per-task-delay-accounting: add memory reclaim delay

Sometimes, application responses become bad under heavy memory load.
Applications take a bit time to reclaim memory.  The statistics, how long
memory reclaim takes, will be useful to measure memory usage.

This patch adds accounting memory reclaim to per-task-delay-accounting for
accounting the time of do_try_to_free_pages().

<i.e>

- When System is under low memory load,
  memory reclaim may not occur.

$ free
             total       used       free     shared    buffers     cached
Mem:       8197800    1577300    6620500          0       4808    1516724
-/+ buffers/cache:      55768    8142032
Swap:     16386292          0   16386292

$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0      0 5069748  10612 3014060    0    0     0     0    3   26  0  0 100  0
 0  0      0 5069748  10612 3014060    0    0     0     0    4   22  0  0 100  0
 0  0      0 5069748  10612 3014060    0    0     0     0    3   18  0  0 100  0

Measure the time of tar command.

$ ls -s test.dat
1501472 test.dat

$ time tar cvf test.tar test.dat
real    0m13.388s
user    0m0.116s
sys     0m5.304s

$ ./delayget -d -p <pid>
CPU             count     real total  virtual total    delay total
                  428     5528345500     5477116080       62749891
IO              count    delay total
                  338     8078977189
SWAP            count    delay total
                    0              0
RECLAIM         count    delay total
                    0              0

- When system is under heavy memory load
  memory reclaim may occur.

$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0 7159032  49724   1812   3012    0    0     0     0    3   24  0  0 100  0
 0  0 7159032  49724   1812   3012    0    0     0     0    4   24  0  0 100  0
 0  0 7159032  49848   1812   3012    0    0     0     0    3   22  0  0 100  0

In this case, one process uses more 8G memory
by execution of malloc() and memset().

$ time tar cvf test.tar test.dat
real    1m38.563s        <-  increased by 85 sec
user    0m0.140s
sys     0m7.060s

$ ./delayget -d -p <pid>
CPU             count     real total  virtual total    delay total
                 9021     7140446250     7315277975      923201824
IO              count    delay total
                 8965    90466349669
SWAP            count    delay total
                    3       21036367
RECLAIM         count    delay total
                  740    61011951153

In the later case, the value of RECLAIM is increasing.
So, taskstats can show how much memory reclaim influences TAT.

Signed-off-by: Keika Kobayashi <kobayashi.kk@ncos.nec.co.jp>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujistu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>