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

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Date:   Mon, 20 Jan 2020 22:57:06 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Phil Auld <pauld@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Hillf Danton <hdanton@...a.com>,
        Parth Shah <parth@...ux.ibm.com>,
        Rik van Riel <riel@...riel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small load imbalance between low
 utilisation SD_NUMA domains v4

> And this is why I'm curious as to why your workload is affected at all
> because it uses many tasks.  I stopped allowing an imbalance for higher
> task counts partially on the basis of your previous report.
> 

With this hunk on top of your patch and 5 runs of numa02, there were 0
traces.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ade7a8dca5e4..7506cf67bde8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8714,8 +8714,10 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 			 * the risk that lower domains have to be rebalanced.
 			 */
 			imbalance_min = 2;
-			if (busiest->sum_nr_running <= imbalance_min)
+			if (busiest->sum_nr_running <= imbalance_min) {
+				trace_printk("Reseting imbalance: busiest->sum_nr_running=%d, local->sum_nr_running=%d\n", busiest->sum_nr_irunning, local->sum_nr_running);
 				env->imbalance = 0;
+			}
 		}
 
 		return;


perf stat for the 5 iterations this time shows: 
77.817 +- 0.995 seconds time elapsed  ( +-  1.28% )
which I think is significantly less than last time around.

So I think it may be some other noise that could have contributed to the
jump last time. Also since the time consumption of numa02 is very small, a
small disturbance can show up as a big number from a percentage perspective.

-- 
Thanks and Regards
Srikar Dronamraju