Messages in this thread | | | Subject | Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task() | From | Vineeth Pillai <> | Date | Sat, 24 Oct 2020 08:27:16 -0400 |
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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
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