From: Sha Zhengju <redacted>

It's inspired by a similar optimization from Glauber Costa
(memcg: make it suck faster; https://lkml.org/lkml/2012/9/25/154).
Here we use jump label to patch the memcg page stat accounting code
in or out when not used. when the first non-root memcg comes to
life the code is patching in otherwise it is out.

Signed-off-by: Sha Zhengju <redacted>
---
  include/linux/memcontrol.h |    9 +++++++++
  mm/memcontrol.c            |    8 ++++++++
  2 files changed, 17 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 1d22b81..3c4430c 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h

@@ -56,6 +56,9 @@ struct mem_cgroup_reclaim_cookie {
  };
  
  #ifdef CONFIG_MEMCG
+
+extern struct static_key memcg_in_use_key;
+
  /*
   * All "charge" functions with gfp_mask should use GFP_KERNEL or
   * (gfp_mask & GFP_RECLAIM_MASK). In current implementatin, memcg doesn't

@@ -158,6 +161,9 @@ extern atomic_t memcg_moving;
  static inline void mem_cgroup_begin_update_page_stat(struct page *page,
  					bool *locked, unsigned long *flags)
  {
+	if (!static_key_false(&memcg_in_use_key))
+		return;
+
  	if (mem_cgroup_disabled())
  		return;
  	rcu_read_lock();

@@ -171,6 +177,9 @@ void __mem_cgroup_end_update_page_stat(struct page *page,
  static inline void mem_cgroup_end_update_page_stat(struct page *page,
  					bool *locked, unsigned long *flags)
  {
+	if (!static_key_false(&memcg_in_use_key))
+		return;
+
  	if (mem_cgroup_disabled())
  		return;
  	if (*locked)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0cb5187..a2f73d7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -531,6 +531,8 @@ enum res_type {
  #define MEM_CGROUP_RECLAIM_SHRINK_BIT	0x1
  #define MEM_CGROUP_RECLAIM_SHRINK	(1 << MEM_CGROUP_RECLAIM_SHRINK_BIT)
  
+struct static_key memcg_in_use_key;
+
  static void mem_cgroup_get(struct mem_cgroup *memcg);
  static void mem_cgroup_put(struct mem_cgroup *memcg);
  

@@ -2226,6 +2228,9 @@ void mem_cgroup_update_page_stat(struct page *page,
  	struct page_cgroup *pc = lookup_page_cgroup(page);
  	unsigned long uninitialized_var(flags);
  
+	if (!static_key_false(&memcg_in_use_key))
+		return;
+
  	if (mem_cgroup_disabled())
  		return;
  

@@ -6340,6 +6345,8 @@ mem_cgroup_css_alloc(struct cgroup *cont)
  		parent = mem_cgroup_from_cont(cont->parent);
  		memcg->use_hierarchy = parent->use_hierarchy;
  		memcg->oom_kill_disable = parent->oom_kill_disable;
+
+		static_key_slow_inc(&memcg_in_use_key);
  	}
  
  	if (parent && parent->use_hierarchy) {

@@ -6407,6 +6414,7 @@ static void mem_cgroup_css_free(struct cgroup *cont)
  	kmem_cgroup_destroy(memcg);
  
  	memcg_dangling_add(memcg);
+	static_key_slow_dec(&memcg_in_use_key);