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

[Joel Fernandes]

On Sat, Oct 24, 2020 at 08:27:16AM -0400, Vineeth Pillai wrote:
> 
> 
> 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 a lot Vineeth!
Just add, the pick_next_entity() returning NULL still causes pick_task_fair()
to behave correctly. Because pick_task_fair() will return 'curr' to the
core-wide picking code, not NULL. The problem is CFS pick_next_entity() is
not able to handle it though and crash,  as Vineeth found the below hunk got
lost in the rebase:

@@ -4464,13 +4464,13 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 	/*
 	 * Prefer last buddy, try to return the CPU to a preempted task.
 	 */
-	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
+	if (left && cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
 		se = cfs_rq->last;

 	/*
 	 * Someone really wants this to run. If it's not unfair, run it.
 	 */
-	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)
 		se = cfs_rq->next;

 	clear_buddies(cfs_rq, se);

Peter's fix in the other email looks good and I will include that for testing
before the next posting.

thanks,

 - Joel