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

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Date:   Tue, 26 Oct 2021 12:41:36 +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 12:15 +0200, Mike Galbraith wrote:
>
...

Well now, that interruption didn't go as planned. The briefer edit
would have been preferred, but you get the drift, so moving on...

> > > 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.
>

wake_wide()'s proper test environment is NUMA, not a desktop box, so
patchlet has yet to meet a real world load that qualifies as such.
That it could detect the test load doing nutty stuff like waking a
thread pool three times the size of the box is all well and good, but
not the point.

$.02 WRT poor abused hackbench: if it happens to benefit that's fine,
but I don't think it should ever be considered change validation.  It's
a useful tool, but the common massive overload use is just nuts IMO.

	-Mike