linux-kernel - Re: [PATCH 2/2 v2] sched: use load

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Date:   Fri, 2 Dec 2016 19:31:04 -0800
From:   Brendan Gregg <brendan.d.gregg@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, matt@...eblueprint.co.uk,
        Morten.Rasmussen@....com, dietmar.eggemann@....com,
        kernellwp@...il.com, yuyang.du@...el.com, umgwanakikbuti@...il.com
Subject: Re: [PATCH 2/2 v2] sched: use load_avg for selecting idlest group

On Fri, Nov 25, 2016 at 7:34 AM, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> find_idlest_group() only compares the runnable_load_avg when looking for
> the least loaded group. But on fork intensive use case like hackbench
> where tasks blocked quickly after the fork, this can lead to selecting the
> same CPU instead of other CPUs, which have similar runnable load but a
> lower load_avg.
>
> When the runnable_load_avg of 2 CPUs are close, we now take into account
> the amount of blocked load as a 2nd selection factor. There is now 3 zones
> for the runnable_load of the rq:
> -[0 .. (runnable_load - imbalance)] : Select the new rq which has
> significantly less runnable_load
> -](runnable_load - imbalance) .. (runnable_load + imbalance)[ : The
> runnable load are close so we use load_avg to chose between the 2 rq
> -[(runnable_load + imbalance) .. ULONG_MAX] : Keep the current rq which
> has significantly less runnable_load
>

For background, is this from the "A decade of wasted cores" paper's
patches? What's the expected typical gain? Thanks,

Brendan