Merge branch 'for-6.14-fixes' into for-6.15

Pull to receive f3f08c3acfb8 ("sched_ext: Fix incorrect assumption about
migration disabled tasks in task_can_run_on_remote_rq()") which conflicts
with 26176116d931 ("sched_ext: Count SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE in
the right spot") in for-6.15.

1 files changed, 21 insertions, 8 deletions

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 90a66a2a43d5..98d5f2f68f38 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2447,6 +2447,25 @@ static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq,
 	SCHED_WARN_ON(task_cpu(p) == cpu);
 
 	/*
+	 * If @p has migration disabled, @p->cpus_ptr is updated to contain only
+	 * the pinned CPU in migrate_disable_switch() while @p is being switched
+	 * out. However, put_prev_task_scx() is called before @p->cpus_ptr is
+	 * updated and thus another CPU may see @p on a DSQ inbetween leading to
+	 * @p passing the below task_allowed_on_cpu() check while migration is
+	 * disabled.
+	 *
+	 * Test the migration disabled state first as the race window is narrow
+	 * and the BPF scheduler failing to check migration disabled state can
+	 * easily be masked if task_allowed_on_cpu() is done first.
+	 */
+	if (unlikely(is_migration_disabled(p))) {
+		if (enforce)
+			scx_ops_error("SCX_DSQ_LOCAL[_ON] cannot move migration disabled %s[%d] from CPU %d to %d",
+				      p->comm, p->pid, task_cpu(p), cpu);
+		return false;
+	}
+
+	/*
 	 * We don't require the BPF scheduler to avoid dispatching to offline
 	 * CPUs mostly for convenience but also because CPUs can go offline
 	 * between scx_bpf_dsq_insert() calls and here. Trigger error iff the
@@ -2454,17 +2473,11 @@ static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq,
 	 */
 	if (!task_allowed_on_cpu(p, cpu)) {
 		if (enforce)
-			scx_ops_error("SCX_DSQ_LOCAL[_ON] verdict target cpu %d not allowed for %s[%d]",
-				      cpu_of(rq), p->comm, p->pid);
+			scx_ops_error("SCX_DSQ_LOCAL[_ON] target CPU %d not allowed for %s[%d]",
+				      cpu, p->comm, p->pid);
 		return false;
 	}
 
-	/*
-	 * If @p has migration disabled, @p->cpus_ptr only contains its current
-	 * CPU and the above task_allowed_on_cpu() test should have failed.
-	 */
-	SCHED_WARN_ON(is_migration_disabled(p));
-
 	if (!scx_rq_online(rq)) {
 		if (enforce)
 			__scx_add_event(SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE, 1);