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

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Date:   Tue, 26 Oct 2021 12:15:31 +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 Tue, 2021-10-26 at 09:18 +0100, Mel Gorman wrote:
> On Mon, Oct 25, 2021 at 08:35:52AM +0200, Mike Galbraith wrote:
>
> >
> > 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.
>
> Another interpretation is that it's simply neutral and does no harm.

Yes, patchlet not completely countermanding PELT is a good sign. Had it
continuously fired and countermanded PELT entirely, that would mean
either that it was busted, or that the desktop load is so damn over-
threaded as to be continuously waking more damn threads than can
possibly have any benefit whatsoever.  Neither of those having happened
happening is a good thing.  While I really dislike PELT's evil side,
it's other face is rather attractive... rob

> > It benefiting NUMA box
> > hackbench is a valid indicator, but one that is IMO too disconnected
> > from the real world to carry much weight.
> >
>
> I think if it's not shown to be harmful to a realistic workload but helps
> an overloaded example then it should be ok. While excessive overload is
> rare in a realistic workload, it does happen. There are a few workloads
> I've seen bugs for that were triggered when an excessive number of worker
> threads get spawned and compete for CPU access which in turns leads more
> worker threads get spawned. There are application workarounds for this
> corner case but it still triggers bugs.
>