[PATCH 1/1] [PATCH v3]sched/pelt: Fix the attach_entity_load_avg calculate method

[Kuyo Chang]

On Thu, 2022-04-14 at 16:44 +0200, Peter Zijlstra wrote:
> I've taken the liberty of carrying over the tags from v2 and reworked
> the Changelog a little.

Thank you for all your assistance.

> ---
> Subject: sched/pelt: Fix attach_entity_load_avg() corner case
> From: kuyo chang <kuyo.chang at mediatek.com>
> Date: Thu, 14 Apr 2022 17:02:20 +0800
> 
> From: kuyo chang <kuyo.chang at mediatek.com>
> 
> The warning in cfs_rq_is_decayed() triggered:
> 
>     SCHED_WARN_ON(cfs_rq->avg.load_avg ||
> 		  cfs_rq->avg.util_avg ||
> 		  cfs_rq->avg.runnable_avg)
> 
> There exists a corner case in attach_entity_load_avg() which will
> cause load_sum to be zero while load_avg will not be.
> 
> Consider se_weight is 88761 as per the sched_prio_to_weight[] table.
> Further assume the get_pelt_divider() is 47742, this gives:
> se->avg.load_avg is 1.
> 
> However, calculating load_sum results in 0:
> 
>   se->avg.load_sum = div_u64(se->avg.load_avg * se->avg.load_sum,
> se_weight(se));
>   se->avg.load_sum = 1*47742/88761 = 0.
> 
> Then enqueue_load_avg() adds this to the cfs_rq totals:
> 
>   cfs_rq->avg.load_avg += se->avg.load_avg;
>   cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> 
> Resulting in load_avg being 1 with load_sum is 0, which will trigger
> the WARN.
> 
> Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq
> and its se")
> Signed-off-by: kuyo chang <kuyo.chang at mediatek.com>
> [peterz: massage changelog]
> Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
> Reviewed-by: Vincent Guittot <vincent.guittot at linaro.org>
> Tested-by: Dietmar Eggemann <dietmar.eggemann at arm.com>
> Link: 
> https://urldefense.com/v3/__https://lkml.kernel.org/r/20220414090229.342-1-kuyo.chang@mediatek.com__;!!CTRNKA9wMg0ARbw!35Im02xxIuUZdLYpPng37Yk7oVNJVJ1tfbu4XRzlq-6VhH3K29Por0gJCFlslT_CMgA$
>  
> ---
>  kernel/sched/fair.c |   10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3829,11 +3829,11 @@ static void attach_entity_load_avg(struc
>  
>  	se->avg.runnable_sum = se->avg.runnable_avg * divider;
>  
> -	se->avg.load_sum = divider;
> -	if (se_weight(se)) {
> -		se->avg.load_sum =
> -			div_u64(se->avg.load_avg * se->avg.load_sum,
> se_weight(se));
> -	}
> +	se->avg.load_sum = se->avg.load_avg * divider;
> +	if (se_weight(se) < se->avg.load_sum)
> +		se->avg.load_sum = div_u64(se->avg.load_sum,
> se_weight(se));
> +	else
> +		se->avg.load_sum = 1;
>  
>  	enqueue_load_avg(cfs_rq, se);
>  	cfs_rq->avg.util_avg += se->avg.util_avg;