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

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Message-ID: <87zh37lldt.fsf@nanos.tec.linutronix.de>
Date:   Tue, 24 Nov 2020 00:10:54 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.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 Mon, Nov 23 2020 at 14:07, Andy Lutomirski wrote:
>> On Nov 23, 2020, at 1:25 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> On Mon, Nov 23 2020 at 22:15, Thomas Gleixner wrote:
>>>> On Sun, Nov 22 2020 at 15:16, Andy Lutomirski wrote:
>>>>
>>>> The common case of a CPU switching back and forth between a small
>>>> number of mms would have no significant overhead.
>>> 
>>> For CPUs which do not support PCID this sucks, which is everything pre
>>> Westmere and all of 32bit. Yes, 32bit. If we go there then 32bit has to
>>> bite the bullet and use the very same mechanism. Not that I care much
>>> TBH.
>> 
>> Bah, I completely forgot that AMD does not support PCID before Zen3
>> which is a major showstopper.
>
> Why?  Couldn’t we rig up the code so we still track all the ASIDs even
> if there is no CPU support?  We would take the TLB flush hit on every
> context switch, but we pay that cost anyway. We would avoid the extra
> copy in the same cases in which we would avoid it if we had PCID.

Did not think about that indeed. Yes, that should do the trick and
should not be worse than what we have now.

Sometimes one just can't see the forest for the trees :)

Thanks,

        tglx