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SubjectRe: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field
On Fri, 06 May 2016, Waiman Long wrote:

>Currently, it is not possible to determine for sure if a reader
>owns a rwsem by looking at the content of the rwsem data structure.
>This patch adds a new state RWSEM_READER_OWNED to the owner field
>to indicate that readers currently own the lock. This enables us to
>address the following 2 issues in the rwsem optimistic spinning code:
>
> 1) rwsem_can_spin_on_owner() will disallow optimistic spinning if
> the owner field is NULL which can mean either the readers own
> the lock or the owning writer hasn't set the owner field yet.
> In the latter case, we miss the chance to do optimistic spinning.
>
> 2) While a writer is spinning and a reader takes the lock, the writer
> will continue to spin in the main rwsem_optimistic_spin() loop as
> the owner is NULL.
>
>Adding the new state will allow optimistic spinning to go forward as
>long as the owner field is not RWSEM_READER_OWNED and the owner is
>running, if set, but stop immediately when that state has been reached.
>
>On a 4-socket Haswell machine running on a 4.6-rc1 based kernel, the
>fio test with multithreaded randrw and randwrite tests on the same
>file on a XFS partition on top of a NVDIMM were run, the aggregated
>bandwidths before and after the patch were as follows:
>
> Test BW before patch BW after patch % change
> ---- --------------- -------------- --------
> randrw 988 MB/s 1192 MB/s +21%
> randwrite 1513 MB/s 1623 MB/s +7.3%
>
>The perf profile of the rwsem_down_write_failed() function in randrw
>before and after the patch were:
>
> 19.95% 5.88% fio [kernel.vmlinux] [k] rwsem_down_write_failed
> 14.20% 1.52% fio [kernel.vmlinux] [k] rwsem_down_write_failed
>
>The actual CPU cycles spend in rwsem_down_write_failed() dropped from
>5.88% to 1.52% after the patch.
>
>The xfstests was also run and no regression was observed.
>
>Signed-off-by: Waiman Long <Waiman.Long@hpe.com>

Acked-by: Davidlohr Bueso <dave@stgolabs.net>

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