[PATCH] sched_ext: Clear direct dispatch state on dequeue when dsq is NULL

[Daniel Hodges <hodgesd@meta.com>]

When a task is direct-dispatched from ops.select_cpu() or ops.enqueue(),
ddsp_dsq_id is set to indicate the target DSQ. If the task is dequeued
before dispatch_enqueue() completes (e.g., task killed or receives a
signal), dispatch_dequeue() is called with dsq == NULL.

In this case, the task is unlinked from ddsp_deferred_locals and
holding_cpu is cleared, but ddsp_dsq_id and ddsp_enq_flags are left
stale. On the next wakeup, when ops.select_cpu() calls
scx_bpf_dsq_insert(), mark_direct_dispatch() finds ddsp_dsq_id already
set and triggers:

  WARNING: CPU: 56 PID: 2323042 at kernel/sched/ext.c:2157
           scx_bpf_dsq_insert+0x16b/0x1d0

Fix this by clearing ddsp_dsq_id and ddsp_enq_flags in dispatch_dequeue()
when dsq is NULL, ensuring clean state for subsequent wakeups.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Daniel Hodges <hodgesd@meta.com>
---
 kernel/sched/ext.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index b563b8c3fd24..fdfef3fd8814 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1143,20 +1143,28 @@ static void dispatch_dequeue(struct rq *rq, struct task_struct *p)
 
 		/*
 		 * When dispatching directly from the BPF scheduler to a local
 		 * DSQ, the task isn't associated with any DSQ but
 		 * @p->scx.holding_cpu may be set under the protection of
 		 * %SCX_OPSS_DISPATCHING.
 		 */
 		if (p->scx.holding_cpu >= 0)
 			p->scx.holding_cpu = -1;
 
+		/*
+		 * Clear direct dispatch state. The task may have been
+		 * direct-dispatched from ops.select_cpu() or ops.enqueue()
+		 * but dequeued before the dispatch completed.
+		 */
+		p->scx.ddsp_dsq_id = SCX_DSQ_INVALID;
+		p->scx.ddsp_enq_flags = 0;
+
 		return;
 	}
 
 	if (!is_local)
 		raw_spin_lock(&dsq->lock);
 
 	/*
 	 * Now that we hold @dsq->lock, @p->holding_cpu and @p->scx.dsq_* can't
 	 * change underneath us.
 	*/
-- 
2.47.3