linux-kernel - Re: find_new

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Date:   Wed, 9 Jun 2021 09:12:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julia Lawall <julia.lawall@...ia.fr>
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 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?