linux-kernel - Re: [RFC PATCH 0/4] Reduce worst-case scanning of runqueues in select_idle

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Date:   Mon, 7 Dec 2020 16:04:41 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 0/4] Reduce worst-case scanning of runqueues in select_idle_sibling

On Mon, 7 Dec 2020 at 10:15, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> This is a minimal series to reduce the amount of runqueue scanning in
> select_idle_sibling in the worst case.
>
> Patch 1 removes SIS_AVG_CPU because it's unused.
>
> Patch 2 improves the hit rate of p->recent_used_cpu to reduce the amount
>         of scanning. It should be relatively uncontroversial
>
> Patch 3-4 scans the runqueues in a single pass for select_idle_core()
>         and select_idle_cpu() so runqueues are not scanned twice. It's
>         a tradeoff because it benefits deep scans but introduces overhead
>         for shallow scans.
>
> Even if patch 3-4 is rejected to allow more time for Aubrey's idle cpu mask

patch 3 looks fine and doesn't collide with Aubrey's work. But I don't
like patch 4  which manipulates different cpumask including
load_balance_mask out of LB and I prefer to wait for v6 of Aubrey's
patchset which should fix the problem of possibly  scanning twice busy
cpus in select_idle_core and select_idle_cpu



> approach to stand on its own, patches 1-2 should be fine. The main decision
> with patch 4 is whether select_idle_core() should do a full scan when searching
> for an idle core, whether it should be throttled in some other fashion or
> whether it should be just left alone.
>
> --
> 2.26.2
>