linux-kernel - Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr

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Date:   Thu, 23 Sep 2021 15:14:12 +0200
From:   Mike Galbraith <efault@....de>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.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 2/2] sched/fair: Scale wakeup granularity relative to
 nr_running

On Thu, 2021-09-23 at 14:41 +0200, Vincent Guittot wrote:
> On Thu, 23 Sept 2021 at 11:22, Mike Galbraith <efault@....de> wrote:
> >
> > On Thu, 2021-09-23 at 10:40 +0200, Vincent Guittot wrote:
> > >
> > > a 100us value should even be enough to fix Mel's problem without
> > > impacting common wakeup preemption cases.
> >
> > It'd be nice if it turn out to be something that simple, but color me
> > skeptical.  I've tried various preemption throttling schemes, and while
>
> Let's see what the results will show. I tend to agree that this will
> not be enough to cover all use cases and I don't see any other way to
> cover all cases than getting some inputs from the threads about their
> latency fairness which bring us back to some kind of latency niceness
> value

What dooms the dirt simple solution is: tasks that dip lightly but
frequently are a thing ;-) Take nfs threads, tell 'em frequent
preemption ain't cool, and no matter how you diplomatically you say it,
they react in the only way they can, by sucking at their job.

	-Mike