Re: [PATCH v2 2/5] sched/fair: Implement throttle task work and related helpers

Chengming Zhou
2025-06-18T09:03:56Z

On 2025/6/18 16:19, Aaron Lu wrote:
> From: Valentin Schneider <vschneid@redhat.com>
>
> Implement throttle_cfs_rq_work() task work which gets executed on task's
> ret2user path where the task is dequeued and marked as throttled.
>
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>

LGTM,

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks!

> ---
> kernel/sched/fair.c | 65 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 65 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f7b8597bc95ac..8226120b8771a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5757,8 +5757,51 @@ static inline int throttled_lb_pair(struct task_group *tg,
> throttled_hierarchy(dest_cfs_rq);
> }
>
> +static inline bool task_is_throttled(struct task_struct *p)
> +{
> + return p->throttled;
> +}
> +
> +static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags);
> static void throttle_cfs_rq_work(struct callback_head *work)
> {
> + struct task_struct *p = container_of(work, struct task_struct, sched_throttle_work);
> + struct sched_entity *se;
> + struct cfs_rq *cfs_rq;
> + struct rq *rq;
> +
> + WARN_ON_ONCE(p != current);
> + p->sched_throttle_work.next = &p->sched_throttle_work;
> +
> + /*
> + * If task is exiting, then there won't be a return to userspace, so we
> + * don't have to bother with any of this.
> + */
> + if ((p->flags & PF_EXITING))
> + return;
> +
> + scoped_guard(task_rq_lock, p) {
> + se = &p->se;
> + cfs_rq = cfs_rq_of(se);
> +
> + /* Raced, forget */
> + if (p->sched_class != &fair_sched_class)
> + return;
> +
> + /*
> + * If not in limbo, then either replenish has happened or this
> + * task got migrated out of the throttled cfs_rq, move along.
> + */
> + if (!cfs_rq->throttle_count)
> + return;
> + rq = scope.rq;
> + update_rq_clock(rq);
> + WARN_ON_ONCE(p->throttled || !list_empty(&p->throttle_node));
> + dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
> + list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
> + p->throttled = true;
> + resched_curr(rq);
> + }
> }
>
> void init_cfs_throttle_work(struct task_struct *p)
> @@ -5798,6 +5841,26 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
> return 0;
> }
>
> +static inline bool task_has_throttle_work(struct task_struct *p)
> +{
> + return p->sched_throttle_work.next != &p->sched_throttle_work;
> +}
> +
> +static inline void task_throttle_setup_work(struct task_struct *p)
> +{
> + if (task_has_throttle_work(p))
> + return;
> +
> + /*
> + * Kthreads and exiting tasks don't return to userspace, so adding the
> + * work is pointless
> + */
> + if ((p->flags & (PF_EXITING | PF_KTHREAD)))
> + return;
> +
> + task_work_add(p, &p->sched_throttle_work, TWA_RESUME);
> +}
> +
> static int tg_throttle_down(struct task_group *tg, void *data)
> {
> struct rq *rq = data;
> @@ -6668,6 +6731,8 @@ static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq) { return false; }
> static void check_enqueue_throttle(struct cfs_rq *cfs_rq) {}
> 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 inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
> {