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

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Date:   Thu, 6 Oct 2016 11:17:18 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <Waiman.Long@....com>
Cc:     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>,
        Dave Chinner <david@...morbit.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 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