[patch 062/262] mm: memcontrol: remove kmemcg_id reparenting

Andrew Morton
2021-11-05T20:37:56Z

From: Muchun Song <songmuchun@bytedance.com>
Subject: mm: memcontrol: remove kmemcg_id reparenting

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 its parent cgroup. This already makes further
list_lru_add()'s add elements to the parent's list. So it is unnecessary
to change kmemcg_id of an offline cgroup to its parent's id. It just
wastes CPU cycles. Just to remove those redundant code.

Link: https://lkml.kernel.org/r/20211025125102.56533-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/memcontrol.c | 19 ++++---------------
1 file changed, 4 insertions(+), 15 deletions(-)

--- a/mm/memcontrol.c~mm-memcontrol-remove-kmemcg_id-reparenting
+++ a/mm/memcontrol.c
@@ -3650,8 +3650,7 @@ static int memcg_online_kmem(struct mem_

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)
@@ -3669,21 +3668,11 @@ static void memcg_offline_kmem(struct me
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
+ * After we have finished memcg_reparent_objcgs(), 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_drain_all_list_lrus(kmemcg_id, parent);

memcg_free_cache_id(kmemcg_id);
_