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

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Message-Id: <20191220130056.GA13192@linux.vnet.ibm.com>
Date:   Fri, 20 Dec 2019 18:30:56 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, pauld@...hat.com,
        valentin.schneider@....com, quentin.perret@....com,
        dietmar.eggemann@....com, Morten.Rasmussen@....com,
        hdanton@...a.com, parth@...ux.ibm.com, riel@...riel.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small degree of load imbalance
 between SD_NUMA domains

* Vincent Guittot <vincent.guittot@...aro.org> [2019-12-19 15:45:39]:

> Hi Mel,
> 
> Thanks for looking at this NUMA locality vs spreading tasks point.
> 
> 
> Shouldn't you consider the number of busiest->idle_cpus instead of the busiest->sum_nr_running ?
> and you could simplify by 
> 
> 
> 	if ((env->sd->flags & SD_NUMA) &&
> 		((100 * busiest->group_weight) <= (env->sd->imbalance_pct * (busiest->idle_cpus << 1)))) {
> 			env->imbalance = 0;
> 			return;
> 	}

Are idle_cpus and sum_nr_running good enough metrics to look at a NUMA
level? We could have asymmetric NUMA topology where one DIE/MC/groups may
have more cores than the other. In such a case looking at idle_cpus (or
sum_nr_running) of the group may not always lead us to the right load
balancing solution.


-- 
Thanks and Regards
Srikar Dronamraju