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    Subject[RFC PATCH-tip v4 00/10] locking/rwsem: Enable reader optimistic spinning
    Date
    v3->v4:
    - Rebased to the latest tip tree due to changes to rwsem-xadd.c.
    - Update the OSQ patch to fix race condition.

    v2->v3:
    - Used smp_acquire__after_ctrl_dep() to provide acquire barrier.
    - Added the following new patches:
    1) make rwsem_spin_on_owner() return a tristate value.
    2) reactivate reader spinning when there is a large number of
    favorable writer-on-writer spinnings.
    3) move all the rwsem macros in arch-specific rwsem.h files
    into a common asm-generic/rwsem_types.h file.
    4) add a boot parameter to specify the reader spinning threshold.
    - Updated some of the patches as suggested by PeterZ and adjusted
    some of the reader spinning parameters.

    v1->v2:
    - Fixed a 0day build error.
    - Added a new patch 1 to make osq_lock() a proper acquire memory
    barrier.
    - Replaced the explicit enabling of reader spinning by an autotuning
    mechanism that disable reader spinning for those rwsems that may
    not benefit from reader spinning.
    - Remove the last xfs patch as it is no longer necessary.

    This patchset enables more aggressive optimistic spinning on
    both readers and writers waiting on a writer or reader owned
    lock. Spinning on writer is done by looking at the on_cpu flag of the
    lock owner. Spinning on readers, on the other hand, is count-based as
    there is no easy way to figure out if all the readers are running. The
    spinner will stop spinning once the count goes to 0. Because of that,
    spinning on readers may hurt performance in some cases.

    An autotuning mechanism is used to determine if a rwsem can benefit
    from reader optimistic spinning. It will maintain reader spinning as
    long as no less than 80% of the spins are successful.

    Patch 1 updates the osq_lock() function to make it a proper acquire
    memory barrier.

    Patch 2 reduces the length of the blocking window after a read locking
    attempt where writer lock stealing is disabled because of the active
    read lock. It can improve rwsem performance for contended lock. It is
    independent of the rest of the patchset.

    Patch 3 modifies rwsem_spin_on_owner() to return a tri-state value
    that can be used in later patch.

    Patch 4 puts in place the autotuning mechanism to check if reader
    optimistic spinning should be used or not.

    Patch 5 moves down the rwsem_down_read_failed() function for later
    patches.

    Patch 6 moves the macro definitions in various arch-specific rwsem.h
    header files into a commont asm-generic/rwsem_types.h file.

    Patch 7 changes RWSEM_WAITING_BIAS to simpify reader trylock code.

    Patch 8 enables readers to do optimistic spinning.

    Patch 9 allows reactivation of reader spinning when a lot of
    writer-on-writer spins are successful.

    Patch 10 adds a new boot parameter to change the reader spinning
    threshold which can be system specific.

    Waiman Long (10):
    locking/osq: Make lock/unlock proper acquire/release barrier
    locking/rwsem: Stop active read lock ASAP
    locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value
    locking/rwsem: Enable count-based spinning on reader
    locking/rwsem: move down rwsem_down_read_failed function
    locking/rwsem: Move common rwsem macros to asm-generic/rwsem_types.h
    locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation
    locking/rwsem: Enable spinning readers
    locking/rwsem: Enable reactivation of reader spinning
    locking/rwsem: Add a boot parameter to reader spinning threshold

    Documentation/kernel-parameters.txt | 3 +
    arch/alpha/include/asm/rwsem.h | 11 +-
    arch/ia64/include/asm/rwsem.h | 9 +-
    arch/s390/include/asm/rwsem.h | 9 +-
    arch/x86/include/asm/rwsem.h | 22 +---
    include/asm-generic/rwsem.h | 20 +--
    include/asm-generic/rwsem_types.h | 28 ++++
    include/linux/rwsem.h | 23 +++-
    kernel/locking/osq_lock.c | 24 ++-
    kernel/locking/rwsem-xadd.c | 296 ++++++++++++++++++++++++++---------
    10 files changed, 308 insertions(+), 137 deletions(-)
    create mode 100644 include/asm-generic/rwsem_types.h

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