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: 4c30f5ce4f7a ("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>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index c78efa99406fffc8649c33d00b3dd0b559e7031a..695503a2f7d1ed572745cfdf0b1fe7fcc48663e4 100644 (file)
--- 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);
 }
 
 /**