[PATCH 6/6] memcg: fix performance of mem_cgroup_begin_update_page_stat()

[KAMEZAWA Hiroyuki]

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.

Acked-by: Greg Thelen <gthelen@xxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Changelog since v5
 - fixed patch description.
---
 include/linux/memcontrol.h |    5 +++--
 mm/memcontrol.c            |    7 ++++++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 75b6307..0710116 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -143,7 +143,7 @@ static inline bool mem_cgroup_disabled(void)
 
 void __mem_cgroup_begin_update_page_stat(struct page *page,
 					bool *lock, unsigned long *flags);
-
+extern atomic_t memcg_moving;
 static inline void mem_cgroup_begin_update_page_stat(struct page *page,
 					bool *lock, unsigned long *flags)
 {
@@ -151,7 +151,8 @@ static inline void mem_cgroup_begin_update_page_stat(struct page *page,
 		return;
 	rcu_read_lock();
 	*lock = false;
-	return __mem_cgroup_begin_update_page_stat(page, lock, flags);
+	if (atomic_read(&memcg_moving))
+		return __mem_cgroup_begin_update_page_stat(page, lock, flags);
 }
 
 void __mem_cgroup_end_update_page_stat(struct page *page,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e19cffb..7dbaa3f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1273,8 +1273,11 @@ int mem_cgroup_swappiness(struct mem_cgroup *memcg)
  *         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();
 }
@@ -1285,8 +1288,10 @@ 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);
+	}
 }
 
 /*
-- 
1.7.4.1


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