[v6,0/4] make vm_committed_as_batch aware of vm overcommit policy

Message

Feng Tang July 10, 2020, 2:01 p.m. UTC

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 keep 'batch' number unchanged for strict OVERCOMMIT_NEVER policy,
and enlarge it for not-so-strict  OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS
policies.

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 for that
case, whether it shows improvements depends on if the test mmap size
is bigger than the batch number computed.

We tested 10+ platforms in 0day (server, desktop and laptop). If we
lift it to 64X, 80%+ platforms show improvements, and for 16X lift,
1/3 of the platforms will show improvements.

And generally it should help the mmap/unmap usage,as Michal Hocko
mentioned:
"
I believe that there are non-synthetic worklaods which would benefit
from a larger batch. E.g. large in memory databases which do large
mmaps during startups from multiple threads.
"

Note: There are some style complain from checkpatch for patch 4,
as sysctl handler declaration follows the similar format of sibling
functions

[1] https://lore.kernel.org/lkml/20200305062138.GI5972@shao2-debian/

patch1: a cleanup for /proc/meminfo
patch2: a preparation patch which also improve the accuracy of
        vm_memory_committed
patch3: add a percpu_counter sync func 
patch4: main change

Please help to review, thanks!

- Feng

----------------------------------------------------------------
Changelog:

  v6: 
      * fix the ltp vm-overcommit test case fail reported
        by 0day test robot, by syncing the percpu-counter
	when changing policy to OVERCOMMIT_NEVER

  v5:
      * rebase after 5.8-rc1
      * remove the 3/4 patch in v4 which is merged in v5.7
      * add code comments for vm_memory_committed()

  v4:
    * Remove the VM_WARN_ONCE check for vm_committed_as underflow,
      thanks to Qian Cai for finding and testing the warning

  v3:
    * refine commit log and cleanup code, according to comments
      from Michal Hocko and Matthew Wilcox
    * change the lift from 16X and 64X after test 
  
  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 


*** BLURB HERE ***

Feng Tang (4):
  proc/meminfo: avoid open coded reading of vm_committed_as
  mm/util.c: make vm_memory_committed() more accurate
  percpu_counter: add percpu_counter_sync()
  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 ++++
 include/linux/percpu_counter.h |  4 ++++
 kernel/sysctl.c                |  2 +-
 lib/percpu_counter.c           | 19 +++++++++++++++++
 mm/mm_init.c                   | 22 +++++++++++++------
 mm/util.c                      | 48 +++++++++++++++++++++++++++++++++++++++++-
 8 files changed, 94 insertions(+), 9 deletions(-)