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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Tue, 10 Oct 2023 13:55:00 -0400
Message-ID: <CAEXW_YSJTShQoOCdTOHamatV2VFFpK5=GZ40=he5Egz5TRBMkg@mail.gmail.com>
Subject: Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Vineeth Pillai <vineethrp@google.com>,
        Suleiman Souhlal <suleiman@google.com>,
        Hsin Yi <hsinyi@google.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>
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On Mon, Oct 9, 2023 at 7:25=E2=80=AFAM Ingo Molnar <mingo@kernel.org> wrote=
:
>
>
> * Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > > > Fixes: 7fd7a9e0caba ("sched/fair: Trigger nohz.next_balance updates=
 when a CPU goes NOHZ-idle")
> > >
> > > Hurm.. does this really warrant a Fixes tag? Afaict nothing is curren=
tly
> > > broken -- this is a pure optimization question, no?
> >
> > IMHO it is a breakage as it breaks NOHZ -- a lot of times the ILB kicks
> > back the CPU stopping the tick out of idle (effectively breaking NOHZ).
> > The large number of IPIs also wrecks power and it happens only on 6.1 a=
nd
> > after. Having the fixes tag means it will also goto all stable kernels =
>=3D
> > 6.1. Hope that sounds reasonable and thank you for taking a look!
>
> So it's basically a fix of a NOHZ performance regression, introduced by
> 7fd7a9e0caba or so, correct?

Yes.

> As long as the fixes have a good hope of being backported with a low amou=
nt
> of overhead, a Fixes: tag for a ~2 years old performance regression is
> unusual but not unprecedented.
>
> We just need to make sure we don't put too much of a burden on the
> shoulders of -stable maintainers ...

Sure, that sounds good. After we upstream, I am happy to assist in any
stable backporting work related to this as well. In any case, I have
to backport it to ChromeOS kernels which are based on stable. There's
only the 6.1 stable kernel at the moment that is affected.

thanks,

 - Joel