[PATCH v2] sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

[Tejun Heo <tj@kernel.org>]

put_prev_task_scx() warns when a runnable task drops to a lower
sched_class without SCX_OPS_ENQ_LAST, assuming balance_one() would
otherwise keep it running.

Under core scheduling that assumption is wrong: a forced-idle SMT sibling
reschedules through the core_pick fast path in pick_next_task(), which skips
pick_task_scx() for the CPU, so balance_one() never runs and a runnable task
can drop to idle with ENQ_LAST unset. Skip the warning when core scheduling
is enabled.

v2: Reworded the description per Kuba Piecuch's review. No code change.

Fixes: 7c65ae81ea86 ("sched_ext: Don't call put_prev_task_scx() before picking the next task")
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/ext.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 9c9cb9d08bca..503c4d2105ee 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -3092,7 +3092,9 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 		 * which should trigger an explicit follow-up scheduling event.
 		 */
 		if (next && sched_class_above(&ext_sched_class, next->sched_class)) {
-			WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_LAST));
+			/* core-sched can force cpu idle while @p is runnable */
+			if (!sched_core_enabled(rq))
+				WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_LAST));
 			do_enqueue_task(rq, p, SCX_ENQ_LAST, -1);
 		} else {
 			do_enqueue_task(rq, p, 0, -1);