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Message-ID: <4105fd08f84c60698b38efcb4d22e999de187d6e.camel@gmx.de>
Subject: Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers
From:   Mike Galbraith <efault@gmx.de>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>
Date:   Tue, 26 Oct 2021 12:15:31 +0200
In-Reply-To: <20211026081817.GM3959@techsingularity.net>
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         <37d8c167df66a1ead16b699115548ca376494c0c.camel@gmx.de>
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         <496d495b290ac69fed75d02ab5915a7871243321.camel@gmx.de>
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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 help=
s
> 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 worke=
r
> 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.
>