[PATCH] sched/ext: Suppress warning in __this_cpu_write() by disabling preemption

[Breno Leitao]

__this_cpu_write() emits a warning if used with preemption enabled.

Function update_locked_rq() might be called with preemption enabled,
which causes the following warning:

	BUG: using __this_cpu_write() in preemptible [00000000] code: scx_layered_6-9/68770

Disable preemption around the __this_cpu_write() call in
update_locked_rq() to suppress the warning, without affecting behavior.

If preemption triggers a  jump to another CPU during the callback it's
fine, since we would track the rq state on the other CPU with its own
local variable.

Suggested-by: Andrea Righi <arighi@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Fixes: 18853ba782bef ("sched_ext: Track currently locked rq")
Acked-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/ext.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index b498d867ba210..24fcbd7331f73 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1258,7 +1258,14 @@ static inline void update_locked_rq(struct rq *rq)
 	 */
 	if (rq)
 		lockdep_assert_rq_held(rq);
+	/*
+	 * __this_cpu_write() emits a warning when used with preemption enabled.
+	 * While there's no functional issue if the callback runs on another
+	 * CPU, we disable preemption here solely to suppress that warning.
+	 */
+	preempt_disable();
 	__this_cpu_write(locked_rq, rq);
+	preempt_enable();
 }
 
 /*

---
base-commit: 155a3c003e555a7300d156a5252c004c392ec6b0
change-id: 20250716-scx_warning-5143cf17f806

Best regards,
--  
Breno Leitao <leitao@xxxxxxxxxx>