linux-kernel - Re: [patch v3.18+ regression fix] sched: Further improve spurious CPU

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Date:   Wed, 31 Aug 2016 12:01:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Galbraith <mgalbraith@...e.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [patch v3.18+ regression fix] sched: Further improve spurious
 CPU_IDLE active migrations

On Tue, Aug 30, 2016 at 07:42:55AM +0200, Mike Galbraith wrote:
> 
> 43f4d666 partially cured spurious migrations, but when there are
> completely idle groups on a lightly loaded processor, and there is
> a buddy pair occupying the busiest group, we will not attempt to
> migrate due to select_idle_sibling() buddy placement, leaving the
> busiest queue with one task.  We skip balancing, but increment
> nr_balance_failed until we kick active balancing, and bounce a
> buddy pair endlessly, demolishing throughput.

Have you ran this patch through other benchmarks? It looks like
something that might make something else go funny.

> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7249,11 +7249,12 @@ static struct sched_group *find_busiest_
>  		 * This cpu is idle. If the busiest group is not overloaded
>  		 * and there is no imbalance between this and busiest group
>  		 * wrt idle cpus, it is balanced. The imbalance becomes
> -		 * significant if the diff is greater than 1 otherwise we
> -		 * might end up to just move the imbalance on another group
> +		 * significant if the diff is greater than 2 otherwise we
> +		 * may end up merely moving the imbalance to another group,
> +		 * or bouncing a buddy pair needlessly.
>  		 */
>  		if ((busiest->group_type != group_overloaded) &&
> -				(local->idle_cpus <= (busiest->idle_cpus + 1)))
> +				(local->idle_cpus <= (busiest->idle_cpus + 2)))
>  			goto out_balanced;

So 43f4d66637bc ("sched: Improve sysbench performance by fixing spurious
active migration") 's +1 made sense in that its a tie breaker. If you
have 3 tasks on 2 groups, one group will have to have 2 tasks, and
bouncing the one task around just isn't going to help _anything_.

Incrementing that to +2 has the effect that if you have two tasks on two
groups, 0,2 is a valid distribution. Which I understand is exactly what
you want for this workload. But if the two tasks are unrelated, 1,1
really is a better spread.