linux-kernel - Re: EEVDF and NUMA balancing

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Date:   Wed, 4 Oct 2023 18:24:39 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: EEVDF and NUMA balancing



On Wed, 4 Oct 2023, Peter Zijlstra wrote:

> On Wed, Oct 04, 2023 at 02:01:26PM +0200, Julia Lawall wrote:
> >
> >
> > On Tue, 3 Oct 2023, Peter Zijlstra wrote:
> >
> > > On Tue, Oct 03, 2023 at 10:25:08PM +0200, Julia Lawall wrote:
> > > > Is it expected that the commit e8f331bcc270 should have an impact on the
> > > > frequency of NUMA balancing?
> > >
> > > Definitely not expected. The only effect of that commit was supposed to
> > > be the runqueue order of tasks. I'll go stare at it in the morning --
> > > definitely too late for critical thinking atm.
> >
> > Maybe it's just randomly making a bad situation worse rather than directly
> > introduing a problem.  There is a high standard deviatind in the
> > performance.  Here are some results with hyperfine.  The general trends
> > are reproducible.
>
> OK,. I'm still busy trying to bring a 4 socket machine up-to-date...
> gawd I hate the boot times on those machines :/
>
> But yeah, I was thinking similar things, I really can't spot an obvious
> fail in that commit.
>
> I'll go have a poke once the darn machine is willing to submit :-)

I tried a two-socket machine, but in 50 runs the problem doesn't show up.

The commit e8f331bcc270 starts with

-       if (sched_feat(PLACE_LAG) && cfs_rq->nr_running > 1) {
+       if (sched_feat(PLACE_LAG) && cfs_rq->nr_running) {

This seemed like a big change - cfs_rq->nr_running > 1 should be rarely
true in ua, while cfs_rq->nr_running should always be true.  Adding back
the > 1 and simply replacing the test by 0 both had no effect, though.

julia