linux-kernel - Re: [PATCH] sched/eas: Don't update misfit status if the task is pinned

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Message-ID: <YAcO9HHHVBx7oBG/@google.com>
Date:   Tue, 19 Jan 2021 16:55:16 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] sched/eas: Don't update misfit status if the task is
 pinned

On Tuesday 19 Jan 2021 at 16:40:27 (+0000), Qais Yousef wrote:
> On 01/19/21 15:35, Quentin Perret wrote:
> > Do you mean failing the sched_setaffinity syscall if e.g. the task
> > has a min clamp that is higher than the capacity of the CPUs to which it
> > will be pinned? If so, I'm not sure if we really want that.
> 
> No. In Android for instance, I'm worried a background task affined to little
> cores that has a utilization > capacity_of(little) will trigger the same
> problem. It'll be affined to more than just 1 cpu, but none of the little cpus
> will actually fit.
> 
> Makes sense?

Now yes.

I agree this may be a real problem, but capacity_of() very much is a
per-CPU thing, because of RT pressure and such, and that is not a static
thing by any mean. So, even if the task doesn't fit on any CPU _now_ we
might still want to mark it misfit, just so it can be picked up by a
potential idle balance on another CPU later on. Maybe capacity_orig_of
would be preferable?

Thanks,
Quentin