[PATCH 0/3] make vm_committed_as_batch aware of vm overcommit policy

[Feng Tang]

When checking a performance change for will-it-scale scalability
mmap test [1], we found very high lock contention for spinlock of
percpu counter 'vm_committed_as':

    94.14%     0.35%  [kernel.kallsyms]         [k] _raw_spin_lock_irqsave
    48.21% _raw_spin_lock_irqsave;percpu_counter_add_batch;__vm_enough_memory;mmap_region;do_mmap;
    45.91% _raw_spin_lock_irqsave;percpu_counter_add_batch;__do_munmap;

Actually this heavy lock contention is not always necessary. The
'vm_committed_as' needs to be very precise when the strict
OVERCOMMIT_NEVER policy is set, which requires a rather small batch
number for the percpu counter.

So lift the batch number to 16X for OVERCOMMIT_ALWAYS and
OVERCOMMIT_GUESS policies, and add a sysctl handler to adjust it
when the policy is reconfigured.

Benchmark with the same testcase in [1] shows 53% improvement on a
8C/16T desktop, and 2097%(20X) on a 4S/72C/144T server. And no change
for some platforms, due to the test mmap size of the case is bigger
than the batch number computed, though the patch will help mmap/munmap
generally.

[1] https://lkml.org/lkml/2020/3/5/57

There are some style complain from checkpatch for patch 3, as it
follows the similar format of sibling functions

patch1: a cleanup for /proc/meminfo
patch2: a preparation patch which also improve the accuracy of
        vm_memory_committed
patch3: the main change

Please help to review, thanks!

- Feng

----------------------------------------------------------------
Changelog:
  v2:
     * add the sysctl handler to cover runtime overcommit policy
       change, as suggested by Andres Morton 
     * address the accuracy concern of vm_memory_committed()
       from Andi Kleen 

Feng Tang (3):
  proc/meminfo: avoid open coded reading of vm_committed_as
  mm/util.c: make vm_memory_committed() more accurate
  mm: adjust vm_committed_as_batch according to vm overcommit policy

 fs/proc/meminfo.c    |  2 +-
 include/linux/mm.h   |  2 ++
 include/linux/mman.h |  4 ++++
 kernel/sysctl.c      |  2 +-
 mm/mm_init.c         | 19 +++++++++++++++----
 mm/util.c            | 15 ++++++++++++++-
 6 files changed, 37 insertions(+), 7 deletions(-)

-- 
2.7.4