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

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Message-ID: <20191219100232.GY2844@hirez.programming.kicks-ass.net>
Date:   Thu, 19 Dec 2019 11:02:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>, pauld@...hat.com,
        srikar@...ux.vnet.ibm.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

On Wed, Dec 18, 2019 at 06:50:52PM +0000, Valentin Schneider wrote:
> I'm quite sure you have reasons to have written it that way, but I was
> hoping we could squash it down to something like:
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..f05d09a8452e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8680,16 +8680,27 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  			env->migration_type = migrate_task;
>  			lsub_positive(&nr_diff, local->sum_nr_running);
>  			env->imbalance = nr_diff >> 1;
> -			return;
> +		} else {
> +
> +			/*
> +			 * If there is no overload, we just want to even the number of
> +			 * idle cpus.
> +			 */
> +			env->migration_type = migrate_task;
> +			env->imbalance = max_t(long, 0, (local->idle_cpus -
> +							 busiest->idle_cpus) >> 1);
>  		}
>  
>  		/*
> -		 * If there is no overload, we just want to even the number of
> -		 * idle cpus.
> +		 * Allow for a small imbalance between NUMA groups; don't do any
> +		 * of it if there is at least half as many tasks / busy CPUs as
> +		 * there are available CPUs in the busiest group
>  		 */
> -		env->migration_type = migrate_task;
> -		env->imbalance = max_t(long, 0, (local->idle_cpus -
> -						 busiest->idle_cpus) >> 1);
> +		if (env->sd->flags & SD_NUMA &&
> +		    (busiest->sum_nr_running < busiest->group_weight >> 1) &&
> +		    (env->imbalance < busiest->group_weight * (env->sd->imbalance_pct - 100) / 100))

Note that this form allows avoiding the division. Every time I see that
/100 I'm thinking we should rename and make imbalance_pct a base-2
thing.

> +				env->imbalance = 0;
> +
>  		return;
>  	}
>