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

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Date: Tue, 12 Mar 2024 14:29:36 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Dawei Li <daweilics@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, 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] sched/fair: fix initial util_avg calculation

On Fri, 8 Mar 2024 at 21:23, Dawei Li <daweilics@...il.com> wrote:
>
> According to the comment for post_init_entity_util_avg(), it seems that
> we are assuming se->load.weight has the same scale/unit as that of
> cfs_rq->avg.load_avg.
>
> As far as I understand, se->load.weight is the scaled-up load, instead
> of the true weight (as mapped directly from the nice value) of a task.
> When CONFIG_32BIT is set, we have load == weight; when CONFIG_64BIT is
> set, we have load == weight * 1024. However, cfs_rq->avg.load_avg is
> the sum of true weights of tasks, as se->avg.load_avg corresponds to
> the true weight of a task.
>
> Based on how sa->util_avg is calculated in the code, we could be
> inflating sa->util_avg by 1024 times? Could this be the reason why
> "However, in many cases, the above util_avg does not give a desired
> value. ... "?

No, this is about the fact that this estimation of util_avg can give
insane value because even the correct se_weight(se) can go up to 88761
whereas util_avg should never go above 1024

>
> I'm not entirely sure about it. Posting this for clarification and
> comments.

Good catch.

The commit message needs to be updated to remove your question above
but otherwise looks good

>
> Signed-off-by: Dawei Li <daweilics@...il.com>
> ---
>  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 6a16129f9a5c..0d13e52e1a92 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);
>                         sa->util_avg /= (cfs_rq->avg.load_avg + 1);
>
>                         if (sa->util_avg > cap)
> --
> 2.40.1
>