Re: [PATCH V3 6/8] memcg: Don't account root_mem_cgroup page statistics

From: Sha Zhengju
Date: Sat Jan 05 2013 - 02:38:23 EST

On Fri, Dec 28, 2012 at 9:04 AM, Kamezawa Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> (2012/12/26 2:27), Sha Zhengju wrote:
>> From: Sha Zhengju <handai.szj@xxxxxxxxxx>
>> If memcg is enabled and no non-root memcg exists, all allocated pages
>> belongs to root_mem_cgroup and go through root memcg statistics routines
>> which brings some overheads. So for the sake of performance, we can give
>> up accounting stats of root memcg for MEM_CGROUP_STAT_FILE_MAPPED/FILE_DIRTY
>> /WRITEBACK and instead we pay special attention while showing root
>> memcg numbers in memcg_stat_show(): as we don't account root memcg stats
>> anymore, the root_mem_cgroup->stat numbers are actually 0. But because of
>> hierachy, figures of root_mem_cgroup may just represent numbers of pages
>> used by its own tasks(not belonging to any other child cgroup). So here we
>> fake these root numbers by using stats of global state and all other memcg.
>> That is for root memcg:
>> nr(MEM_CGROUP_STAT_FILE_MAPPED) = global_page_state(NR_FILE_MAPPED) -
>> sum_of_all_memcg(MEM_CGROUP_STAT_FILE_MAPPED);
>> Dirty/Writeback pages accounting are in the similar way.
>> Signed-off-by: Sha Zhengju <handai.szj@xxxxxxxxxx>
> isn't it better to use mem_cgroup_is_root() call rather than
> direct comparison (memcg == root_mem_cgroup) ?

Okay, it's better to use the wrapper.

> Anyway, Ack to this approach.

Thanks for reviewing!

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