[RFC] More deterministic SLOB for real time embedded systems

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

I've been reading SLUB/SLOB code for a while. SLUB recently became
real time compatible by reducing its locking area.

for now, SLUB is the only slab allocator for PREEMPT_RT because
it works better than SLAB on RT and SLOB uses non-deterministic method,
sequential fit.

But memory usage of SLUB is too high for systems with low memory.
So In my local repository I made SLOB to use segregated free list
method, which is more more deterministic, to provide bounded latency.

This can be done by managing list of partial pages globally
for every power of two sizes (8, 16, 32, ..., PAGE_SIZE) per NUMA nodes.
minimal allocation size is size of pointers to keep pointer of next free object
like SLUB.

By making size of objects in same page to have same size, there's no
need to iterate free blocks in a page. (Also iterating pages isn't needed)

Some cleanups and more tests (especially with NUMA/RT configs) needed,
but want to hear your opinion about the idea. Did not test on RT yet.

Below is result of benchmarks and memory usage. (on !RT)
with 13% increase in memory usage, it's nine times faster and
bounded fragmentation, and importantly provides predictable execution time.

current SLOB:
memory usage:
            Slab:               7936 kB
hackbench:
    Time: 263.900
    Performance counter stats for 'hackbench -g 4 -l 10000':

	527649.37 msec cpu-clock                 #    1.999 CPUs utilized
              12451963      context-switches          #   23.599 K/sec
                251231      cpu-migrations            #  476.132 /sec
                  4112      page-faults               #    7.793 /sec
          342196899596      cycles                    #    0.649 GHz
          228439896295      instructions              #    0.67  insn per cycle
            3228211614      branch-misses
           65667138978      cache-references          #  124.452 M/sec
            7406902357      cache-misses              #   11.279 % of all cache refs

	263.956975106 seconds time elapsed

           5.213166000 seconds user
         521.716737000 seconds sys

SLOB with segregated free list:
memory usage:
            Slab:               8976 kB
hackbench:

    Time: 28.896
    Performance counter stats for 'hackbench -g 4 -l 10000':

              57669.66 msec cpu-clock                 #    1.995 CPUs utilized
                902343      context-switches          #   15.647 K/sec
                 10569      cpu-migrations            #  183.268 /sec
                  4116      page-faults               #   71.372 /sec
           72101524728      cycles                    #    1.250 GHz
           68780577270      instructions              #    0.95  insn per cycle
             230133481      branch-misses
           23610741192      cache-references          #  409.414 M/sec
             896060729      cache-misses              #    3.795 % of all cache refs

	28.909188264 seconds time elapsed

           1.521686000 seconds user
          56.105718000 seconds sys