linux-kernel - Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se

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Message-ID: <20231011131454.GN14330@noisy.programming.kicks-ass.net>
Date:   Wed, 11 Oct 2023 15:14:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     Benjamin Segall <bsegall@...gle.com>, mingo@...nel.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bristot@...hat.com,
        corbet@....net, qyousef@...alina.io, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, pjt@...gle.com, pavel@....cz,
        qperret@...gle.com, tim.c.chen@...ux.intel.com, joshdon@...gle.com,
        timj@....org, kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org, joel@...lfernandes.org, efault@....de,
        tglx@...utronix.de
Subject: Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se

On Wed, Oct 11, 2023 at 08:12:09PM +0800, Abel Wu wrote:

> The implementation of __pick_eevdf() now is quite complicated which
> makes it really hard to maintain. I'm trying my best to refactor this
> part, hopefully can do some help. Below is only for illustration, I
> need to test more.

Well, the form it has now is somewhat close to the one in the paper. I
think Ben added one early break it doesn't have or something.

As the paper explains, you get two walks, one down the eligibility path,
and then one down the heap. I think the current code structure
represents that fairly well.

Very vague memories seem to suggest I thought to be clever some 10+
years ago when I wrote that other decent loop that Ben showed to be
broken (and woe on me for not verifying it when I resurrected the
patches, I rewrote pretty much everything else, except this lookup :/).