linux-kernel - Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB

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Message-ID: <ZSPjIwWxSdKAsKZD@gmail.com>
Date:   Mon, 9 Oct 2023 13:25:23 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        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>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vineeth Pillai <vineethrp@...gle.com>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Hsin Yi <hsinyi@...gle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB


* Joel Fernandes <joel@...lfernandes.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 currently
> > 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 and 
> after. Having the fixes tag means it will also goto all stable kernels >= 
> 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?

As long as the fixes have a good hope of being backported with a low amount 
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 ...

Thanks,

	Ingo