Re: [PATCH] sched/fair: Prevent cfs_rq from being unthrottled with zero runtime_remaining

[Aaron Lu <ziqianlu@bytedance.com>]

On Mon, Sep 29, 2025 at 03:04:03PM +0530, K Prateek Nayak wrote:
... ...
> Can we instead do a check_enqueue_throttle() in enqueue_throttled_task()
> if we find cfs_rq->throttled_limbo_list to be empty?
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 18a30ae35441..fd2d4dad9c27 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5872,6 +5872,8 @@ static bool enqueue_throttled_task(struct task_struct *p)
>  	 */
>  	if (throttled_hierarchy(cfs_rq) &&
>  	    !task_current_donor(rq_of(cfs_rq), p)) {
                /*
                 * Make sure to throttle this cfs_rq or it can be unthrottled
                 * with no runtime_remaining and gets throttled again on its
                 * unthrottle path.
                 */
> +		if (list_empty(&cfs_rq->throttled_limbo_list))
> +			check_enqueue_throttle(cfs_rq);

BTW, do you think a comment is needed? Something like the above, not
sure if it's too redundant though, feel free to let me know your
thoughts, thanks.

>  		list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
>  		return true;
>  	}
> ---