From 7a9df76679633cf26e0fea882a684012c6ec245a Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 7 Oct 2026 06:26:47 -1000 Subject: [PATCH] sched_ext: Fix stall when a task enqueued with SCX_ENQ_LAST lands back on its CPU's local DSQ With SCX_OPS_ENQ_LAST, the last runnable task on a CPU is passed to ops.enqueue() instead of being kept running. Exiting tasks without SCX_OPS_ENQ_EXITING, migration disabled tasks without SCX_OPS_ENQ_MIGRATION_DISABLED and tasks on an offline rq skip ops.enqueue() and go straight to the local DSQ. This happens in put_prev_task_scx() after the pick has settled on idle, so nothing reschedules the CPU, which idles with the task queued until an unrelated wakeup lands on it or the watchdog fires. scx_mitosis, which sets SCX_OPS_ENQ_LAST without SCX_OPS_ENQ_EXITING, hit this during mass cgroup teardown: exiting tasks stuck in exit_mmap() for over 40 seconds. A task put on the local DSQ runs, as anywhere else. If the SCX_ENQ_LAST enqueue left the task on this CPU's local DSQ, set need_resched on the task being switched in, as proxy_resched_idle() does. resched_curr() would flag the task being put instead, and __schedule() clears that right after the pick. Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class") Cc: stable@vger.kernel.org # v6.12+ Reported-by: Xiangyu Bu Link: https://github.com/sched-ext/scx/pull/3870 Signed-off-by: Tejun Heo Cc: Dan Schatzberg --- kernel/sched/ext/ext.c | 13 +++++++++++-- kernel/sched/ext/internal.h | 5 +++-- 2 files changed, 14 insertions(+), 4 deletions(-) diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c index ac8fe70a9fb6..64d937fb2b24 100644 --- a/kernel/sched/ext/ext.c +++ b/kernel/sched/ext/ext.c @@ -3213,8 +3213,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p, /* * If @p is runnable but we're about to enter a lower * sched_class, %SCX_OPS_ENQ_LAST must be set. Tell - * ops.enqueue() that @p is the only one available for this cpu, - * which should trigger an explicit follow-up scheduling event. + * ops.enqueue() that @p is the only one available for this cpu. * This doesn't apply if the baseline access on the CPU is lost. * * Under core scheduling, a pick dispatches only when nothing is @@ -3226,6 +3225,16 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p, WARN_ON_ONCE(!sched_core_enabled(rq) && !(sch->ops.flags & SCX_OPS_ENQ_LAST)); scx_do_enqueue_task(rq, p, SCX_ENQ_LAST, -1); + + /* + * A task put on the local DSQ runs, as anywhere else. + * Here the insert can't reschedule on its own: the pick + * has settled on @next and @p is still curr, so + * resched_curr() would flag @p and __schedule() clears + * that right after the pick. Flag @next instead. + */ + if (p->scx.dsq == &rq->scx.local_dsq) + set_tsk_need_resched(next); } else { scx_do_enqueue_task(rq, p, 0, -1); } diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h index 5b37faa532d6..85579fcabf52 100644 --- a/kernel/sched/ext/internal.h +++ b/kernel/sched/ext/internal.h @@ -1774,8 +1774,9 @@ enum scx_enq_flags { * %SCX_OPS_ENQ_LAST is specified, they're ops.enqueue()'d with the * %SCX_ENQ_LAST flag set. * - * The BPF scheduler is responsible for triggering a follow-up - * scheduling event. Otherwise, Execution may stall. + * If the task is queued on the local DSQ of the CPU it was running on, + * it continues to run. Otherwise, the CPU goes idle. A scheduler that + * wants a full dispatch cycle on the CPU should kick it. */ SCX_ENQ_LAST = 1LLU << 41,