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

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Date:   Fri, 7 Oct 2016 17:45:29 -0400
From:   Waiman Long <waiman.long@....com>
To:     Dave Chinner <david@...morbit.com>
CC:     Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>, <linux-alpha@...r.kernel.org>,
        <linux-ia64@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        Jason Low <jason.low2@...com>,
        Jonathan Corbet <corbet@....net>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock
 ASAP

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