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| author | K Prateek Nayak <kprateek.nayak@amd.com> | 2025-10-15 06:03:59 +0000 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2025-10-19 16:34:06 +0200 |
| commit | e67e3e738f088e6c5ccfab618a29318a3f08db41 (patch) | |
| tree | 66947f8cf7b59b8b68f99d7e3f5d04e3c96688ff | |
| parent | 496b5ef11dc316953a25086b38a5df1891196030 (diff) | |
| download | linux-e67e3e738f088e6c5ccfab618a29318a3f08db41.tar.gz linux-e67e3e738f088e6c5ccfab618a29318a3f08db41.tar.bz2 linux-e67e3e738f088e6c5ccfab618a29318a3f08db41.zip | |
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 <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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
| -rw-r--r-- | kernel/sched/fair.c | 9 |
1 files changed, 6 insertions, 3 deletions
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index af61769b1d50..b3d9826e25b0 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7187,6 +7187,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags) int h_nr_delayed = 0; struct cfs_rq *cfs_rq; u64 slice = 0; + int ret = 0; if (entity_is_task(se)) { p = task_of(se); @@ -7218,7 +7219,7 @@ 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) { @@ -7261,7 +7262,7 @@ 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); @@ -7273,6 +7274,8 @@ 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); @@ -7288,7 +7291,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags) __block_task(rq, p); } - return 1; + return ret; } /* |