Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock ASAP

[Davidlohr Bueso <dave@stgolabs.net>]

On Thu, 18 Aug 2016, Waiman Long wrote:

>Currently, when down_read() fails, the active read locking isn't undone
>until the rwsem_down_read_failed() function grabs the wait_lock. If the
>wait_lock is contended, it may takes a while to get the lock. During
>that period, writer lock stealing will be disabled because of the
>active read lock.
>
>This patch will release the active read lock ASAP so that writer lock
>stealing can happen sooner. The only downside is when the reader is
>the first one in the wait queue as it has to issue another atomic
>operation to update the count.
>
>On a 4-socket Haswell machine running on a 4.7-rc1 tip-based kernel,
>the fio test with multithreaded randrw and randwrite tests on the
>same file on a XFS partition on top of a NVDIMM with DAX were run,
>the aggregated bandwidths before and after the patch were as follows:
>
>  Test      BW before patch     BW after patch  % change
>  ----      ---------------     --------------  --------
>  randrw        1210 MB/s          1352 MB/s      +12%
>  randwrite     1622 MB/s          1710 MB/s      +5.4%

Yeah, this is really a bad workload to make decisions on locking
heuristics imo - if I'm thinking of the same workload. Mainly because
concurrent buffered io to the same file isn't very realistic and you
end up pathologically pounding on i_rwsem (which used to be until
recently i_mutex until Al's parallel lookup/readdir). Obviously write
lock stealing wins in this case.

Thanks,
Davidlohr