linux-kernel - Re: [PATCH] sched, fair: Allow a small load imbalance between low utilisation SD

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Message-ID: <03ad3a0a1d8e84c12ad958e291040a32a49c9f0f.camel@surriel.com>
Date:   Mon, 06 Jan 2020 11:44:57 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, pauld@...hat.com,
        valentin.schneider@....com, srikar@...ux.vnet.ibm.com,
        quentin.perret@....com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com, hdanton@...a.com, parth@...ux.ibm.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small load imbalance between low
 utilisation SD_NUMA domains v3

On Mon, 2020-01-06 at 16:33 +0000, Mel Gorman wrote:
> On Mon, Jan 06, 2020 at 10:47:18AM -0500, Rik van Riel wrote:
> > > +			imbalance_adj = (100 / (env->sd->imbalance_pct
> > > - 100)) - 1;
> > > +
> > > +			/*
> > > +			 * Allow small imbalances when the busiest
> > > group has
> > > +			 * low utilisation.
> > > +			 */
> > > +			imbalance_max = imbalance_adj << 1;
> > > +			if (busiest->sum_nr_running < imbalance_max)
> > > +				env->imbalance -= min(env->imbalance,
> > > imbalance_adj);
> > > +		}
> > > +
> > 
> > Wait, so imbalance_max is a function only of
> > env->sd->imbalance_pct, and it gets compared
> > against busiest->sum_nr_running, which is related
> > to the number of CPUs in the node?
> > 
> 
> It's not directly related to the number of CPUs in the node. Are you
> thinking of busiest->group_weight?

I am, because as it is right now that if condition
looks like it might never be true for imbalance_pct 115.

Presumably you put that check there for a reason, and
would like it to trigger when the amount by which a node
is busy is less than 2 * (imbalance_pct - 100).

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