[PATCH 0/6] page cgroup diet v5

[PATCH 0/6] page cgroup diet v5

[KAMEZAWA Hiroyuki]

This patch set is for removing 2 flags PCG_FILE_MAPPED and PCG_MOVE_LOCK on
page_cgroup->flags. After this, page_cgroup has only 3bits of flags.
And, this set introduces a new method to update page status accounting per memcg.
With it, we don't have to add new flags onto page_cgroup if 'struct page' has
information. This will be good for avoiding a new flag for page_cgroup.

Fixed pointed out parts.
 - added more comments
 - fixed texts
 - removed redundant arguments.

Passed some tests on 3.3.0-rc3-next-20120216.

Thanks,
-Kame

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[PATCH 1/6] memcg: remove EXPORT_SYMBOL(mem_cgroup_update_page_stat)

[KAMEZAWA Hiroyuki]

[PATCH 2/6] memcg: simplify move_account() check.

[KAMEZAWA Hiroyuki]

[PATCH 3/6] memcg: remove PCG_MOVE_LOCK flag from page_cgroup

[KAMEZAWA Hiroyuki]

[PATCH 4/6] memcg: use new logic for page stat accounting.

[KAMEZAWA Hiroyuki]

[PATCH 5/6] memcg: remove PCG_FILE_MAPPED

[KAMEZAWA Hiroyuki]

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

[KAMEZAWA Hiroyuki]

Re: [PATCH 0/6] page cgroup diet v5

[KAMEZAWA Hiroyuki]

[-- Attachment #1: Type: text/plain, Size: 1452 bytes --]

On Fri, 17 Feb 2012 18:24:26 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> 
> This patch set is for removing 2 flags PCG_FILE_MAPPED and PCG_MOVE_LOCK on
> page_cgroup->flags. After this, page_cgroup has only 3bits of flags.
> And, this set introduces a new method to update page status accounting per memcg.
> With it, we don't have to add new flags onto page_cgroup if 'struct page' has
> information. This will be good for avoiding a new flag for page_cgroup.
> 
> Fixed pointed out parts.
>  - added more comments
>  - fixed texts
>  - removed redundant arguments.
> 
> Passed some tests on 3.3.0-rc3-next-20120216.
> 

Here is a micro benchmark test before/after this series.
mmap 1G bytes twice and repeat fault->drop repeatedly. (test program is attached)

== Before == 3 runs after 1st run
[root@bluextal test]# time ./mmap 1G

real    0m21.053s
user    0m6.046s
sys     0m14.743s
[root@bluextal test]# time ./mmap 1G

real    0m21.302s
user    0m6.027s
sys     0m14.979s
[root@bluextal test]# time ./mmap 1G

real    0m21.061s
user    0m6.020s
sys     0m14.722s

== After == 3 runs after 1st run
[root@bluextal test]# time ./mmap 1G

real    0m20.969s
user    0m5.960s
sys     0m14.777s
[root@bluextal test]# time ./mmap 1G

real    0m20.968s
user    0m6.069s
sys     0m14.650s
[root@bluextal test]# time ./mmap 1G

real    0m21.164s
user    0m6.152s
sys     0m14.707s


I think there is no regression.


Thanks,
-Kame




[-- Attachment #2: mmap.c --]
[-- Type: text/x-csrc, Size: 818 bytes --]

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#include <fcntl.h>

void reader(int fd, int size)
{
	int i, off, x;
	char *addr;

	addr = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
	for (i = 0; i < 100; i++) {
		for(off = 0; off < size; off += 4096) {
			x += *(addr + off);
		}
		madvise(addr, size, MADV_DONTNEED);
	}
}

int main(int argc, char *argv[])
{
	int fd;
	char *addr, *c;
	unsigned long size;
	struct stat statbuf;

	fd = open(argv[1], O_RDONLY);
	if (fd < 0) {
		perror("cannot open file");
		return 1;
	}

	if (fstat(fd, &statbuf)) {
		perror("fstat failed");
		return 1;
	}
	size = statbuf.st_size;
	/* mmap in 2 place. */
	addr = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
	mlock(addr, size);
	reader(fd, size);
}

Re: [PATCH 0/6] page cgroup diet v5

[Andrew Morton]

On Fri, 17 Feb 2012 18:24:26 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> 
> This patch set is for removing 2 flags PCG_FILE_MAPPED and PCG_MOVE_LOCK on
> page_cgroup->flags. After this, page_cgroup has only 3bits of flags.
> And, this set introduces a new method to update page status accounting per memcg.
> With it, we don't have to add new flags onto page_cgroup if 'struct page' has
> information. This will be good for avoiding a new flag for page_cgroup.
> 
> Fixed pointed out parts.
>  - added more comments
>  - fixed texts
>  - removed redundant arguments.
> 

I tweaked a few things here.  Renamed "bool lock;" to "bool locked" in
several places.  Also the void-returning
mem_cgroup_begin_update_page_stat() was doing an explicit return which
is OK C but pointless and misleading.


Also, this has been bugging me for a while ;)

From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm/memcontrol.c: s/stealed/stolen/

A grammatical fix.

Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff -puN mm/memcontrol.c~a mm/memcontrol.c
--- a/mm/memcontrol.c~a
+++ a/mm/memcontrol.c
@@ -1299,8 +1299,8 @@ static void mem_cgroup_end_move(struct m
 /*
  * 2 routines for checking "mem" is under move_account() or not.
  *
- * mem_cgroup_stealed() - checking a cgroup is mc.from or not. This is used
- *			  for avoiding race in accounting. If true,
+ * mem_cgroup_stolen() -  checking whether a cgroup is mc.from or not. This
+ *			  is used for avoiding races in accounting.  If true,
  *			  pc->mem_cgroup may be overwritten.
  *
  * mem_cgroup_under_move() - checking a cgroup is mc.from or mc.to or
@@ -1308,7 +1308,7 @@ static void mem_cgroup_end_move(struct m
  *			  waiting at hith-memory prressure caused by "move".
  */
 
-static bool mem_cgroup_stealed(struct mem_cgroup *memcg)
+static bool mem_cgroup_stolen(struct mem_cgroup *memcg)
 {
 	VM_BUG_ON(!rcu_read_lock_held());
 	return atomic_read(&memcg->moving_account) > 0;
@@ -1356,7 +1356,7 @@ static bool mem_cgroup_wait_acct_move(st
  * Take this lock when
  * - a code tries to modify page's memcg while it's USED.
  * - a code tries to modify page state accounting in a memcg.
- * see mem_cgroup_stealed(), too.
+ * see mem_cgroup_stolen(), too.
  */
 static void move_lock_mem_cgroup(struct mem_cgroup *memcg,
 				  unsigned long *flags)
@@ -1899,9 +1899,9 @@ again:
 	 * If this memory cgroup is not under account moving, we don't
 	 * need to take move_lock_page_cgroup(). Because we already hold
 	 * rcu_read_lock(), any calls to move_account will be delayed until
-	 * rcu_read_unlock() if mem_cgroup_stealed() == true.
+	 * rcu_read_unlock() if mem_cgroup_stolen() == true.
 	 */
-	if (!mem_cgroup_stealed(memcg))
+	if (!mem_cgroup_stolen(memcg))
 		return;
 
 	move_lock_mem_cgroup(memcg, flags);
_

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Re: [PATCH 5/6] memcg: remove PCG_FILE_MAPPED

[Johannes Weiner]

On Fri, Feb 17, 2012 at 06:28:18PM +0900, KAMEZAWA Hiroyuki wrote:
> >From a19a8eae9c2a3dabb25a50c7a1b2e3a183935260 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Date: Thu, 2 Feb 2012 15:02:18 +0900
> Subject: [PATCH 5/6] memcg: remove PCG_FILE_MAPPED
> 
> with new lock scheme for updating memcg's page stat, we don't need
> a flag PCG_FILE_MAPPED which was duplicated information of page_mapped().
> 
> Acked-by: Greg Thelen <gthelen@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  include/linux/page_cgroup.h |    6 ------
>  mm/memcontrol.c             |    6 +-----
>  2 files changed, 1 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> index 7a3af74..a88cdba 100644
> --- a/include/linux/page_cgroup.h
> +++ b/include/linux/page_cgroup.h
> @@ -6,8 +6,6 @@ enum {
>  	PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */
>  	PCG_USED, /* this object is in use. */
>  	PCG_MIGRATION, /* under page migration */
> -	/* flags for mem_cgroup and file and I/O status */
> -	PCG_FILE_MAPPED, /* page is accounted as "mapped" */
>  	__NR_PCG_FLAGS,
>  };
>  
> @@ -66,10 +64,6 @@ TESTPCGFLAG(Used, USED)
>  CLEARPCGFLAG(Used, USED)
>  SETPCGFLAG(Used, USED)
>  
> -SETPCGFLAG(FileMapped, FILE_MAPPED)
> -CLEARPCGFLAG(FileMapped, FILE_MAPPED)
> -TESTPCGFLAG(FileMapped, FILE_MAPPED)
> -
>  SETPCGFLAG(Migration, MIGRATION)
>  CLEARPCGFLAG(Migration, MIGRATION)
>  TESTPCGFLAG(Migration, MIGRATION)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 86bdde0..e19cffb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1935,10 +1935,6 @@ void mem_cgroup_update_page_stat(struct page *page,
>  
>  	switch (idx) {
>  	case MEMCG_NR_FILE_MAPPED:
> -		if (val > 0)
> -			SetPageCgroupFileMapped(pc);
> -		else if (!page_mapped(page))
> -			ClearPageCgroupFileMapped(pc);
>  		idx = MEM_CGROUP_STAT_FILE_MAPPED;
>  		break;
>  	default:
> @@ -2559,7 +2555,7 @@ static int mem_cgroup_move_account(struct page *page,
>  
>  	move_lock_mem_cgroup(from, &flags);
>  
> -	if (PageCgroupFileMapped(pc)) {
> +	if (page_mapped(page)) {

As opposed to update_page_stat(), this runs against all types of
pages, so I think it should be

	if (!PageAnon(page) && page_mapped(page))

instead.

>  		/* Update mapped_file data for mem_cgroup */
>  		preempt_disable();
>  		__this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);

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Re: [PATCH 5/6] memcg: remove PCG_FILE_MAPPED

[Hillf Danton]

On Sat, Feb 18, 2012 at 9:39 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Fri, Feb 17, 2012 at 06:28:18PM +0900, KAMEZAWA Hiroyuki wrote:
>> @@ -2559,7 +2555,7 @@ static int mem_cgroup_move_account(struct page *page,
>>
>>       move_lock_mem_cgroup(from, &flags);
>>
>> -     if (PageCgroupFileMapped(pc)) {
>> +     if (page_mapped(page)) {
>
> As opposed to update_page_stat(), this runs against all types of
> pages, so I think it should be
>
>        if (!PageAnon(page) && page_mapped(page))
>
> instead.
>
Perhaps the following helper or similar needed,
along with page_mapped()

static inline bool page_is_file_mapping(struct page *page)
{
	struct address_space *mapping = page_mapping(page);

	return mapping && mapping != &swapper_space &&
		((unsigned long)mapping & PAGE_MAPPING_FLAGS) == 0;
}

>>               /* Update mapped_file data for mem_cgroup */
>>               preempt_disable();
>>               __this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);

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Re: [PATCH 5/6] memcg: remove PCG_FILE_MAPPED

[KAMEZAWA Hiroyuki]

On Sat, 18 Feb 2012 22:43:58 +0800
Hillf Danton <dhillf@gmail.com> wrote:

> On Sat, Feb 18, 2012 at 9:39 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > On Fri, Feb 17, 2012 at 06:28:18PM +0900, KAMEZAWA Hiroyuki wrote:
> >> @@ -2559,7 +2555,7 @@ static int mem_cgroup_move_account(struct page *page,
> >>
> >> A  A  A  move_lock_mem_cgroup(from, &flags);
> >>
> >> - A  A  if (PageCgroupFileMapped(pc)) {
> >> + A  A  if (page_mapped(page)) {
> >
> > As opposed to update_page_stat(), this runs against all types of
> > pages, so I think it should be
> >
> > A  A  A  A if (!PageAnon(page) && page_mapped(page))
> >
> > instead.
> >
> Perhaps the following helper or similar needed,
> along with page_mapped()
> 
> static inline bool page_is_file_mapping(struct page *page)
> {
> 	struct address_space *mapping = page_mapping(page);
> 
> 	return mapping && mapping != &swapper_space &&
> 		((unsigned long)mapping & PAGE_MAPPING_FLAGS) == 0;
> }
> 

Ah, thank you. I'll post a fix soon.

Ragard,
-Kame

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Re: [PATCH 1/6] memcg: remove EXPORT_SYMBOL(mem_cgroup_update_page_stat)

[Johannes Weiner]

On Fri, Feb 17, 2012 at 06:25:35PM +0900, KAMEZAWA Hiroyuki wrote:
> >From 7075e575cc6ee383f8fd161ba44c44a60c295d5f Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Date: Thu, 2 Feb 2012 12:05:41 +0900
> Subject: [PATCH 1/6] memcg: remove EXPORT_SYMBOL(mem_cgroup_update_page_stat)
> 
> >From the log, I guess EXPORT was for preparing dirty accounting.
> But _now_, we don't need to export this. Remove this for now.
> 
> Reviewed-by: Greg Thelen <gthelen@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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

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Re: [PATCH 2/6] memcg: simplify move_account() check.

[Johannes Weiner]

On Fri, Feb 17, 2012 at 06:26:12PM +0900, KAMEZAWA Hiroyuki wrote:
> >From 3b6620772d7fd7b2126d5253eafb6afaf4ed6e34 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Date: Thu, 2 Feb 2012 10:02:39 +0900
> Subject: [PATCH 2/6] memcg: simplify move_account() check.
> 
> In memcg, for avoiding take-lock-irq-off at accessing page_cgroup,
> a logic, flag + rcu_read_lock(), is used. This works as following
> 
>      CPU-A                     CPU-B
>                              rcu_read_lock()
>     set flag
>                              if(flag is set)
>                                    take heavy lock
>                              do job.
>     synchronize_rcu()        rcu_read_unlock()
>     take heavy lock.
> 
> In recent discussion, it's argued that using per-cpu value for this
> flag just complicates the code because 'set flag' is very rare.
> 
> This patch changes 'flag' implementation from percpu to atomic_t.
> This will be much simpler.
> 
> Changelog v5.
>  - removed redundant ().
>  - updated patch description.
> 
> Changelog: v4
>  - fixed many typos.
>  - fixed return value to be bool
>  - add comments.
> Changelog: v3
>  - this is a new patch since v3.
> 
> Acked-by: Greg Thelen <gthelen@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Much better!

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

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Re: [PATCH 3/6] memcg: remove PCG_MOVE_LOCK flag from page_cgroup

[Johannes Weiner]

On Fri, Feb 17, 2012 at 06:26:51PM +0900, KAMEZAWA Hiroyuki wrote:
> >From 62a96c625be0c30fc5828d88685b6873ed254060 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Date: Thu, 2 Feb 2012 11:49:59 +0900
> Subject: [PATCH 3/6] memcg: remove PCG_MOVE_LOCK flag from page_cgroup.
> 
> PCG_MOVE_LOCK is used for bit spinlock to avoid race between overwriting
> pc->mem_cgroup and page statistics accounting per memcg.
> This lock helps to avoid the race but the race is very rare because moving
> tasks between cgroup is not a usual job.
> So, it seems using 1bit per page is too costly.
> 
> This patch changes this lock as per-memcg spinlock and removes PCG_MOVE_LOCK.
> 
> If smaller lock is required, we'll be able to add some hashes but
> I'd like to start from this.
> 
> Changelog:
>   - fixed to pass memcg as an argument rather than page_cgroup.
>     and renamed from move_lock_page_cgroup() to move_lock_mem_cgroup()
> 
> Acked-by: Greg Thelen <gthelen@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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

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Re: [PATCH 4/6] memcg: use new logic for page stat accounting.

[Johannes Weiner]

On Fri, Feb 17, 2012 at 06:27:43PM +0900, KAMEZAWA Hiroyuki wrote:
> >From 5bf592d432e4552db74e94a575afcd83aa652a9d Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Date: Thu, 2 Feb 2012 11:49:59 +0900
> Subject: [PATCH 4/6] memcg: use new logic for page stat accounting.
> 
> Now, page-stat-per-memcg is recorded into per page_cgroup flag by
> duplicating page's status into the flag. The reason is that memcg
> has a feature to move a page from a group to another group and we
> have race between "move" and "page stat accounting",
> 
> Under current logic, assume CPU-A and CPU-B. CPU-A does "move"
> and CPU-B does "page stat accounting".
> 
> When CPU-A goes 1st,
> 
>             CPU-A                           CPU-B
>                                     update "struct page" info.
>     move_lock_mem_cgroup(memcg)
>     see pc->flags
>     copy page stat to new group
>     overwrite pc->mem_cgroup.
>     move_unlock_mem_cgroup(memcg)
>                                     move_lock_mem_cgroup(mem)
>                                     set pc->flags
>                                     update page stat accounting
>                                     move_unlock_mem_cgroup(mem)
> 
> stat accounting is guarded by move_lock_mem_cgroup() and "move"
> logic (CPU-A) doesn't see changes in "struct page" information.
> 
> But it's costly to have the same information both in 'struct page' and
> 'struct page_cgroup'. And, there is a potential problem.
> 
> For example, assume we have PG_dirty accounting in memcg.
> PG_..is a flag for struct page.
> PCG_ is a flag for struct page_cgroup.
> (This is just an example. The same problem can be found in any
>  kind of page stat accounting.)
> 
> 	  CPU-A                               CPU-B
>       TestSet PG_dirty
>       (delay)                        TestClear PG_dirty
>                                      if (TestClear(PCG_dirty))
>                                           memcg->nr_dirty--
>       if (TestSet(PCG_dirty))
>           memcg->nr_dirty++
> 
> Here, memcg->nr_dirty = +1, this is wrong.
> This race was reported by  Greg Thelen <gthelen@google.com>.
> Now, only FILE_MAPPED is supported but fortunately, it's serialized by
> page table lock and this is not real bug, _now_,
> 
> If this potential problem is caused by having duplicated information in
> struct page and struct page_cgroup, we may be able to fix this by using
> original 'struct page' information.
> But we'll have a problem in "move account"
> 
> Assume we use only PG_dirty.
> 
>          CPU-A                   CPU-B
>     TestSet PG_dirty
>     (delay)                    move_lock_mem_cgroup()
>                                if (PageDirty(page))
>                                       new_memcg->nr_dirty++
>                                pc->mem_cgroup = new_memcg;
>                                move_unlock_mem_cgroup()
>     move_lock_mem_cgroup()
>     memcg = pc->mem_cgroup
>     new_memcg->nr_dirty++
> 
> accounting information may be double-counted. This was original
> reason to have PCG_xxx flags but it seems PCG_xxx has another problem.
> 
> I think we need a bigger lock as
> 
>      move_lock_mem_cgroup(page)
>      TestSetPageDirty(page)
>      update page stats (without any checks)
>      move_unlock_mem_cgroup(page)
> 
> This fixes both of problems and we don't have to duplicate page flag
> into page_cgroup. Please note: move_lock_mem_cgroup() is held
> only when there are possibility of "account move" under the system.
> So, in most path, status update will go without atomic locks.
> 
> This patch introduce mem_cgroup_begin_update_page_stat() and
> mem_cgroup_end_update_page_stat() both should be called at modifying
> 'struct page' information if memcg takes care of it. as
> 
>      mem_cgroup_begin_update_page_stat()
>      modify page information
>      mem_cgroup_update_page_stat()
>      => never check any 'struct page' info, just update counters.
>      mem_cgroup_end_update_page_stat().
> 
> This patch is slow because we need to call begin_update_page_stat()/
> end_update_page_stat() regardless of accounted will be changed or not.
> A following patch adds an easy optimization and reduces the cost.
> 
> Changes since v4
>  - removed unused argument *lock
>  - added more comments.
> Changes since v3
>  - fixed typos
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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

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

[Johannes Weiner]

On Fri, Feb 17, 2012 at 06:28:51PM +0900, KAMEZAWA Hiroyuki wrote:
> >From 07d3ce332ee4bc1eaef4b8fb2019b0c06bd7afb1 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 "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@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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

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