memcg: fix performance of mem_cgroup_begin_update_page_stat()

mem_cgroup_begin_update_page_stat() should be very fast because it's
called very frequently.  Now, it needs to look up page_cgroup and its
memcg....this is slow.

This patch adds a global variable to check "any memcg is moving or not".
With this, the caller doesn't need to visit page_cgroup and memcg.

Here is a test result.  A test program makes page faults onto a file,
MAP_SHARED and makes each page's page_mapcount(page) > 1, and free the
range by madvise() and page fault again.  This program causes 26214400
times of page fault onto a file(size was 1G.) and shows shows the cost of
mem_cgroup_begin_update_page_stat().

Before this patch for mem_cgroup_begin_update_page_stat()

    [kamezawa@bluextal test]$ time ./mmap 1G

    real    0m21.765s
    user    0m5.999s
    sys     0m15.434s

    27.46%     mmap  mmap               [.] reader
    21.15%     mmap  [kernel.kallsyms]  [k] page_fault
     9.17%     mmap  [kernel.kallsyms]  [k] filemap_fault
     2.96%     mmap  [kernel.kallsyms]  [k] __do_fault
     2.83%     mmap  [kernel.kallsyms]  [k] __mem_cgroup_begin_update_page_stat

After this patch

    [root@bluextal test]# time ./mmap 1G

    real    0m21.373s
    user    0m6.113s
    sys     0m15.016s

In usual path, calls to __mem_cgroup_begin_update_page_stat() goes away.

Note: we may be able to remove this optimization in future if
      we can get pointer to memcg directly from struct page.

[akpm@linux-foundation.org: don't return a void]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Greg Thelen <gthelen@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -144,6 +144,8 @@ static inline bool mem_cgroup_disabled(void)
 void __mem_cgroup_begin_update_page_stat(struct page *page, bool *locked,
 					 unsigned long *flags);
 
+extern atomic_t memcg_moving;
+
 static inline void mem_cgroup_begin_update_page_stat(struct page *page,
 					bool *locked, unsigned long *flags)
 {
@@ -151,7 +153,8 @@ static inline void mem_cgroup_begin_update_page_stat(struct page *page,
 		return;
 	rcu_read_lock();
 	*locked = false;
-	return __mem_cgroup_begin_update_page_stat(page, locked, flags);
+	if (atomic_read(&memcg_moving))
+		__mem_cgroup_begin_update_page_stat(page, locked, flags);
 }
 
 void __mem_cgroup_end_update_page_stat(struct page *page,

mm/memcontrol.c

@@ -1306,8 +1306,13 @@ int mem_cgroup_swappiness(struct mem_cgroup *memcg)
  *                                              rcu_read_unlock()
  *         start move here.
  */
+
+/* for quick checking without looking up memcg */
+atomic_t memcg_moving __read_mostly;
+
 static void mem_cgroup_start_move(struct mem_cgroup *memcg)
 {
+	atomic_inc(&memcg_moving);
 	atomic_inc(&memcg->moving_account);
 	synchronize_rcu();
 }
@@ -1318,8 +1323,10 @@ static void mem_cgroup_end_move(struct mem_cgroup *memcg)
 	 * Now, mem_cgroup_clear_mc() may call this function with NULL.
 	 * We check NULL in callee rather than caller.
 	 */
-	if (memcg)
+	if (memcg) {
+		atomic_dec(&memcg_moving);
 		atomic_dec(&memcg->moving_account);
+	}
 }
 
 /*