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 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().

Thanks!

> 
> Thanks,
> Aaron
>   
>>>>>
>>>>> Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>
>>>>> ---
>>>>>     kernel/sched/fair.c | 24 ++++++++++++++++++++++++
>>>>>     1 file changed, 24 insertions(+)
>>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index 74bc320cbc238..4c66fd8d24389 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -5866,6 +5866,10 @@ static void throttle_cfs_rq_work(struct callback_head *work)
>>>>>     		update_rq_clock(rq);
>>>>>     		WARN_ON_ONCE(!list_empty(&p->throttle_node));
>>>>>     		dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
>>>>> +		/*
>>>>> +		 * Must not add it to limbo list before dequeue or dequeue will
>>>>> +		 * mistakenly regard this task as an already throttled one.
>>>>> +		 */
>>>>>     		list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
>>>>>     		resched_curr(rq);
>>>>>     	}
>>>>> @@ -5881,6 +5885,20 @@ void init_cfs_throttle_work(struct task_struct *p)
>>>>>     	INIT_LIST_HEAD(&p->throttle_node);
>>>>>     }
>>>>> +static void dequeue_throttled_task(struct task_struct *p, int flags)
>>>>> +{
>>>>> +	/*
>>>>> +	 * Task is throttled and someone wants to dequeue it again:
>>>>> +	 * it must be sched/core when core needs to do things like
>>>>> +	 * task affinity change, task group change, task sched class
>>>>> +	 * change etc.
>>>>> +	 */
>>>>> +	WARN_ON_ONCE(p->se.on_rq);
>>>>> +	WARN_ON_ONCE(flags & DEQUEUE_SLEEP);
>>>>> +
>>>>> +	list_del_init(&p->throttle_node);
>>>>> +}
>>>>> +
>>>>>     static void enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags);
>>>>>     static int tg_unthrottle_up(struct task_group *tg, void *data)
>>>>>     {
>>>>> @@ -6834,6 +6852,7 @@ static inline void sync_throttle(struct task_group *tg, int cpu) {}
>>>>>     static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
>>>>>     static void task_throttle_setup_work(struct task_struct *p) {}
>>>>>     static bool task_is_throttled(struct task_struct *p) { return false; }
>>>>> +static void dequeue_throttled_task(struct task_struct *p, int flags) {}
>>>>>     static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
>>>>>     {
>>>>> @@ -7281,6 +7300,11 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
>>>>>      */
>>>>>     static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>>>>>     {
>>>>> +	if (unlikely(task_is_throttled(p))) {
>>>>> +		dequeue_throttled_task(p, flags);
>>>>> +		return true;
>>>>> +	}
>>>>> +
>>>>>     	if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & DEQUEUE_SAVE))))
>>>>>     		util_est_dequeue(&rq->cfs, p);