linux-kernel - Re: [PATCH v1] sched/fair: update_pick_idlest() Select group with lowest group_util when idle

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Date:   Fri, 6 Nov 2020 17:06:59 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Phil Auld <pauld@...hat.com>, Peter Puhov <peter.puhov@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Robert Foley <robert.foley@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Jirka Hladky <jhladky@...hat.com>
Subject: Re: [PATCH v1] sched/fair: update_pick_idlest() Select group with
 lowest group_util when idle_cpus are equal

On Fri, 6 Nov 2020 at 17:00, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Fri, Nov 06, 2020 at 02:33:56PM +0100, Vincent Guittot wrote:
> > On Fri, 6 Nov 2020 at 13:03, Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > On Wed, Nov 04, 2020 at 09:42:05AM +0000, Mel Gorman wrote:
> > > > While it's possible that some other factor masked the impact of the patch,
> > > > the fact it's neutral for two workloads in 5.10-rc2 is suspicious as it
> > > > indicates that if the patch was implemented against 5.10-rc2, it would
> > > > likely not have been merged. I've queued the tests on the remaining
> > > > machines to see if something more conclusive falls out.
> > > >
> > >
> > > It's not as conclusive as I would like. fork_test generally benefits
> > > across the board but I do not put much weight in that.
> > >
> > > Otherwise, it's workload and machine-specific.
> > >
> > > schbench: (wakeup latency sensitive), all machines benefitted from the
> > >         revert at the low utilisation except one 2-socket haswell machine
> > >         which showed higher variability when the machine was fully
> > >         utilised.
> >
> > There is a pending patch to should improve this bench:
> > https://lore.kernel.org/patchwork/patch/1330614/
> >
>
> Ok, I've slotted this one in with a bunch of other stuff I wanted to run
> over the weekend. That particular patch was on my radar anyway. It just
> got bumped up the schedule a little bit.
>
> > > hackbench: Neutral except for the same 2-socket Haswell machine which
> > >         took an 8% performance penalty of 8% for smaller number of groups
> > >         and 4% for higher number of groups.
> > >
> > > pipetest: Mostly neutral except for the *same* machine showing an 18%
> > >         performance gain by reverting.
> > >
> > > kernbench: Shows small gains at low job counts across the board -- 0.84%
> > >         lowest gain up to 5.93% depending on the machine
> > >
> > > gitsource: low utilisation execution of the git test suite. This was
> > >         mostly a win for the revert. For the list of machines tested it was
> > >
> > >          14.48% gain (2 socket but SNC enabled to 4 NUMA nodes)
> > >         neutral      (2 socket broadwell)
> > >         36.37% gain  (1 socket skylake machine)
> > >          3.18% gain  (2 socket broadwell)
> > >          4.4%        (2 socket EPYC 2)
> > >          1.85% gain  (2 socket EPYC 1)
> > >
> > > While it was clear-cut for 5.9, it's less clear-cut for 5.10-rc2 although
> > > the gitsource shows some severe differences depending on the machine that
> > > is worth being extremely cautious about. I would still prefer a revert
> > > but I'm also extremely biased and I know there are other patches in the
> >
> > This one from Julia can also impact
> >
>
> Which one? I'm guessing "[PATCH v2] sched/fair: check for idle core"

Yes, Sorry I sent my answer before adding the link

>
> --
> Mel Gorman
> SUSE Labs