The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: 9f6d929ee2c6f0266edb564bcd2bd47fd6e884a8
Gitweb: https://git.kernel.org/tip/9f6d929ee2c6f0266edb564bcd2bd47fd6e884a8
Author: Vincent Guittot <vincent.guittot@linaro.org>
AuthorDate: Sun, 03 May 2026 12:45:03 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 06 May 2026 17:41:18 +02:00
sched/fair: Fix wakeup_preempt_fair() for not waking up task
Make sure to only call pick_next_entity() on an non-empty cfs_rq.
The assumption that p is always enqueued and not delayed, is only true for
wakeup. If p was moved while delayed, pick_next_entity() will dequeue it and
the cfs might become empty. Test if there are still queued tasks before trying
again to determine if p could be the next one to be picked.
There are at least 2 cases:
When cfs becomes idle, it tries to pull tasks but if those pulled tasks are
delayed, they will be dequeued when attached to cfs. attach_tasks() ->
attach_task() -> wakeup_preempt(rq, p, 0);
A misfit task running on cfs A triggers a load balance to be pulled on a better
cpu, the load balance on cfs B starts an active load balance to pulled the
running misfit task. If there is a delayed dequeue task on cfs A, it can be
pulled instead of the previously running misfit task. attach_one_task() ->
attach_task() -> wakeup_preempt(rq, p, 0);
Fixes: ac8e69e69363 ("sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260503104503.1732682-1-vincent.guittot@linaro.org
---
kernel/sched/fair.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b91c8b2..3ebec18 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9174,9 +9174,10 @@ pick:
/*
* Because p is enqueued, nse being null can only mean that we
- * dequeued a delayed task.
+ * dequeued a delayed task. If there are still entities queued in
+ * cfs, check if the next one will be p.
*/
- if (!nse)
+ if (!nse && cfs_rq->nr_queued)
goto pick;
if (sched_feat(RUN_TO_PARITY))