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

[Li, Aubrey]

On 2020/10/26 17:01, Peter Zijlstra wrote:
> 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'.
> 
> Quite correct. Hurmph.. the reason we do this is because... we do the
> update_curr() the wrong way around. And I can't seem to remember why we
> do that (it was in my original patches).
> 
> Something like so seems the obvious thing to do, but I can't seem to
> remember why we're not doing it :-(
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6950,15 +6950,10 @@ static struct task_struct *pick_task_fai
>  	do {
>  		struct sched_entity *curr = cfs_rq->curr;
>  
> -		se = pick_next_entity(cfs_rq, NULL);
> +		if (curr && curr->on_rq)
> +			update_curr(cfs_rq);
>  
> -		if (curr) {
> -			if (se && curr->on_rq)
> -				update_curr(cfs_rq);
> -
> -			if (!se || entity_before(curr, se))
> -				se = curr;
> -		}
> +		se = pick_next_entity(cfs_rq, curr);
>  
>  		cfs_rq = group_cfs_rq(se);
>  	} while (cfs_rq);
> 

This patch works too for my benchmark, thanks Peter!