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Subject[PATCH 6/6 v4] memcg: fix performance of mem_cgroup_begin_update_page_stat()
From 3377fd7b6e23a5d2a368c078eae27e2b49c4f4aa Mon Sep 17 00:00:00 2001
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Date: Mon, 6 Feb 2012 12:14:47 +0900
Subject: [PATCH 6/6] 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 "a memcg is moving or not".
By 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.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
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 3f3ef33..3df9979 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, bool *lock,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ec55880..fd02fab 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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