sched/fair: Block delayed tasks on throttled hierarchy during dequeue

JIRA: https://issues.redhat.com/browse/RHEL-69492
Upstream Status: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git (v6.12.54)

CONFLICTS: minor context conflict in the first chunk of the patch.

commit e67e3e7
Author: K Prateek Nayak <kprateek.nayak@amd.com>
Date:   Wed Oct 15 06:03:59 2025 +0000

    sched/fair: Block delayed tasks on throttled hierarchy during dequeue

    Dequeuing a fair task on a throttled hierarchy returns early on
    encountering a throttled cfs_rq since the throttle path has already
    dequeued the hierarchy above and has adjusted the h_nr_* accounting till
    the root cfs_rq.

    dequeue_entities() crucially misses calling __block_task() for delayed
    tasks being dequeued on the throttled hierarchies, but this was mostly
    harmless until commit b7ca574 ("sched/core: Tweak
    wait_task_inactive() to force dequeue sched_delayed tasks") since all
    existing cases would re-enqueue the task if task_on_rq_queued() returned
    true and the task would eventually be blocked at pick after the
    hierarchy was unthrottled.

    wait_task_inactive() is special as it expects the delayed task on
    throttled hierarchy to reach the blocked state on dequeue but since
    __block_task() is never called, task_on_rq_queued() continues to return
    true. Furthermore, since the task is now off the hierarchy, the pick
    never reaches it to fully block the task even after unthrottle leading
    to wait_task_inactive() looping endlessly.

    Remedy this by calling __block_task() if a delayed task is being
    dequeued on a throttled hierarchy.

    This fix is only required for stabled kernels implementing delay dequeue
    (>= v6.12) before v6.18 since upstream commit e1fad12 ("sched/fair:
    Switch to task based throttle model") indirectly fixes this by removing
    the early return conditions in dequeue_entities() as part of the per-task
    throttle feature.

    Cc: stable@vger.kernel.org
    Reported-by: Matt Fleming <matt@readmodwrite.com>
    Closes: https://lore.kernel.org/all/20250925133310.1843863-1-matt@readmodwrite.com/
    Fixes: b7ca574 ("sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks")
    Tested-by: Matt Fleming <mfleming@cloudflare.com>
    Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>

Author: lclaudio

kernel/sched/fair.c

@@ -7064,6 +7064,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
 	int h_nr_runnable = 0;
 	struct cfs_rq *cfs_rq;
 	u64 slice = 0;
+	int ret = 0;
 
 	if (entity_is_task(se)) {
 		p = task_of(se);
@@ -7093,7 +7094,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
 
 		/* end evaluation on encountering a throttled cfs_rq */
 		if (cfs_rq_throttled(cfs_rq))
-			return 0;
+			goto out;
 
 		/* Don't dequeue parent if it has other entities besides us */
 		if (cfs_rq->load.weight) {
@@ -7132,7 +7133,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
 
 		/* end evaluation on encountering a throttled cfs_rq */
 		if (cfs_rq_throttled(cfs_rq))
-			return 0;
+			goto out;
 	}
 
 	sub_nr_running(rq, h_nr_queued);
@@ -7141,6 +7142,8 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
 	if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
 		rq->next_balance = jiffies;
 
+	ret = 1;
+out:
 	if (p && task_delayed) {
 		SCHED_WARN_ON(!task_sleep);
 		SCHED_WARN_ON(p->on_rq != 1);
@@ -7156,7 +7159,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
 		__block_task(rq, p);
 	}
 
-	return 1;
+	return ret;
 }
 
 /*