On Fri, Feb 17, 2012 at 06:28:51PM +0900, KAMEZAWA Hiroyuki wrote:

quoted

From 07d3ce332ee4bc1eaef4b8fb2019b0c06bd7afb1 Mon Sep 17 00:00:00 2001

From: KAMEZAWA Hiroyuki <redacted>
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 "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 <redacted>
Signed-off-by: KAMEZAWA Hiroyuki <redacted>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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