mm: Dynamic management of dirty page writebacks

This feature allows to have two different intervals for dirty page
writebacks and to dynamically manage them when the system has been
resumed (it's active) or when has been suspended.

Three new procfs parameters are exposed inside /proc/sys/vm path:
- "dynamic_dirty_writeback" is the activation status of this feature,
  set 1 to enable it, set 0 to disable it and use the standard behaviour
- "dirty_writeback_active_centisecs" is the interval for the dirty page
  writebacks when the system is active (screen on)
- "dirty_writeback_suspend_centisecs" is the interval for the dirty page
  writebacks when the system is suspended (screen off)

This feature is in part inspired on Francisco Franco's patch:
https://github.com/franciscofranco/mako/commit/34d79548cf16b69c7d0154836018e7f258a22947

Signed-off-by: Cristoforo Cataldo <cristoforo.cataldo@gmail.com>

Conflicts:
	mm/Kconfig
mm: Added filesystem dynamic read-ahead (Credits to Chad Gooldman)
mm: Lowered swappiness 60->45
mm: Optimized SLUB memory allocator
fs: Asynchronous I/O latency to a solid-state disk greatly increased

between the 2.6.32 and 3.0 kernels. By removing the plug from
do_io_submit(), we observed a 34% improvement in the I/O latency.

Unfortunately, at this level, we don't know if the request is to
a rotating disk or not.
[PATCH] lib/int_sqrt: optimize square root algorithm

From: Davidlohr Bueso <davidlohr.bueso@hp.com>

This patch optimizes the current version of the shift-and-subtract
(hardware) algorithm, described by John von Newmann[1] and Guy L. Steele.

Iterating 1,000,000 times, perf shows for the current version:

 Performance counter stats for './sqrt-curr' (10 runs):

         27.170996 task-clock                #    0.979 CPUs utilized            ( +-  3.19% )
                 3 context-switches          #    0.103 K/sec                    ( +-  4.76% )
                 0 cpu-migrations            #    0.004 K/sec                    ( +-100.00% )
               104 page-faults               #    0.004 M/sec                    ( +-  0.16% )
        64,921,199 cycles                    #    2.389 GHz                      ( +-  0.03% )
        28,967,789 stalled-cycles-frontend   #   44.62% frontend cycles idle     ( +-  0.18% )
   <not supported> stalled-cycles-backend
       104,502,623 instructions              #    1.61  insns per cycle
                                             #    0.28  stalled cycles per insn  ( +-  0.00% )
        34,088,368 branches                  # 1254.587 M/sec                    ( +-  0.00% )
             4,901 branch-misses             #    0.01% of all branches          ( +-  1.32% )

       0.027763015 seconds time elapsed                                          ( +-  3.22% )

And for the new version:

Performance counter stats for './sqrt-new' (10 runs):

          0.496869 task-clock                #    0.519 CPUs utilized            ( +-  2.38% )
                 0 context-switches          #    0.000 K/sec
                 0 cpu-migrations            #    0.403 K/sec                    ( +-100.00% )
               104 page-faults               #    0.209 M/sec                    ( +-  0.15% )
           590,760 cycles                    #    1.189 GHz                      ( +-  2.35% )
           395,053 stalled-cycles-frontend   #   66.87% frontend cycles idle     ( +-  3.67% )
   <not supported> stalled-cycles-backend
           398,963 instructions              #    0.68  insns per cycle
                                             #    0.99  stalled cycles per insn  ( +-  0.39% )
            70,228 branches                  #  141.341 M/sec                    ( +-  0.36% )
             3,364 branch-misses             #    4.79% of all branches          ( +-  5.45% )

       0.000957440 seconds time elapsed                                          ( +-  2.42% )

Furthermore, this saves space in instruction text:

   text    data     bss     dec     hex filename
    111       0       0     111      6f lib/int_sqrt-baseline.o
     89       0       0      89      59 lib/int_sqrt.o

[1] http://en.wikipedia.org/wiki/First_Draft_of_a_Report_on_the_EDVAC

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Reviewed-and-tested-by: Jonathan Gonzalez <jgonzlez@linets.cl>