Re: [PATCH 1/1] [PATCH v2]sched/pelt: Refine the enqueue_load_avg calculate method

[Vincent Guittot]

On Thu, 14 Apr 2022 at 03:59, Kuyo Chang <kuyo.chang@xxxxxxxxxxxx> wrote:
>
> From: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
>
> I meet the warning message at cfs_rq_is_decayed at below code.
>
> SCHED_WARN_ON(cfs_rq->avg.load_avg ||
>                     cfs_rq->avg.util_avg ||
>                     cfs_rq->avg.runnable_avg)
>
> Following is the calltrace.
>
> Call trace:
> __update_blocked_fair
> update_blocked_averages
> newidle_balance
> pick_next_task_fair
> __schedule
> schedule
> pipe_read
> vfs_read
> ksys_read
>
> After code analyzing and some debug messages, I found it exits a corner
> case at attach_entity_load_avg which will cause load_sum is null but
> load_avg is not.
> Consider se_weight is 88761 according by sched_prio_to_weight table.
> And assume the get_pelt_divider() is 47742, se->avg.load_avg is 1.
> By the calculating for se->avg.load_sum as following will become zero
> as following.
> 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.
>
> After enqueue_load_avg code as below.
> cfs_rq->avg.load_avg += se->avg.load_avg;
> cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
>
> Then the load_sum for cfs_rq will be 1 while the load_sum for cfs_rq is 0.
> So it will hit the warning message.
>
> In order to fix the corner case, make sure the se->load_avg|sum is correct
> before enqueue_load_avg.
>
> After long time testing, the kernel warning was gone and the system runs
> as well as before.

This needs a fix tag:
Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq and its se")

>
> Signed-off-by: kuyo chang <kuyo.chang@xxxxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>
> v1->v2:
>
> (1)Thanks for suggestion from Peter Zijlstra & Vincent Guittot.
> (2)By suggestion from Vincent Guittot,
> rework the se->load_sum calculation method for fix the corner case,
> make sure the se->load_avg|sum is correct before enqueue_load_avg.
> (3)Rework changlog.
>
>  kernel/sched/fair.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d4bd299d67ab..159274482c4e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3829,10 +3829,12 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>
>         se->avg.runnable_sum = se->avg.runnable_avg * divider;
>
> -       se->avg.load_sum = divider;
> -       if (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_avg * se->avg.load_sum, se_weight(se));
> +                       div_u64(se->avg.load_sum, se_weight(se));
> +       } else {
> +               se->avg.load_sum = 1;
>         }
>
>         enqueue_load_avg(cfs_rq, se);
> --
> 2.18.0
>