[PATCH 2/2] sched_ext: Make scx_bpf_reenqueue_local() skip tasks that are being migrated

[Tejun Heo]

When a running task is migrated to another CPU, the stop_task is used to
preempt the running task and migrate it. This, expectedly, invokes
ops.cpu_release(). If the BPF scheduler then calls
scx_bpf_reenqueue_local(), it re-enqueues all tasks on the local DSQ
including the task which is being migrated.

This creates an unnecessary re-enqueue of a task which is about to be
deactivated and re-activated for migration anyway. It can also cause
confusion for the BPF scheduler as scx_bpf_task_cpu() of the task and its
allowed CPUs may not agree while migration is pending.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Fixes: 245254f7081d ("sched_ext: Implement sched_ext_ops.cpu_acquire/release()")
Cc: David Vernet <void@xxxxxxxxxxxxx>
---
 kernel/sched/ext.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 0fabfe19327c..f4a7abcec793 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5646,6 +5646,20 @@ __bpf_kfunc u32 scx_bpf_reenqueue_local(void)
 	 */
 	list_for_each_entry_safe(p, n, &rq->scx.local_dsq.list,
 				 scx.dsq_list.node) {
+		/*
+		 * If @p is being migrated, @p's current CPU may not agree with
+		 * its allowed CPUs and the migration_cpu_stop is about to
+		 * deactivate and re-activate @p anyway. Skip re-enqueueing.
+		 *
+		 * While racing sched property changes may also dequeue and
+		 * re-enqueue a migrating task while its current CPU and allowed
+		 * CPUs disagree, they use %ENQUEUE_RESTORE which is bypassed to
+		 * the current local DSQ for running tasks and thus are not
+		 * visible to the BPF scheduler.
+		 */
+		if (p->migration_pending)
+			continue;
+
 		dispatch_dequeue(rq, p);
 		list_add_tail(&p->scx.dsq_list.node, &tasks);
 	}
-- 
2.45.2