timers/migration: Turn tmigr_hierarchy level_list into a flexible array

The level_list array is allocated separately right after the parent
struct. The size of the array is already known.

Move level_list to the struct tail as a flexible array member and fold the
two allocations into a single kzalloc_flex().

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: Claude:Opus-4.7
Link: https://patch.msgid.link/20260522231618.41622-1-rosenp@gmail.com

Author: neheb

kernel/time/timer_migration.c

@@ -1963,17 +1963,15 @@ static struct tmigr_hierarchy *tmigr_get_hierarchy(int cpu)
 	if (hier)
 		return hier;
 
-	hier = kzalloc(sizeof(*hier), GFP_KERNEL);
+	hier = kzalloc_flex(*hier, level_list, tmigr_hierarchy_levels);
 	if (!hier)
 		return ERR_PTR(-ENOMEM);
 
 	hier->cpumask = kzalloc(cpumask_size(), GFP_KERNEL);
-	if (!hier->cpumask)
-		goto err;
-
-	hier->level_list = kzalloc_objs(struct list_head, tmigr_hierarchy_levels);
-	if (!hier->level_list)
-		goto err;
+	if (!hier->cpumask) {
+		kfree(hier);
+		return ERR_PTR(-ENOMEM);
+	}
 
 	for (int i = 0; i < tmigr_hierarchy_levels; i++)
 		INIT_LIST_HEAD(&hier->level_list[i]);
@@ -1982,10 +1980,6 @@ static struct tmigr_hierarchy *tmigr_get_hierarchy(int cpu)
 	list_add_tail(&hier->node, &tmigr_hierarchy_list);
 
 	return hier;
-err:
-	kfree(hier->cpumask);
-	kfree(hier);
-	return ERR_PTR(-ENOMEM);
 }
 
 static int tmigr_connect_old_root(struct tmigr_hierarchy *hier, int cpu,

kernel/time/timer_migration.h

@@ -9,19 +9,18 @@
  * struct tmigr_hierarchy - a hierarchy associated to a given CPU capacity.
  *                          Homogeneous systems have only one hierarchy.
  *                          Heterogenous have one hierarchy per CPU capacity.
- * @level_list:	Per level lists of tmigr groups
  * @cpumask:	CPUs belonging to this hierarchy
  * @root:	The current root of the hierarchy
  * @capacity:	CPU capacity associated to this hierarchy
  * @node:	Node in the global hierarchy list
+ * @level_list:	Per level lists of tmigr groups
  */
 struct tmigr_hierarchy {
-	struct list_head	*level_list;
 	struct cpumask		*cpumask;
 	struct tmigr_group	*root;
 	unsigned long		capacity;
 	struct list_head	node;
-
+	struct list_head	level_list[];
 };
 
 /**