linux-kernel - Re: [RFC][PATCH 2/2] memcg: new lock for mutual execution of account

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Date:	Sun, 17 Oct 2010 14:33:50 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	Greg Thelen <gthelen@...gle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 2/2] memcg: new lock for mutual execution of
 account_move and file stats

On Fri, Oct 15, 2010 at 5:12 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> When we try to enhance page's status update to support other flags,
> one of problem is updating status from IRQ context.
>
> Now, mem_cgroup_update_file_stat() takes lock_page_cgroup() to avoid
> race with _account move_. IOW, there are no races with charge/uncharge
> in nature. Considering an update from IRQ context, it seems better
> to disable IRQ at lock_page_cgroup() to avoid deadlock.
>
> But lock_page_cgroup() is used too widerly and adding IRQ disable
> there makes the performance bad. To avoid the big hammer, this patch
> adds a new lock for update_stat().
>
> This lock is for mutual execustion of updating stat and accout moving.
> This adds a new lock to move_account..so, this makes move_account slow.
> But considering trade-off, I think it's acceptable.
>
> A score of moving 8GB anon pages, 8cpu Xeon(3.1GHz) is here.
>
> [before patch] (mmotm + optimization patch (#1 in this series)
> [root@...extal kamezawa]# time echo 2257 > /cgroup/B/tasks
>
> real    0m0.694s
> user    0m0.000s
> sys     0m0.683s
>
> [After patch]
> [root@...extal kamezawa]# time echo 2238 > /cgroup/B/tasks
>
> real    0m0.741s
> user    0m0.000s
> sys     0m0.730s
>
> This moves 8Gbytes == 2048k pages. But no bad effects to codes
> other than "move".
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

It looks good than old approach.
Just a below nitpick.

> ---
>  include/linux/page_cgroup.h |   29 +++++++++++++++++++++++++++++
>  mm/memcontrol.c             |   11 +++++++++--
>  2 files changed, 38 insertions(+), 2 deletions(-)
>
> Index: mmotm-1013/include/linux/page_cgroup.h
> ===================================================================
> --- mmotm-1013.orig/include/linux/page_cgroup.h
> +++ mmotm-1013/include/linux/page_cgroup.h
> @@ -36,6 +36,7 @@ struct page_cgroup *lookup_page_cgroup(s
>  enum {
>        /* flags for mem_cgroup */
>        PCG_LOCK,  /* page cgroup is locked */
> +       PCG_LOCK_STATS, /* page cgroup's stat accounting flags are locked */

Hmm, I think naming isn't a good. Aren't both for stat?
As I understand, Both are used for stat.
One is just used by charge/uncharge and the other is used by
pdate_file_stat/move_account.
If you guys who are expert in mcg feel it with easy, I am not against.
But at least, mcg-not-familiar people like me don't feel it comfortable.





-- 
Kind regards,
Minchan Kim
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