memcg background reclaim , yet another one.

This patch is based on Ying Han's one....at its origin, but I changed too much ;)
Then, start this as new thread.

(*) This work is not related to the topic "rewriting global LRU using memcg"
    discussion, at all. This kind of hi/low watermark has been planned since
    memcg was born. 

At first, per-memcg background reclaim is used for
  - helping memory reclaim and avoid direct reclaim.
  - set a not-hard limit of memory usage.

For example, assume a memcg has its hard-limit as 500M bytes.
Then, set high-watermark as 400M. Here, memory usage can exceed 400M up to 500M
but memory usage will be reduced automatically to 400M as time goes by.

This is useful when a user want to limit memory usage to 400M but don't want to
see big performance regression by hitting limit when memory usage spike happens.

1) == hard limit = 400M ==
[root@rhel6-test hilow]# time cp ./tmpfile xxx                
real    0m7.353s
user    0m0.009s
sys     0m3.280s

2) == hard limit 500M/ hi_watermark = 400M ==
[root@rhel6-test hilow]# time cp ./tmpfile xxx

real    0m6.421s
user    0m0.059s
sys     0m2.707s

Above is a brief result on VM and needs more study. But my impression is positive.
I'd like to use bigger real machine in the next time.

Here is a short list of updates from Ying Han's one.

 1. use workqueue and visit memcg in round robin.
 2. only allow setting hi watermark. low-watermark is automatically determined.
    This is good for avoiding bad cpu usage by background reclaim.
 3. totally rewrite algorithm of shrink_mem_cgroup for round-robin.
 4. fixed get_scan_count() , this was problematic.
 5. added some statistics, which I think necessary.
 6. added documenation

Then, the algorithm is not a cut-n-paste from kswapd. I thought kswapd should be
updated...and 'priority' in vmscan.c seems to be an enemy of memcg ;)


Thanks
-Kame





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