linux-kernel - Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers

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Date:   Mon, 25 Oct 2021 08:35:52 +0200
From:   Mike Galbraith <efault@....de>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers

On Fri, 2021-10-22 at 12:05 +0100, Mel Gorman wrote:
> On Fri, Oct 22, 2021 at 12:26:08PM +0200, Mike Galbraith wrote:
>
> >
> > Patchlet helped hackbench?  That's.. unexpected (at least by me).
> >
>
> I didn't analyse in depth and other machines do not show as dramatic
> a difference but it's likely due to timings of tasks getting wakeup
> preempted.

Wakeup tracing made those hackbench numbers less surprising. There's
tons of wake-many going on. At a glance, it appears to already be bi-
directional though, so patchlet helping seemingly means that there's
just not quite enough to tickle the heuristic without a little help.
Question is, is the potential reward of strengthening that heuristic
yet again, keeping in mind that "heuristic" tends to not play well with
"deterministic", worth the risk?

My desktop trace session said distribution improved a bit, but there
was no meaningful latency or throughput improvement, making for a
pretty clear "nope" to the above question.  It benefiting NUMA box
hackbench is a valid indicator, but one that is IMO too disconnected
from the real world to carry much weight.

	-Mike