Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

[Jason Low <jason.low2@hp.com>]

On Fri, 2016-05-06 at 20:20 -0400, 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: Jason Low <jason.low2@hp.com>