Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()

[Vineeth Pillai]

On 10/24/20 7:10 AM, Vineeth Pillai wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 93a3b874077d..4cae5ac48b60 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4428,12 +4428,14 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct 
> sched_entity *curr)
>                         se = second;
>         }
>
> -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) 
> < 1) {
> +       if (left && cfs_rq->next &&
> +                       wakeup_preempt_entity(cfs_rq->next, left) < 1) {
>                 /*
>                  * Someone really wants this to run. If it's not 
> unfair, run it.
>                  */
>                 se = cfs_rq->next;
> -       } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, 
> left) < 1) {
> +       } else if (left && cfs_rq->last &&
> +                       wakeup_preempt_entity(cfs_rq->last, left) < 1) {
>                 /*
>                  * Prefer last buddy, try to return the CPU to a 
> preempted task.
>
>
> There reason for left being NULL needs to be investigated. This was
> there from v1 and we did not yet get to it. I shall try to debug later
> this week.

Thinking more about it and looking at the crash, I think that
'left == NULL' can happen in pick_next_entity for core scheduling.
If a cfs_rq has only one task that is running, then it will be
dequeued and 'left = __pick_first_entity()' will be NULL as the
cfs_rq will be empty. This would not happen outside of coresched
because we never call pick_tack() before put_prev_task() which
will enqueue the task back.

With core scheduling, a cpu can call pick_task() for its sibling while
the sibling is still running the active task and put_prev_task has yet
not been called. This can result in 'left == NULL'. So I think the
above fix is appropriate when core scheduling is active. It could be
cleaned up a bit though.

Thanks,
Vineeth