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

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/12/26 2:27), Sha Zhengju wrote:
> From: Sha Zhengju <handai.szj@taobao.com>
> 
> 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@taobao.com>

isn't it better to use mem_cgroup_is_root() call rather than
direct comparison (memcg == root_mem_cgroup) ?

Anyway, Ack to this approach. 

Thanks,
-Kame

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