linux-kernel - Re: [PATCH v2 6/8] sched/fair: use load instead of runnable load

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Message-ID: <CAKfTPtCVqGZUyu7Xj33jcypJfZbn-4aHuTyN3YNKDL+p-zhq6A@mail.gmail.com>
Date:   Mon, 26 Aug 2019 17:45:53 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 6/8] sched/fair: use load instead of runnable load

On Tue, 6 Aug 2019 at 18:07, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Aug 01, 2019 at 04:40:22PM +0200, Vincent Guittot wrote:
> > runnable load has been introduced to take into account the case
> > where blocked load biases the load balance decision which was selecting
> > underutilized group with huge blocked load whereas other groups were
> > overloaded.
> >
> > The load is now only used when groups are overloaded. In this case,
> > it's worth being conservative and taking into account the sleeping
> > tasks that might wakeup on the cpu.
>
> This one scares me a little. I have the feeling I'm missing/forgetting
> something.
>
> Also; while the regular load-balance (find-busiest) stuff is now all
> aware of idle, this change also impacts wake_affine and find_idlest, and
> they've not changed.

Yes. I thought about this a bit before applying this changes to all
cpu_runnable_load.
-For wake_affine, it starts by looking at an idle cpu with
wake_affine_idle which is similar in some way to the new load balance
approach even if it might need more changes to align more closely both
paths.
-For find_idlest, it looks at the group with most spare capacity in
priority and fall back to load. That being said I have overlooked that
 both load and runnable load are already used and cpu.load is saved
twice instead of one.

I can't remember detailed results but this patch is responsible of
part of perf improvements described in the cover letter

Also, I haven't touch the numa stats which still use runnable_load.
But this should be addressed too