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

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Date:   Thu, 19 Nov 2020 19:28:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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 09:23:47AM -0800, Linus Torvalds wrote:
> 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.

Well, I did do write the patches.

> 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.

It is for this instance; but sure, it's come up before in other
contexts.

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

ASMP is different in that it is a hardware constraint, you're just not
going to be able to run more of X than there's X capable hardware units
on (be if FPUs, Vector units, 32bit or whatever)

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

But, but, my windmills! :-)

> 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.

The first stage of throwing it out is understanding the problem, which
is just about where we're at. Next is creating a new formalism (if
possible) that covers this new issue. That might take a while.

Thing is though; without a formalism to reason about timeliness
guarantees, there is no Real-Time.

So sure, I've written the patches, doesn't mean I have to like the place
we're in due to it.