linux-kernel - Re: find_new

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Date:   Wed, 9 Jun 2021 09:26:00 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: find_new_ilb



On Wed, 9 Jun 2021, Peter Zijlstra wrote:

> On Tue, Jun 08, 2021 at 09:51:30PM +0200, Julia Lawall wrote:
> > Starting from the following commit:
> >
> > commit 45da7a2b0af8fa29dff2e6ba8926322068350fce
> > Author: Peter Zijlstra <peterz@...radead.org>
> > Date:   Tue Aug 18 10:48:17 2020 +0200
> >
> >     sched/fair: Exclude the current CPU from find_new_ilb()
> >
> > up through Linux 5.12, I observed that often when most of the machine was
> > idle, there could be many (thousands) of sched_wake_idle_without_ipi
> > events, typically between cores 0 and 1.  I don't see this any more in
> > Linux v5.13-rc1.  I looked through the patches to fair.c and core.c
> > subsequent to v5.12, and I didn't see anything that explicitly addresses
> > this issue.  Before I plunge into another set of rounds of bisecting, I
> > wonder if anyone knows whether and how this problem was resolved?
>
> Hurmph.. that patch was preparation for a later change that never seems
> to have happened. If it is causing trouble for you, I think you can
> savely revert it.
>
> At the time I thought it was very strange that new_idle would select
> itself as idle-balancer, doubly so, because the only way to get there
> would be with NEED_RESCHED already set, so the IPI wouldn't in fact do
> anything.
>
> Looking again, the difference is ofcourse that previously we'd select
> self and NO-OP, but now we'll potentially select another CPU and
> actually do something.
>
> This is arguably an improvement, because we did want to do something.
>
>  I can't quite remember what would've change here since, Vincent, can
>  you remember?
>
> Anyway, is this actually causing you trouble, or are you just going on
> the increased number of events?

Thanks for the feedback.  The scenarios I saw wouldn't have caused trouble
for anyone, because the machine was highly idle (maybe a couple of cores
in use).  And the problem seems to be gone as well in v5.13.

julia