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

Waiman Long
2016-10-07T21:45:29Z

On 10/06/2016 05:47 PM, Dave Chinner wrote:
> On Thu, Oct 06, 2016 at 11:17:18AM -0700, Davidlohr Bueso wrote:
>> 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.
> Except that it's DAX, and in 4.7-rc1 that used shared locking at the
> XFS level and never took exclusive locks.
>
> *However*, the DAX IO path locking in XFS has changed in 4.9-rc1 to
> match the buffered IO single writer POSIX semantics - the test is a
> bad test based on the fact it exercised a path that is under heavy
> development and so can't be used as a regression test across
> multiple kernels.
>
> If you want to stress concurrent access to a single file, please
> use direct IO, not DAX or buffered IO.

Thanks for the update. I will change the test when I update this patch.

Cheers,
Longman