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

Aaron Lu
2025-06-18T08:20:19Z

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>
---
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)
{
--
2.39.5