linux-kernel - Re: [PATCH 2/2] sched_ext: Fix missing post-enqueue handling in move_local_task_to_local

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Message-Id: <DEW09WJ1U1FU.IQ9B68XD9VAS@etsalapatis.com>
Date: Fri, 12 Dec 2025 00:57:42 -0500
From: "Emil Tsalapatis" <emil@...alapatis.com>
To: "Tejun Heo" <tj@...nel.org>, "Andrea Righi" <arighi@...dia.com>,
 "Changwoo Min" <changwoo@...lia.com>, "David Vernet" <void@...ifault.com>
Cc: <linux-kernel@...r.kernel.org>, <sched-ext@...ts.linux.dev>,
 <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched_ext: Fix missing post-enqueue handling in
 move_local_task_to_local_dsq()

On Thu Dec 11, 2025 at 8:45 PM EST, 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 idle
> task is running.
>
> The function is used by move_task_between_dsqs() which backs
> scx_bpf_dsq_move() and 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.
>
> Fixes: 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()")
> Cc: stable@...r.kernel.org # v6.12+
> Reviewed-by: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Tejun Heo <tj@...nel.org>

Reviewed-by: Emil Tsalapatis <emil@...alapatis.com>

> ---
>  kernel/sched/ext.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index c78efa99406f..695503a2f7d1 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -988,6 +988,14 @@ static void local_dsq_post_enq(struct scx_dispatch_q *dsq, struct task_struct *p
>  	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;
> @@ -1636,6 +1644,8 @@ static void move_local_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
>  
>  	dsq_mod_nr(dst_dsq, 1);
>  	p->scx.dsq = dst_dsq;
> +
> +	local_dsq_post_enq(dst_dsq, p, enq_flags);
>  }
>  
>  /**