linux-kernel - Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched

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Message-ID: <20250522120336.GI39944@noisy.programming.kicks-ass.net>
Date: Thu, 22 May 2025 14:03:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Aaron Lu <ziqianlu@...edance.com>
Cc: Valentin Schneider <vschneid@...hat.com>,
	Ben Segall <bsegall@...gle.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Xi Wang <xii@...gle.com>, linux-kernel@...r.kernel.org,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Chuyi Zhou <zhouchuyi@...edance.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Florian Bezdeka <florian.bezdeka@...mens.com>
Subject: Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched_class
 change for throttled task

On Tue, May 20, 2025 at 06:41:07PM +0800, 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.
> 
> Signed-off-by: Aaron Lu <ziqianlu@...edance.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);

This is asymmetric -- dequeue removes it from that throttle list, but
the corresponding enqueue will not add it back, what gives?

Because now we have:

 p->on_rq=1
 p->throttle_node on list

move_queued_task()
  deactivate_task()
    dequeue_task_fair()
      list_del_init(throttle_node)
    p->on_rq = 2

  activate_task()
    enqueue_task_fair()
      // nothing special, makes the thing runnable
    p->on_rq = 1;

and we exit with a task that is on-rq and not throttled ?!?

Why is this? Are we relying on pick_task_fair() to dequeue it again and
fix up our inconsistencies? If so, that had better have a comment on.