While running workloads that do lots of forking processes, exiting
processes and page reclamation(AIM 7) on large systems very high system
time(100%) and lots of lock contention was observed.



CPU5:
[<ffffffff814afb48>] ? _spin_lock+0x27/0x48
 [<ffffffff81101deb>] ? anon_vma_link+0x2a/0x5a
 [<ffffffff8105d3d8>] ? dup_mm+0x242/0x40c
 [<ffffffff8105e0a9>] ? copy_process+0xab1/0x12be
 [<ffffffff8105ea07>] ? do_fork+0x151/0x330
 [<ffffffff81058407>] ? default_wake_function+0x0/0x36
 [<ffffffff814b0243>] ? _spin_lock_irqsave+0x2f/0x68
 [<ffffffff810121d3>] ? stub_clone+0x13/0x20
[<ffffffff81011e02>] ? system_call_fastpath+0x16/0x1b

CPU4:
[<ffffffff814afb4a>] ? _spin_lock+0x29/0x48
 [<ffffffff81103062>] ? anon_vma_unlink+0x2a/0x84
 [<ffffffff810fbab7>] ? free_pgtables+0x3c/0xe1
 [<ffffffff810fd8b1>] ? exit_mmap+0xc5/0x110
 [<ffffffff8105ce4c>] ? mmput+0x55/0xd9
 [<ffffffff81061afd>] ? exit_mm+0x109/0x129
 [<ffffffff81063846>] ? do_exit+0x1d7/0x712
 [<ffffffff814b0243>] ? _spin_lock_irqsave+0x2f/0x68
 [<ffffffff81063e07>] ? do_group_exit+0x86/0xb2
 [<ffffffff81063e55>] ? sys_exit_group+0x22/0x3e
[<ffffffff81011e02>] ? system_call_fastpath+0x16/0x1b

CPU0:
[<ffffffff814afb4a>] ? _spin_lock+0x29/0x48
[<ffffffff81101ad1>] ? page_check_address+0x9e/0x16f
 [<ffffffff81101cb8>] ? page_referenced_one+0x53/0x10b
 [<ffffffff81102f5a>] ? page_referenced+0xcd/0x167
 [<ffffffff810eb32d>] ? shrink_active_list+0x1ed/0x2a3
 [<ffffffff810ebde9>] ? shrink_zone+0xa06/0xa38
 [<ffffffff8108440a>] ? getnstimeofday+0x64/0xce
 [<ffffffff810ecaf9>] ? do_try_to_free_pages+0x1e5/0x362
 [<ffffffff810ecd9f>] ? try_to_free_pages+0x7a/0x94
 [<ffffffff810ea66f>] ? isolate_pages_global+0x0/0x242
 [<ffffffff810e57b9>] ? __alloc_pages_nodemask+0x397/0x572
 [<ffffffff810e3c1e>] ? __get_free_pages+0x19/0x6e
 [<ffffffff8105d6c9>] ? copy_process+0xd1/0x12be
 [<ffffffff81204eb2>] ? avc_has_perm+0x5c/0x84
 [<ffffffff81130db8>] ? user_path_at+0x65/0xa3
 [<ffffffff8105ea07>] ? do_fork+0x151/0x330
 [<ffffffff810b7935>] ? check_for_new_grace_period+0x78/0xab
 [<ffffffff810121d3>] ? stub_clone+0x13/0x20
[<ffffffff81011e02>] ? system_call_fastpath+0x16/0x1b


------------------------------------------------------------------------------
   PerfTop:     864 irqs/sec  kernel:99.7% [100000 cycles],  (all, 8
CPUs)
------------------------------------------------------------------------------

             samples    pcnt         RIP          kernel function
  ______     _______   _____   ________________   _______________

             3235.00 - 75.1% - ffffffff814afb21 : _spin_lock
              670.00 - 15.6% - ffffffff81101a33 : page_check_address
              165.00 -  3.8% - ffffffffa01cbc39 : rpc_sleep_on  [sunrpc]
               40.00 -  0.9% - ffffffff81102113 : try_to_unmap_one
               29.00 -  0.7% - ffffffff81101c65 : page_referenced_one
               27.00 -  0.6% - ffffffff81101964 : vma_address
                8.00 -  0.2% - ffffffff8125a5a0 : clear_page_c
                6.00 -  0.1% - ffffffff8125a5f0 : copy_page_c
                6.00 -  0.1% - ffffffff811023ca : try_to_unmap_anon
                5.00 -  0.1% - ffffffff810fb014 : copy_page_range
                5.00 -  0.1% - ffffffff810e4d18 : get_page_from_freelist



The cause was determined to be the unconditional call to
page_referenced() for every mapped page encountered in
shrink_active_list().  page_referenced() takes the anon_vma->lock and
calls page_referenced_one() for each vma.  page_referenced_one() then
calls page_check_address() which takes the pte_lockptr spinlock.   If
several CPUs are doing this at the same time there is a lot of
pte_lockptr spinlock contention with the anon_vma->lock held.  This
causes contention on the anon_vma->lock, stalling in the fo and very
high system time.

Before the splitLRU patch shrink_active_list() would only call
page_referenced() when reclaim_mapped got set.  reclaim_mapped only got
set when the priority worked its way from 12 all the way to 7. This
prevented page_referenced() from being called from shrink_active_list()
until the system was really struggling to reclaim memory.

On way to prevent this is to change page_check_address() to execute a
spin_trylock(ptl) when it was called by shrink_active_list() and simply
fail if it could not get the pte_lockptr spinlock.  This will make
shrink_active_list() consider the page not referenced and allow the
anon_vma->lock to be dropped much quicker.

The attached patch does just that, thoughts???