(2012/12/26 2:27), Sha Zhengju wrote:

From: Sha Zhengju <redacted>

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 <redacted>

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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