linux-kernel - Re: [POC][RFC][PATCH v2] sched: Extended Scheduler Time Slice

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Message-ID: <CAHk-=whnyt2TccpDaWGTbDsVkKApL3c2FtDPMEwuTmeu_cEL8Q@mail.gmail.com>
Date:   Thu, 26 Oct 2023 08:50:32 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ankur Arora <ankur.a.arora@...cle.com>, linux-mm@...ck.org,
        x86@...nel.org, akpm@...ux-foundation.org, luto@...nel.org,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Youssef Esmat <youssefesmat@...omium.org>,
        Vineeth Pillai <vineethrp@...gle.com>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [POC][RFC][PATCH v2] sched: Extended Scheduler Time Slice

On Thu, 26 Oct 2023 at 08:36, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> >       asm volatile("xchg %b0,%1"
>
> which has an implicit lock prefix (xchg with a memory operand is a
> special-case):

Yeah, this is why we do "percpu_xchg()" - which does not want locked
semantics - as a "cmpxchg" loop.

Steven, check out

    arch/x86/include/asm/percpu.h

for a rough implementation of a 'xchg()' without SMP coherency, just
cpu-local one (ie atomic wrt being preempted by the kernel, but not
atomic wrt other CPU's accessing the same variable concurrently)

             Linus