[RFC][PATCH 0/2] memcg: replace memcg's per cpu status counter with array counter like vmstat

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

Hi,

In current implementation, memcg uses its own percpu counters for counting
evetns and # of RSS, CACHES. Now, counter is maintainer per cpu without
any synchronization as vm_stat[] or percpu_counter. So, this is
 update-is-fast-but-read-is-slow conter.

Because "read" for these counter was only done by memory.stat file, I thought
read-side-slowness was acceptable. Amount of memory usage, which affects
memory limit check, can be read by memory.usage_in_bytes. It's maintained
by res_counter.

But in current -rc, root memcg's memory usage is calcualted by this per cpu
counter and read side slowness may be trouble if it's frequently read.

And, in recent discusstion, I wonder we should maintain NR_DIRTY etc...
in memcg. So, slow-read-counter will not match our requirements, I guess.
I want some counter like vm_stat[] in memcg.

This 2 patches are for using counter like vm_stat[] in memcg.
Just an idea level implementaion but I think this is not so bad.

I confirmed this patch works well. I'm now thinking how to test performance...

Any comments are welcome. 
This patch is onto mmotm + some myown patches...so...this is just an RFC.

Regards,
-Kame

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