From: Vincent Guittot Date: Sun, 3 May 2026 10:45:03 +0000 (+0200) Subject: sched/fair: Fix wakeup_preempt_fair() for not waking up task X-Git-Tag: v7.1-rc3~14^2 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9f6d929ee2c6f0266edb564bcd2bd47fd6e884a8;p=thirdparty%2Fkernel%2Flinux.git 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 Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260503104503.1732682-1-vincent.guittot@linaro.org --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index b91c8b294229..3ebec186f982 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))