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Subject: Re: [PATCH] sched, fair: Allow a small degree of load imbalance
 between SD_NUMA domains
From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ingo Molnar <mingo@kernel.org>, pauld@redhat.com,
        srikar@linux.vnet.ibm.com, quentin.perret@arm.com,
        dietmar.eggemann@arm.com, Morten.Rasmussen@arm.com,
        hdanton@sina.com, parth@linux.ibm.com, riel@surriel.com,
        LKML <linux-kernel@vger.kernel.org>
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Date:   Thu, 19 Dec 2019 14:23:33 +0000
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On 19/12/2019 11:46, Valentin Schneider wrote:
> As for picking values, right now we have
> 
>   125 (default) / 117 (LLC domain) / 110 (SMT domain)
> 
> We could have
> 
>   >> 2 (25%), >> 3 (12.5%), >> 4 (6.25%).
> 


Hmph, I see that task_numa_migrate() starts with a slightly different value
(112), and does the same halving pattern as wake_affine_weight():

  x = 100 + (sd->imbalance_pct - 100) / 2;

The 112 could use >> 3 (12.5%); the halving is just an extra shift with the
suggested changes.

> It's not strictly equivalent but IMO the whole imbalance_pct thing isn't
> very precise anyway; just needs to be good enough on a sufficient number of
> topologies.
> 
> 
> 
>>> +				env->imbalance = 0;
>>> +
>>>  		return;
>>>  	}
>>>