linux-kernel - [PATCH v2 02/21] mm: memcontrol: remove kmemcg

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Message-Id: <20210527062148.9361-3-songmuchun@bytedance.com>
Date:   Thu, 27 May 2021 14:21:29 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     willy@...radead.org, akpm@...ux-foundation.org, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com, shakeelb@...gle.com,
        guro@...com, shy828301@...il.com, alexs@...nel.org,
        richard.weiyang@...il.com, david@...morbit.com,
        trond.myklebust@...merspace.com, anna.schumaker@...app.com
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-nfs@...r.kernel.org,
        zhengqi.arch@...edance.com, duanxiongchun@...edance.com,
        fam.zheng@...edance.com, Muchun Song <songmuchun@...edance.com>
Subject: [PATCH v2 02/21] 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 the parent cgroup. This already makes 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. It is just
waste CPU cycles. Just remove those redundant code.

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

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index db29b96f7311..9add859f69d7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3654,8 +3654,7 @@ 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)
@@ -3672,22 +3671,7 @@ 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