linux-kernel - Re: [PATCH v3 1/3] sched/fair: fix rounding issue for asym packing

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Message-ID: <ec8ccd83-0613-e37a-1a7b-ec680c4c4779@arm.com>
Date:   Thu, 20 Dec 2018 11:16:30 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org
Cc:     Morten.Rasmussen@....com
Subject: Re: [PATCH v3 1/3] sched/fair: fix rounding issue for asym packing

On 20/12/2018 07:55, Vincent Guittot wrote:
> When check_asym_packing() is triggered, the imbalance is set to :
>   busiest_stat.avg_load * busiest_stat.group_capacity / SCHED_CAPACITY_SCALE
> But busiest_stat.avg_load equals
>   sgs->group_load *SCHED_CAPACITY_SCALE / sgs->group_capacity
> These divisions can generate a rounding that will make imbalance slightly
> lower than the weighted load of the cfs_rq.
> But this is enough to skip the rq in find_busiest_queue and prevents asym
> migration to happen.
> 
> Directly set imbalance to sgs->group_load to remove the rounding.
                            ^^^^^^^^^^^^^^^
I see where that's coming from, but using 'sgs' here is tad confusing since
'sds->busiest_stat' is what's actually used.

Maybe just something like 'the busiest's sgs->group_load' would be good
enough to make things explicit.

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ca46964..9b31247 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8476,9 +8476,7 @@ static int check_asym_packing(struct lb_env *env, struct sd_lb_stats *sds)
>  	if (sched_asym_prefer(busiest_cpu, env->dst_cpu))
>  		return 0;
>  
> -	env->imbalance = DIV_ROUND_CLOSEST(
> -		sds->busiest_stat.avg_load * sds->busiest_stat.group_capacity,
> -		SCHED_CAPACITY_SCALE);
> +	env->imbalance = sds->busiest_stat.avg_load;

That should be group_load, not avg_load. With that fixed:

Reviewed-by: Valentin Schneider <valentin.schneider@....com>

>  
>  	return 1;
>  }
>