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

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