linux-kernel - Re: [PATCH] sched/fair: Don't pull task if local group is more loaded than busiest group

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Date:   Wed, 25 Mar 2020 14:58:18 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Aubrey Li <aubrey.li@...el.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, vpillai@...italocean.com,
        joel@...lfernandes.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH] sched/fair: Don't pull task if local group is more
 loaded than busiest group

Le mercredi 25 mars 2020 à 20:46:28 (+0800), Aubrey Li a écrit :
> A huge number of load imbalance was observed when the local group
> type is group_fully_busy, and the average load of local group is
> greater than the selected busiest group, so the imbalance calculation
> returns a negative value actually. Fix this problem by comparing the

Do you see any wrong task migration ? because if imbalance is < 0, detach_tasks should return early

> average load before local group type check.
> 
> Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
> ---
>  kernel/sched/fair.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c1217bf..c524369 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8862,17 +8862,17 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>  		goto out_balanced;
>  
>  	/*
> +	 * If the local group is more loaded than the selected
> +	 * busiest group don't try to pull any tasks.
> +	 */
> +	if (local->avg_load >= busiest->avg_load)

If local is not overloaded, local->avg_load is null because it has not been already computed.

You should better add such test in calculate_imbalance after we computed local->avg_load and set imbalance to NULL

something like:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9e544e702f66..62f0861cefc0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9152,6 +9152,15 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s

                sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
                                sds->total_capacity;
+
+               /*
+                * If the local group is more loaded than the selected
+                * busiest group don't try to pull any tasks.
+                */
+               if (local->avg_load >= busiest->avg_load) {
+                       env->imbalance =0;
+                       return;
+               }
        }

        /*


> +		goto out_balanced;
> +
> +	/*
>  	 * When groups are overloaded, use the avg_load to ensure fairness
>  	 * between tasks.
>  	 */
>  	if (local->group_type == group_overloaded) {
> -		/*
> -		 * If the local group is more loaded than the selected
> -		 * busiest group don't try to pull any tasks.
> -		 */
> -		if (local->avg_load >= busiest->avg_load)
> -			goto out_balanced;
> -
>  		/* XXX broken for overlapping NUMA groups */
>  		sds.avg_load = (sds.total_load * SCHED_CAPACITY_SCALE) /
>  				sds.total_capacity;
> -- 
> 2.7.4
>