This is version 3 or 4....for rss counting. removed RFC.

My purpose is gathering more (rss-related) information per process without
scalability impact. (and improve oom-killer etc..)
The whole patch series is organized as

 [1/5] clean-up per mm stat counting.
 [2/5] making counter (a bit) more scalable with per-thread counting.
 [3/5] adding swap counter per mm
 [4/5] adding lowmem detection logic
 [5/5] adding lowmem usage counter per mm.

Big changes from previous one are...
  - removed per-cpu counter. added per-thread counter
  - synchronization point of a counter is moved to memory.c
    no hooks to ticks and scheduler.

Now, this patch is not very invasive as previous ones.

cache-miss/page fault with my benchmark on my box is

 [Before patch] 4.55 cache-miss/fault
 [After patch 2] 3.99 cache-miss/fault
 [After all patch] 4.06 cache-miss/fault

>From this numbers, I think swap/lowmem counters can be added.

My test program is attached (this is not modified from previous one)

[Future Plan]
 - add CONSTRAINT_LOWMEM oom killer.
 - add rss+swap based oom killer (with sysctl ?)
 - add some patch for perf ?
 - add mm_accessor patch.
 - improve page faults scalability, finally.

Thanks,
-Kame