[PATCH sched_ext/for-6.12-fixes] Disable SM_IDLE/rq empty path when scx_enabled

[Pat Somaru <patso@likewhatevs.io>]

Disable the rq empty path when scx is enabled. SCX must consult the
BPF scheduler (via the dispatch path in balance) to determine if rq
is empty.

This fixes stalls when scx is enabled.

Signed-off-by: Pat Somaru <patso@likewhatevs.io>
Fixes: 3dcac251b066 ("sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()")
---
 kernel/sched/core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c415d61a646b5..6a1f662a5e118 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6591,7 +6591,11 @@ static void __sched notrace __schedule(int sched_mode)
 	 */
 	prev_state = READ_ONCE(prev->__state);
 	if (sched_mode == SM_IDLE) {
-		if (!rq->nr_running) {
+		/*
+		 * scx needs to check the bpf scheduler to determine
+		 * if rq is empty, so disable this path for it.
+		 */
+		if (!rq->nr_running && !scx_enabled()) {
 			next = prev;
 			goto picked;
 		}
-- 
2.44.2