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

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Message-ID: <20200107091256.GE3466@techsingularity.net>
Date:   Tue, 7 Jan 2020 09:12:56 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Rik van Riel <riel@...riel.com>,
        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, 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 Tue, Jan 07, 2020 at 09:51:11AM +0800, Hillf Danton wrote:
> 
> Hi Folks
> 
> On Mon, 06 Jan 2020 11:44:57 -0500 Rik van Riel wrote:
> > 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).
> 
> 
> If three per cent can make any sense in helping determine utilisation
> low then the busy load has to meet
> 
> 	busiest->sum_nr_running < max(3, cpus in the node / 32);
> 

Why 3% and why would the low utilisation cut-off depend on the number of
CPUs in the node? That simply means that the cut-off scales to machine
size and does not take into account any consideration between local memory
latency and memory bandwidth.

> And we can't skip pulling tasks from a numa node without comparing it
> to the local load
> 
> 	local->sum_nr_running * env->sd->imbalance_pct <
> 	busiest->sum_nr_running * 100;
> 
> with imbalance_pct taken into account.
> 

Again, why? In this context, an imbalance has already been calculated
and whether based on running tasks or idle CPUs, it's not a negative
number. The imbalance_adj used as already accounted for imbalance_pct
albeit not as a ratio as it's normally used.

-- 
Mel Gorman
SUSE Labs