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

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Message-ID: <CAKfTPtBigdckTCpYyVD9R0U9Mm+ENX7BJ3gqrTkPW3m64iPCVA@mail.gmail.com>
Date:   Thu, 10 Dec 2020 10:38:37 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Ziljstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 0/4] Reduce scanning of runqueues in select_idle_sibling

On Thu, 10 Dec 2020 at 09:00, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Wed, 9 Dec 2020 at 15:37, Mel Gorman <mgorman@...hsingularity.net> wrote:
> >
> > On Tue, Dec 08, 2020 at 03:34:57PM +0000, Mel Gorman wrote:
> > > Changelog since v1
> > > o Drop single-pass patch                                              (vincent)
> > > o Scope variables used for SIS_AVG_CPU                                        (dietmar)
> > > o Remove redundant assignment                                         (dietmar
> > >
> > > This reduces 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 moves all SIS_PROP-related calculations under SIS_PROP
> > >
> > > Patch 3 improves the hit rate of p->recent_used_cpu to reduce the amount
> > >       of scanning. It should be relatively uncontroversial
> > >
> > > Patch 4 returns an idle candidate if one is found while scanning for a
> > >       free core.
> > >
> >
> > Any other objections to the series? Vincent marked 1, 3 and 4 as
> > reviewed. While patch 2 had some mild cosmetic concerns, I think the
> > version and how it treats SIS_PROP is fine as it is to keep it
> > functionally equivalent to !SIS_PROP and without adding too many
> > SIS_PROP checks.
>
> while testing your patchset and Aubrey one on top of tip, I'm facing
> some perf regression on my arm64 numa system on hackbench and reaim.
> The regression seems to comes from your patchset but i don't know
> which patch in particular yet
>
> hackbench -l 256000 -g 1
>
> v5.10-rc7 + tip/sched/core 13,255(+/- 3.22%)
> with your patchset         15.368(+/- 2.74)  -15.9%
>
> I'm also seeing perf regression on reaim but this one needs more
> investigation before confirming
>
> TBH, I was not expecting regressions. I'm running more test to find
> which patch is the culprit

The regression comes from patch 3: sched/fair: Do not replace
recent_used_cpu with the new target


>
>
> >
> > --
> > Mel Gorman
> > SUSE Labs