linux-kernel - Re: [patch V4 4/8] sched: Make migrate

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Message-ID: <CAHk-=wire3dzhHx=KiL_f5Rj0=1u9ustsa33QoR-F9-v-NU9Ng@mail.gmail.com>
Date:   Thu, 19 Nov 2020 09:23:47 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mel Gorman <mgorman@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [patch V4 4/8] sched: Make migrate_disable/enable() independent
 of RT

On Thu, Nov 19, 2020 at 3:14 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> I still hate all of this, and I really fear that with migrate_disable()
> available, people will be lazy and usage will increase :/
>
> Case at hand is this series, the only reason we need it here is because
> per-cpu page-tables are expensive...

No, I think you as a scheduler person just need to accept it.

Because this is certainly not the only time migration limiting has
come up, and no, it has absolutely nothing to do with per-cpu page
tables being completely unacceptable.

The scheduler people need to get used to this. Really. Because ASMP is
just going to be a fact.

There are few things more futile than railing against reality, Peter.

Honestly, the only argument I've ever heard against limiting migration
is the whole "our scheduling theory doesn't cover it".

So either throw the broken theory away, or live with it. Theory that
doesn't match reality isn't theory, it's religion.

          Linus