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

[Li, Aubrey]

On 2020/10/24 20:27, 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.

This patch works, thanks Vineeth for the quick fix!