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

[Vincent Guittot]

On Mon, 11 Apr 2022 at 08:17, 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 zero and
> 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.

The root problem is there, se->avg.load_sum must not be null if
se->avg.load_avg is not null because the correct relation between _avg
and _sum is:

load_avg = weight * load_sum / divider.

so the fix should be attach_entity_load_avg() and probably the below is enough

se->avg.load_sum = div_u64(se->avg.load_avg * se->avg.load_sum,
se_weight(se)) + 1;

>
> 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.
>
> After all, I refer the following commit patch to do the similar thing at
> enqueue_load_avg.
> sched/pelt: Relax the sync of load_sum with load_avg
>
> After long time testing, the kernel warning was gone and the system runs
> as well as before.
>
> Signed-off-by: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d4bd299d67ab..30d8b6dba249 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3074,8 +3074,10 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  static inline void
>  enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -       cfs_rq->avg.load_avg += se->avg.load_avg;
> -       cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> +       add_positive(&cfs_rq->avg.load_avg, se->avg.load_avg);
> +       add_positive(&cfs_rq->avg.load_sum, se_weight(se) * se->avg.load_sum);
> +       cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum,
> +                                         cfs_rq->avg.load_avg * PELT_MIN_DIVIDER);
>  }
>
>  static inline void
> --
> 2.18.0
>