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Date:   Mon, 13 May 2019 13:35:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     songliubraving@fb.com, Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>, tkjos@google.com,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        quentin.perret@linaro.org, chris.redpath@arm.com,
        Dietmar.Eggemann@arm.com, linux-kernel@vger.kernel.org,
        steven.sistare@oracle.com
Subject: Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU
 isn't found
Message-ID: <20190513113518.GQ2623@hirez.programming.kicks-ass.net>
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 <59b37c56b8fcb834f7d3234e776eaeff74ad117f.1556182965.git.viresh.kumar@linaro.org>
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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.