[PATCH 2/9] mm: memcontrol: remove kmemcg_id reparenting

[Muchun Song <songmuchun@bytedance.com>]

Since slab objects and kmem pages are charged to object cgroup instead
of memory cgroup, memcg_reparent_objcgs() will reparent this cgroup and
all its descendants to the parent cgroup. It means that the new parent
memory cgroup can be returned by mem_cgroup_from_obj() which is called
from list_lru_from_kmem(). This can make further list_lru_add()'s add
elements to the parent's list. So we do not need to change kmemcg_id
of an offline cgroup to its parent's id. This is just waste CPU cycles.
Just remove those redundant code.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memcontrol.c | 20 ++------------------
 1 file changed, 2 insertions(+), 18 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 64ada9e650a5..21e12312509c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3485,8 +3485,7 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
-	struct cgroup_subsys_state *css;
-	struct mem_cgroup *parent, *child;
+	struct mem_cgroup *parent;
 	int kmemcg_id;
 
 	if (memcg->kmem_state != KMEM_ONLINE)
@@ -3503,22 +3502,7 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 	kmemcg_id = memcg->kmemcg_id;
 	BUG_ON(kmemcg_id < 0);
 
-	/*
-	 * Change kmemcg_id of this cgroup and all its descendants to the
-	 * parent's id, and then move all entries from this cgroup's list_lrus
-	 * to ones of the parent. After we have finished, all list_lrus
-	 * corresponding to this cgroup are guaranteed to remain empty. The
-	 * ordering is imposed by list_lru_node->lock taken by
-	 * memcg_drain_all_list_lrus().
-	 */
-	rcu_read_lock(); /* can be called from css_free w/o cgroup_mutex */
-	css_for_each_descendant_pre(css, &memcg->css) {
-		child = mem_cgroup_from_css(css);
-		BUG_ON(child->kmemcg_id != kmemcg_id);
-		child->kmemcg_id = parent->kmemcg_id;
-	}
-	rcu_read_unlock();
-
+	/* memcg_reparent_objcgs() must be called before this. */
 	memcg_drain_all_list_lrus(kmemcg_id, parent);
 
 	memcg_free_cache_id(kmemcg_id);
-- 
2.11.0