Description of problem:

Bad scalability with memory cgroups.

Version-Release number of selected component (if applicable):
 2.6.32-131.12.1

How reproducible:

run a program that allocates memory on a 32 cpu system

Steps to Reproduce:
1. create a user Memory Cgroup
2. run 32 occurences of program that allocates memory
3. compare time with same program executed outside Memory cgroup or with  cgroup_disable=memory
  
Actual results:

. in  cgroup memory:  
Time for Allocating Buffers : 100.222279 second(s)
 75.70%  _spin_lock res_counter_uncharge

Expected results:
. outside cgroup memory:
Time for Allocating Buffers : 3.721997 second(s) 


Additional info:
perf profiling shows contention on cgroup spinlock. This has been reworked in recent kernels to coalesce the accounting on cgroup. 2.6.37 kernel does not show the problem of scalability.

    75.70%  malloc_seq  [kernel.kallsyms]                                                                     [k] _s
            |
            --- _spin_lock
               |          
               |--52.11%-- res_counter_uncharge
               |          __mem_cgroup_uncharge_common
               |          mem_cgroup_uncharge_page
               |          page_remove_rmap
               |          unmap_vmas
               |          exit_mmap
               |          mmput
               |          |          
               |          |--100.00%-- exit_mm
               |          |          do_exit
               |          |          do_group_exit
               |          |          sys_exit_group
               |          |          system_call_fastpath
               |           --0.00%-- [...]
               |          
               |--47.86%-- res_counter_charge