linux-kernel - Re: [PATCH v2] sched/fair: fix initial util

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Message-ID: <ZgqOEJ5sCANkkk5N@linux.ibm.com>
Date: Mon, 1 Apr 2024 16:06:00 +0530
From: Vishal Chourasia <vishalc@...ux.ibm.com>
To: Dawei Li <daweilics@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: fix initial util_avg calculation

On Thu, Mar 14, 2024 at 06:59:16PM -0700, Dawei Li wrote:
> Change se->load.weight to se_weight(se) in the calculation for the
> initial util_avg to avoid unnecessarily inflating the util_avg by 1024
> times.
> 
> The reason is that se->load.weight has the unit/scale as the scaled-up
> load, while cfs_rg->avg.load_avg has the unit/scale as the true task
> weight (as mapped directly from the task's nice/priority value). With
> CONFIG_32BIT, the scaled-up load is equal to the true task weight. With
> CONFIG_64BIT, the scaled-up load is 1024 times the true task weight.
> Thus, the current code may inflate the util_avg by 1024 times. The
> follow-up capping will not allow the util_avg value to go wild. But the
> calculation should have the correct logic.
> 
> Signed-off-by: Dawei Li <daweilics@...il.com>
> ---
> Changes in v2:
> - update the commit message
> ---
>  kernel/sched/fair.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a19ea290b790..5f98f639bdb9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1031,7 +1031,8 @@ void init_entity_runnable_average(struct sched_entity *se)
>   * With new tasks being created, their initial util_avgs are extrapolated
>   * based on the cfs_rq's current util_avg:
>   *
> - *   util_avg = cfs_rq->util_avg / (cfs_rq->load_avg + 1) * se.load.weight
> + *   util_avg = cfs_rq->avg.util_avg / (cfs_rq->avg.load_avg + 1)
> + *		* se_weight(se)
>   *
>   * However, in many cases, the above util_avg does not give a desired
>   * value. Moreover, the sum of the util_avgs may be divergent, such
> @@ -1078,7 +1079,7 @@ void post_init_entity_util_avg(struct task_struct *p)
>  
>  	if (cap > 0) {
>  		if (cfs_rq->avg.util_avg != 0) {
> -			sa->util_avg  = cfs_rq->avg.util_avg * se->load.weight;
> +			sa->util_avg  = cfs_rq->avg.util_avg * se_weight(se);
Hi,

The comment above the declaration of se_weight function says we should be
using full load resolution and get rid of this helper.

Should we be adding new user of the helper?

/*
 * XXX we want to get rid of these helpers and use the full load resolution.
 */
static inline long se_weight(struct sched_entity *se)
{
	return scale_load_down(se->load.weight);
}


>  			sa->util_avg /= (cfs_rq->avg.load_avg + 1);
>  
>  			if (sa->util_avg > cap)
> -- 
> 2.40.1
>