Re: [PATCH 2/2] sched_ext: Fix missing post-enqueue handling in move_local_task_to_local_dsq()

[Andrea Righi <arighi@nvidia.com>]

On Thu, Dec 11, 2025 at 12:48:09PM -1000, Tejun Heo wrote:
> move_local_task_to_local_dsq() is used when moving a task from a non-local
> DSQ to a local DSQ on the same CPU. It directly manipulates the local DSQ
> without going through dispatch_enqueue() and was missing the post-enqueue
> handling that triggers preemption when SCX_ENQ_PREEMPT is set or the
> enqueued task should preempt a lower priority task.

Ditto, lower sched class / idle task maybe?

Thanks,
-Andrea

> 
> This was fine when the function was only used from consume_dispatch_q()
> where the CPU is already vacant and looking for the next task. However, the
> function is now also used by move_task_between_dsqs() which backs
> scx_bpf_dsq_move() which may be called while the CPU is busy.
> 
> Add local_dsq_post_enq() call to move_local_task_to_local_dsq(). As the
> dispatch path doesn't need post-enqueue handling, add SCX_RQ_IN_BALANCE
> early exit to keep consume_dispatch_q() behavior unchanged and avoid
> triggering unnecessary resched when scx_bpf_dsq_move() is used from the
> dispatch path.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  kernel/sched/ext.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -981,6 +981,14 @@ static void local_dsq_post_enq(struct sc
>  	struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);
>  	bool preempt = false;
>  
> +	/*
> +	 * If @rq is in balance, the CPU is already vacant and looking for the
> +	 * next task to run. No need to preempt or trigger resched after moving
> +	 * @p into its local DSQ.
> +	 */
> +	if (rq->scx.flags & SCX_RQ_IN_BALANCE)
> +		return;
> +
>  	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
>  	    rq->curr->sched_class == &ext_sched_class) {
>  		rq->curr->scx.slice = 0;
> @@ -1629,6 +1637,8 @@ static void move_local_task_to_local_dsq
>  
>  	dsq_mod_nr(dst_dsq, 1);
>  	p->scx.dsq = dst_dsq;
> +
> +	local_dsq_post_enq(dst_dsq, p, enq_flags);
>  }
>  
>  /**