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

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put_prev_task_scx() warns when a runnable task drops to a lower sched_class
without SCX_OPS_ENQ_LAST, on the assumption that balance_one() would have
kept it running. Core scheduling breaks that: a forced-idle SMT sibling
reschedules through the core_pick fast path in pick_next_task(), which skips
pick_task_scx() and thus balance_one(), so a runnable task can drop to idle
with ENQ_LAST unset.

Gate the warning on sched_cpu_cookie_match(): a cookie mismatch means core
scheduling forced the idle, while a match (or core scheduling off) still
catches a genuine missing-ENQ_LAST drop.

Fixes: 7c65ae81ea ("sched_ext: Don't call put_prev_task_scx() before picking the next task")
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo
2026-06-29 12:55:48 -10:00
parent cb36d81e75
commit b7d9c359e5
1 changed files with 6 additions and 1 deletions
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7
kernel/sched/ext/ext.c
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@@ -3090,9 +3090,14 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
* sched_class, %SCX_OPS_ENQ_LAST must be set. Tell
* ops.enqueue() that @p is the only one available for this cpu,
* which should trigger an explicit follow-up scheduling event.
*
* Core scheduling can force this CPU idle while @p stays
* runnable. @p's cookie then won't match the core's, so skip
* the warning in that case.
*/
if (next && sched_class_above(&ext_sched_class, next->sched_class)) {
WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_LAST));
WARN_ON_ONCE(sched_cpu_cookie_match(rq, p) &&
!(sch->ops.flags & SCX_OPS_ENQ_LAST));
do_enqueue_task(rq, p, SCX_ENQ_LAST, -1);
} else {
do_enqueue_task(rq, p, 0, -1);