[PATCH v2 2/3] mm: memcg: redirect stats updates of dying memcgs for all hierarchies

[Hui Zhu]

From: Hui Zhu <zhuhui@xxxxxxxxxx>

get_non_dying_memcg_start() redirects the stat updates of a dying memcg to
its closest non-dying ancestor, but only on cgroup v1; on cgroup v2 the
stats keep being accounted to the dying memcg itself.

The previous patch restored lruvec_page_state_local() in
count_shadow_nodes(), which reads those state_locals on cgroup v2 too, so
apply the redirection to all hierarchies.  Offlining is rare, so the added
cost on the stat update fast path is limited to an rcu_read_lock() and a
css_is_dying() check; the upward walk happens only while a memcg is dying.

Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
---
 mm/memcontrol.c | 30 +++++-------------------------
 1 file changed, 5 insertions(+), 25 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8319ad8c5c23..b3d1ac3fe0aa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -805,20 +805,14 @@ static long memcg_state_val_in_pages(int idx, long val)
 	return val < 0 ? -res : res;
 }
 
-#ifdef CONFIG_MEMCG_V1
 /*
- * Used in mod_memcg_state() and mod_memcg_lruvec_state() to avoid race with
- * reparenting of non-hierarchical state_locals.
+ * Used in mod_memcg_state() and mod_memcg_lruvec_state() to avoid race
+ * with reparenting of non-hierarchical state_locals.  Offlining a
+ * memcg is rare, so do the redirection for all cgroup hierarchies.
  */
-static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg,
-							   bool *rcu_locked)
+static inline struct mem_cgroup *
+get_non_dying_memcg_start(struct mem_cgroup *memcg, bool *rcu_locked)
 {
-	/* Rebinding can cause this value to be changed at runtime */
-	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
-		*rcu_locked = false;
-		return memcg;
-	}
-
 	rcu_read_lock();
 	*rcu_locked = true;
 
@@ -830,22 +824,8 @@ static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *me
 
 static inline void get_non_dying_memcg_end(bool rcu_locked)
 {
-	if (!rcu_locked)
-		return;
-
 	rcu_read_unlock();
 }
-#else
-static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg,
-							   bool *rcu_locked)
-{
-	return memcg;
-}
-
-static inline void get_non_dying_memcg_end(bool rcu_locked)
-{
-}
-#endif
 
 static void __mod_memcg_state(struct mem_cgroup *memcg,
 			      enum memcg_stat_item idx, long val)
-- 
2.53.0