Re: [PATCH sched_ext/for-7.4-fixes] sched/core: Avoid requeueing a singleton balance callback

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 21, 2026 at 09:05:35AM +0200, Andrea Righi wrote:
> With proxy execution, __schedule() can call set_next_task_scx() with
> SNT_REPICK and then SNT_PICK for the same donor. Both calls queue the
> kick_sync balance callback while kick_sync_pending remains set.
> 
> When the callback is the only list entry, its next pointer is NULL, so
> the existing duplicate check misses it. Queueing it again links it to
> itself and causes the wait to run twice.
> 
> Also check whether the callback is already the head of the list to
> reject the second insertion.
> 
> This covers the kick_sync callback because nothing else is queued
> between SNT_REPICK and SNT_PICK. Requeueing the tail of a longer list
> remains possible and is outside the scope of this fix.

Cute.

> Fixes: e2859566ffbc ("sched_ext: Queue the kick_sync wait from set_next_task_scx()")
> Reported-by: Tejun Heo <tj@kernel.org>
> Link: https://lore.kernel.org/r/08cac02c604dd73ef96fc92eb29d692b@kernel.org
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> ---
> Resending including Peter, this touches kernel/sched/sched.h.
> 
> Original post:
> https://lore.kernel.org/r/20261020084400.845842-2-arighi@nvidia.com
> 
> The triggering commit e2859566ffbc is only available in sched_ext/for-7.4-fixes,
> so this fix is intended for the 7.4 cycle:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git for-7.4-fixes

OK, probably best if TJ routes it then? in which case:

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

>  kernel/sched/sched.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 499a5649582e7..2584d1a7f6d21 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2158,7 +2158,8 @@ queue_balance_callback(struct rq *rq,
>  	 * balance_push() is active, see the comment with
>  	 * balance_push_callback.
>  	 */
> -	if (unlikely(head->next || rq->balance_callback == &balance_push_callback))
> +	if (unlikely(head->next || rq->balance_callback == head ||
> +		     rq->balance_callback == &balance_push_callback))
>  		return;
>  
>  	head->func = func;
> -- 
> 2.56.0
>