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 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: 4c30f5c ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()")
Cc: stable@vger.kernel.org # v6.12+
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

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);
 }
 
 /**