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Subject: Re: [PATCH] sched, fair: Allow a small load imbalance between low
 utilisation SD_NUMA domains v3
To:     Mel Gorman <mgorman@techsingularity.net>,
        Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Hillf Danton <hdanton@sina.com>, Rik van Riel <riel@surriel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Phil Auld <pauld@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        Parth Shah <parth@linux.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Valentin Schneider <valentin.schneider@arm.com>
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Date:   Wed, 8 Jan 2020 15:49:55 +0000
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On 07/01/2020 10:16, Mel Gorman wrote:
> I think running tasks at least is the least bad metric. idle CPUs gets
> caught up in corner cases with bindings and util_avg can be skewed by
> outliers. Running tasks is a sensible starting point until there is a
> concrete use case that shows it is unworkable.

I'd tend to agree with you here. Also; this being in the group_has_spare
imbalance type we have some guarantees that the group is not overutilized.

If we keep some threshold of 'sum_nr_running < group_weight / 2', then this
"naturally" puts a hard limit of 50% total group utilization (corner case
where all tasks are 100% util).

> Lets see what you think of
> the other untested patch I posted that takes the group weight and child
> domain weight into account.
>