diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 8d57f7686bb0..c8a8ea2e5b98 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1487,6 +1487,21 @@ static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,
 	s.seq = 0;
 	atomic_set(&group->migr_state, s.state);
 
+	/*
+	 * If this is a new top-level, prepare its groupmask in advance.
+	 * This avoids accidents where yet another new top-level is
+	 * created in the future and made visible before the current groupmask.
+	 */
+	if (list_empty(&tmigr_level_list[lvl])) {
+		group->groupmask = BIT(0);
+		/*
+		 * The previous top level has prepared its groupmask already,
+		 * simply account it as the first child.
+		 */
+		if (lvl > 0)
+			group->num_children = 1;
+	}
+
 	timerqueue_init_head(&group->events);
 	timerqueue_init(&group->groupevt.nextevt);
 	group->groupevt.nextevt.expires = KTIME_MAX;
@@ -1550,8 +1565,20 @@ static void tmigr_connect_child_parent(struct tmigr_group *child,
 	raw_spin_lock_irq(&child->lock);
 	raw_spin_lock_nested(&parent->lock, SINGLE_DEPTH_NESTING);
 
+	if (activate) {
+		/*
+		 * @child is the old top and @parent the new one. In this
+		 * case groupmask is pre-initialized and @child already
+		 * accounted, along with its new sibling corresponding to the
+		 * CPU going up.
+		 */
+		WARN_ON_ONCE(child->groupmask != BIT(0) || parent->num_children != 2);
+	} else {
+		/* Adding @child for the CPU going up to @parent. */
+		child->groupmask = BIT(parent->num_children++);
+	}
+
 	child->parent = parent;
-	child->groupmask = BIT(parent->num_children++);
 
 	raw_spin_unlock(&parent->lock);
 	raw_spin_unlock_irq(&child->lock);