Re: [PATCH v3] sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

[Andrea Righi <arighi@nvidia.com>]

On Mon, Jun 29, 2026 at 12:55:48PM -1000, Tejun Heo wrote:
> 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.
> 
> v3: Gate the warning on sched_cpu_cookie_match() so it is suppressed only on
>     the forced-idle CPU instead of whenever core scheduling is enabled, so a
>     genuine missing-ENQ_LAST drop is still caught (Andrea Righi).
> v2: Reworded the description (Kuba Piecuch).
> 
> Fixes: 7c65ae81ea86 ("sched_ext: Don't call put_prev_task_scx() before picking the next task")
> Signed-off-by: Tejun Heo <tj@kernel.org>

Looks good.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/sched/ext/ext.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 3b2e13bc924b..e75e2fd5ab7e 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -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);