Re: [RFC][PATCH 2/2] memcg: new lock for mutual execution of account_move and file stats

[Minchan Kim <minchan.kim@gmail.com>]

On Sun, Oct 17, 2010 at 2:33 PM, Minchan Kim <minchan.kim@gmail.com> wrote:
> On Fri, Oct 15, 2010 at 5:12 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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@bluextal kamezawa]# time echo 2257 > /cgroup/B/tasks
>>
>> real    0m0.694s
>> user    0m0.000s
>> sys     0m0.683s
>>
>> [After patch]
>> [root@bluextal 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@jp.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.
>

And I think this patch would be better to be part of Greg Thelen's series.

-- 
Kind regards,
Minchan Kim

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