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

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Message-ID: <20251211224809.3383633-3-tj@kernel.org>
Date: Thu, 11 Dec 2025 12:48:09 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>,
	David Vernet <void@...ifault.com>
Cc: Emil Tsalapatis <emil@...alapatis.com>,
	linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev,
	Tejun Heo <tj@...nel.org>
Subject: [PATCH 2/2] sched_ext: Fix missing post-enqueue handling in move_local_task_to_local_dsq()

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.

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@...nel.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);
 }

 /**