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 matt@readmodwrite.com 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 mfleming@cloudflare.com Signed-off-by: K Prateek Nayak kprateek.nayak@amd.com --- Hello Greg, Sasha,
Please consider the same fix for the v6.17 stable kernel too since there is a report of a similar issue on v6.17.1 based RT kernel at https://lore.kernel.org/lkml/aPN7XBJbGhdWJDb2@uudg.org/ and Luis confirmed that this fix solves the issue for him in https://lore.kernel.org/lkml/aPgm6KvDx5Os2oJS@uudg.org/ --- kernel/sched/fair.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 8ce56a8d507f..f0a4d9d7424d 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; }
/*
base-commit: 6c7871823908a4330e145d635371582f76ce1407