linux-kernel - Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found

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Date:   Mon, 13 May 2019 13:35:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     songliubraving@...com, Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>, tkjos@...gle.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        quentin.perret@...aro.org, chris.redpath@....com,
        Dietmar.Eggemann@....com, linux-kernel@...r.kernel.org,
        steven.sistare@...cle.com
Subject: Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU
 isn't found

On Mon, May 13, 2019 at 03:04:18PM +0530, Viresh Kumar wrote:
> On 10-05-19, 09:21, Peter Zijlstra wrote:

> > I don't hate his per se; but the whole select_idle_sibling() thing is
> > something that needs looking at.
> > 
> > There was the task stealing thing from Steve that looked interesting and
> > that would render your apporach unfeasible.
> 
> I am surely missing something as I don't see how that patchset will
> make this patchset perform badly, than what it already does.

Nah; I just misremembered. I know Oracle has a patch set poking at
select_idle_siblings() _somewhere_ (as do I), and I just found the wrong
one.

Basically everybody is complaining select_idle_sibling() is too
expensive for checking the entire LLC domain, except for FB (and thus
likely some other workloads too) that depend on it to kill their tail
latency.

But I suppose we could still do this, even if we scan only a subset of
the LLC, just keep track of the last !idle CPU running only SCHED_IDLE
tasks and pick that if you do not (in your limited scan) find a better
candidate.