[PATCH v3 1/2] sched_ext: Call ops.dequeue() when a task arrives on a remote local DSQ

[Kuba Piecuch]

When a task in the BPF scheduler's custody is moved to another CPU's
local DSQ, ops.dequeue() is only called once the task is picked for
execution, from set_next_task_scx() with SCX_DEQ_CORE_SCHED_EXEC, even
though no core-sched pick took place. It should instead be called
without flags when the task is inserted into the destination DSQ, as it
is for same-rq dispatches.

move_remote_task_to_local_dsq() sets p->scx.sticky_cpu across the
migration so that deactivate_task() on the source rq doesn't end custody.
Since commit b75aaea24c9f ("sched_ext: Properly mark SCX-internal
migrations via sticky_cpu"), enqueue_task_scx() only clears it after
inserting the task, so task_leave_custody() still sees the migration in
progress and skips the custody exit.

Clear p->scx.sticky_cpu as soon as enqueue_task_scx() has read it, as
was done before that commit. The source side is unaffected.

Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
Cc: stable@xxxxxxxxxxxxxxx # 7.1.x: 18d62044cda7: sched_ext: Preserve rq tracking across local DSQ dispatch
Cc: stable@xxxxxxxxxxxxxxx # 7.1.x
Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-5.5
Signed-off-by: Kuba Piecuch <jpiecuch@xxxxxxxxxx>
---
 kernel/sched/ext/ext.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 493b7aac7087..5209510d6a5e 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -2152,6 +2152,13 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_
 	int sticky_cpu = p->scx.sticky_cpu;
 	u64 enq_flags = core_enq_flags | rq->scx.remote_activate_enq_flags;
 
+	/*
+	 * An SCX-internal migration ends on arrival. Clear sticky_cpu so @p can
+	 * leave custody when inserted into the destination DSQ.
+	 */
+	if (sticky_cpu >= 0)
+		p->scx.sticky_cpu = -1;
+
 	/*
 	 * SCX_RQ_IN_WAKEUP promises a task_woken_scx() call once this enqueue
 	 * returns. Only the core's wakeup path delivers one. The flags stashed
@@ -2190,9 +2197,6 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_
 		dl_server_start(&rq->ext_server);
 
 	scx_do_enqueue_task(rq, p, enq_flags, sticky_cpu);
-
-	if (sticky_cpu >= 0)
-		p->scx.sticky_cpu = -1;
 out:
 	rq->scx.flags &= ~SCX_RQ_IN_WAKEUP;
 
-- 
2.56.0.rc1.315.gc6ed9934b7-goog