dequeue/pick), but that will negatively affect task
> selection and probably not good enough either.
> 
> Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>

Very good, thanks!

It's a bit sad we have to muck about with the tree now, but alas.

If only Google and FB could agree on what to do with this cgroup
nonsense, then maybe we could get rid of all this.