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

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Message-ID: <63237b23-ae10-45f9-abdd-8ea4adb4d15e@linux.dev>
Date: Fri, 23 May 2025 10:43:53 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Aaron Lu <ziqianlu@...edance.com>,
 Valentin Schneider <vschneid@...hat.com>, Ben Segall <bsegall@...gle.com>,
 K Prateek Nayak <kprateek.nayak@....com>,
 Peter Zijlstra <peterz@...radead.org>, Josh Don <joshdon@...gle.com>,
 Ingo Molnar <mingo@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Xi Wang <xii@...gle.com>
Cc: 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>,
 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 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?

Thanks!

> 
> 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);
>