Re: [PATCH 2/2] memcg: add per cgroup dirty pages accounting

[Sha Zhengju <handai.szj@gmail.com>]

On Sat, Jun 16, 2012 at 2:34 PM, Kamezawa Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> (2012/06/16 0:32), Greg Thelen wrote:
>>
>> On Fri, Jun 15 2012, Sha Zhengju wrote:
>>
>>> This patch adds memcg routines to count dirty pages. I notice that
>>> the list has talked about per-cgroup dirty page limiting
>>> (http://lwn.net/Articles/455341/) before, but it did not get merged.
>>
>>
>> Good timing, I was just about to make another effort to get some of
>> these patches upstream.  Like you, I was going to start with some basic
>> counters.
>>
>> Your approach is similar to what I have in mind.  While it is good to
>> use the existing PageDirty flag, rather than introducing a new
>> page_cgroup flag, there are locking complications (see below) to handle
>> races between moving pages between memcg and the pages being {un}marked
>> dirty.
>>
>>> I've no idea how is this going now, but maybe we can add per cgroup
>>> dirty pages accounting first. This allows the memory controller to
>>> maintain an accurate view of the amount of its memory that is dirty
>>> and can provide some infomation while group's direct reclaim is working.
>>>
>>> After commit 89c06bd5 (memcg: use new logic for page stat accounting),
>>> we do not need per page_cgroup flag anymore and can directly use
>>> struct page flag.
>>>
>>>
>>> Signed-off-by: Sha Zhengju<handai.szj@taobao.com>
>>> ---
>>>  include/linux/memcontrol.h |    1 +
>>>  mm/filemap.c               |    1 +
>>>  mm/memcontrol.c            |   32 +++++++++++++++++++++++++-------
>>>  mm/page-writeback.c        |    2 ++
>>>  mm/truncate.c              |    1 +
>>>  5 files changed, 30 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>> index a337c2e..8154ade 100644
>>> --- a/include/linux/memcontrol.h
>>> +++ b/include/linux/memcontrol.h
>>> @@ -39,6 +39,7 @@ enum mem_cgroup_stat_index {
>>>        MEM_CGROUP_STAT_FILE_MAPPED,  /* # of pages charged as file rss */
>>>        MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
>>>        MEM_CGROUP_STAT_DATA, /* end of data requires synchronization */
>>> +       MEM_CGROUP_STAT_FILE_DIRTY,  /* # of dirty pages in page cache */
>>>        MEM_CGROUP_STAT_NSTATS,
>>>  };
>>>
>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>> index 79c4b2b..5b5c121 100644
>>> --- a/mm/filemap.c
>>> +++ b/mm/filemap.c
>>> @@ -141,6 +141,7 @@ void __delete_from_page_cache(struct page *page)
>>>         * having removed the page entirely.
>>>         */
>>>        if (PageDirty(page)&&  mapping_cap_account_dirty(mapping)) {
>>> +               mem_cgroup_dec_page_stat(page,
>>> MEM_CGROUP_STAT_FILE_DIRTY);
>>
>>
>> You need to use mem_cgroup_{begin,end}_update_page_stat around critical
>> sections that:
>> 1) check PageDirty
>> 2) update MEM_CGROUP_STAT_FILE_DIRTY counter
>>
>> This protects against the page from being moved between memcg while
>> accounting.  Same comment applies to all of your new calls to
>> mem_cgroup_{dec,inc}_page_stat.  For usage pattern, see
>> page_add_file_rmap.
>>
>
> If you feel some difficulty with mem_cgroup_{begin,end}_update_page_stat(),
> please let me know...I hope they should work enough....
>

Hi, Kame

While digging into the bigger lock of mem_cgroup_{begin,end}_update_page_stat(),
I find the reality is more complex than I thought. Simply stated,
modifying page info
and update page stat may be wide apart and in different level (eg.
mm&fs), so if we
use the big lock it may lead to scalability and maintainability issues.

For example:
     mem_cgroup_begin_update_page_stat()
     modify page information                 => TestSetPageDirty in
ceph_set_page_dirty() (fs/ceph/addr.c)
     XXXXXX                                         => other fs operations
     mem_cgroup_update_page_stat()   => account_page_dirtied() in
mm/page-writeback.c
     mem_cgroup_end_update_page_stat().

We can choose to get lock in higher level meaning vfs set_page_dirty()
but this may span
too much and can also have some missing cases.
What's your opinion of this problem?


Thanks,
Sha

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