[PATCH 6.12 507/567] sched_ext: Fix missing post-enqueue handling in move_local_task_to_local_dsq()

Greg Kroah-Hartman
2026-01-06T17:42:17Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit f5e1e5ec204da11fa87fdf006d451d80ce06e118 ]

move_local_task_to_local_dsq() is used when moving a task from a non-local
DSQ to a local DSQ on the same CPU. It directly manipulates the local DSQ
without going through dispatch_enqueue() and was missing the post-enqueue
handling that triggers preemption when SCX_ENQ_PREEMPT is set or the idle
task is running.

The function is used by move_task_between_dsqs() which backs
scx_bpf_dsq_move() and may be called while the CPU is busy.

Add local_dsq_post_enq() call to move_local_task_to_local_dsq(). As the
dispatch path doesn't need post-enqueue handling, add SCX_RQ_IN_BALANCE
early exit to keep consume_dispatch_q() behavior unchanged and avoid
triggering unnecessary resched when scx_bpf_dsq_move() is used from the
dispatch path.

Fixes: 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()")
Cc: stable@vger.kernel.org # v6.12+
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/sched/ext.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1682,6 +1682,14 @@ static void local_dsq_post_enq(struct sc
struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);
bool preempt = false;

+ /*
+ * If @rq is in balance, the CPU is already vacant and looking for the
+ * next task to run. No need to preempt or trigger resched after moving
+ * @p into its local DSQ.
+ */
+ if (rq->scx.flags & SCX_RQ_IN_BALANCE)
+ return;
+
if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
rq->curr->sched_class == &ext_sched_class) {
rq->curr->scx.slice = 0;
@@ -2259,6 +2267,8 @@ static void move_local_task_to_local_dsq

dsq_mod_nr(dst_dsq, 1);
p->scx.dsq = dst_dsq;
+
+ local_dsq_post_enq(dst_dsq, p, enq_flags);
}

#ifdef CONFIG_SMP