Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched_class change for throttled task

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2025/5/23 19:59, Aaron Lu wrote:
> On Fri, May 23, 2025 at 05:53:55PM +0800, Chengming Zhou wrote:
>> On 2025/5/23 17:42, Aaron Lu wrote:
>>> On Fri, May 23, 2025 at 05:13:35PM +0800, Chengming Zhou wrote:
>>>> On 2025/5/23 15:56, Aaron Lu wrote:
>>>>> On Fri, May 23, 2025 at 10:43:53AM +0800, Chengming Zhou wrote:
>>>>>> On 2025/5/20 18:41, Aaron Lu wrote:
>>>>>>> On task group change, for tasks whose on_rq equals to TASK_ON_RQ_QUEUED,
>>>>>>> core will dequeue it and then requeued it.
>>>>>>>
>>>>>>> The throttled task is still considered as queued by core because p->on_rq
>>>>>>> is still set so core will dequeue it, but since the task is already
>>>>>>> dequeued on throttle in fair, handle this case properly.
>>>>>>>
>>>>>>> Affinity and sched class change is similar.
>>>>>>
>>>>>> How about setting p->on_rq to 0 when throttled? which is the fact that
>>>>>> the task is not on cfs queue anymore, does this method cause any problem?
>>>>>>
>>>>>
>>>>> On task group change/affinity change etc. if the throttled task is
>>>>> regarded as !on_rq, then it will miss the chance to be enqueued to the
>>>>> new(and correct) cfs_rqs, instead, it will be enqueued back to its
>>>>> original cfs_rq on unthrottle which breaks affinity or task group
>>>>
>>>> Yeah, this is indeed a problem, I was thinking to delete the throttled task
>>>> from the cfs_rq limbo list, then add it to another cfs_rq limbo list or cfs_rq
>>>> runnable tree based on the new cfs_rq's throttle status.
>>>
>>> Only work when the task is still handled by fair :)
>>>
>>>>
>>>> But it's much complex compared with your current method.
>>>>
>>>>> settings. We may be able to do something in tg_unthrottle_up() to take
>>>>> special care of these situations, but it seems a lot of headaches.
>>>>>
>>>>> Also, for task group change, if the new task group does not have throttle
>>>>> setting, that throttled task should be allowed to run immediately instead
>>>>> of waiting for its old cfs_rq's unthrottle event. Similar is true when
>>>>> this throttled task changed its sched class, like from fair to rt.
>>>>>
>>>>> Makes sense?
>>>>
>>>> Ok, the another problem of the current method I can think of is the PELT maintenance,
>>>> we skip the actual dequeue_task_fair() process, which includes PELT detach, we just
>>>> delete it from the cfs_rq limbo list, so it can result in PELT maintenance error.
>>>>
>>>
>>> There are corresponding callbacks that handle this, e.g. for task group
>>> change, there is task_change_group_fair() that handles PELT detach; for
>>> affinity change, there is migrate_task_rq_fair() does that and for sched
>>> class change, there is switched_from/to() does that.
>>>
>>> Or do I miss anything?
>>
>> migrate_task_rq_fair() only do it when !task_on_rq_migrating(p), which is wakeup migrate,
>> because we already do detach in dequeue_task_fair() for on_rq task migration...
>> You can see the DO_DETACH flag in update_load_avg() called from dequeue_entity().
>>
> 
> Understood, thanks for catching this!
> 
> So the code was initially developed on top of v5.15 and there is a
> detach in migrate_task_rq_fair():
> 
> 	if (p->on_rq == TASK_ON_RQ_MIGRATING) {
> 		/*
> 		 * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old'
> 		 * rq->lock and can modify state directly.
> 		 */
> 		lockdep_assert_rq_held(task_rq(p));
> 		detach_entity_cfs_rq(&p->se);
> 	}
> 
> But looks like it's gone now by commit e1f078f50478("sched/fair: Combine
> detach into dequeue when migrating task") and I failed to notice this
> detail...

Yeah..

> 
> Anyway, the task is already dequeued without TASK_ON_RQ_MIGRATING being
> set when throttled and it can't be dequeued again, so perhaps something
> like below could cure this situation?(just to illustrate the idea, not
> even compile tested)

Ok, seems reasonable to me!

> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 89afa472299b7..dc2e9a6bf3fd7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5868,6 +5868,9 @@ static void dequeue_throttled_task(struct task_struct *p, int flags)
> 	WARN_ON_ONCE(flags & DEQUEUE_SLEEP);
> 
> 	list_del_init(&p->throttle_node);
> +
> +       if (task_on_rq_migrating(p))
> +               detach_task_cfs_rq(p);
> }
> 
>