From: K Prateek Nayak Date: Thu, 23 Oct 2025 04:03:59 +0000 (+0000) Subject: sched/fair: Block delayed tasks on throttled hierarchy during dequeue X-Git-Tag: v6.17.6~185 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=514784d00832d23a6a7953ed3694306e5af87b03;p=thirdparty%2Fkernel%2Fstable.git 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 b7ca5743a260 ("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 e1fad12dcb66 ("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 Closes: https://lore.kernel.org/all/20250925133310.1843863-1-matt@readmodwrite.com/ Fixes: b7ca5743a260 ("sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks") Tested-by: Matt Fleming Signed-off-by: K Prateek Nayak Signed-off-by: Greg Kroah-Hartman --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 8f0b1acace0ad..4770d25ae2406 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6969,6 +6969,7 @@ 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); @@ -6998,7 +6999,7 @@ 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) { @@ -7039,7 +7040,7 @@ 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); @@ -7048,6 +7049,8 @@ 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) { WARN_ON_ONCE(!task_sleep); WARN_ON_ONCE(p->on_rq != 1); @@ -7063,7 +7066,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags) __block_task(rq, p); } - return 1; + return ret; } /*