linux-kernel - Re: [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP

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Date:	Wed, 15 Jun 2016 15:17:50 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	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>, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP

On 06/15/2016 01:22 PM, Peter Zijlstra wrote:
> On Tue, Jun 14, 2016 at 06:48:05PM -0400, 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%
>>
>> The write-only microbench also showed improvement because some read
>> locking was done by the XFS code.
> How does a reader only micro-bench react? I'm thinking the extra atomic
> might hurt a bit.
>

A reader only benchmark will not go into the slow path at all. It is 
only when there is a mix of readers and writers will the reader slowpath 
be executed.

I think there will be a little bit of performance impact for a workload 
that produce just the right amount of rwsem contentions. However, it is 
hard to produce a microbenchmark to create such a right amount of 
contention. As the amount of contention increases, I believe this patch 
will help performance instead of hurting it. Even then, the amount of 
performance degradation in that particular case will be pretty small.

Cheers,
Longman