linux-kernel - Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Message-ID: <349c5926-cd35-2313-b8a0-ebfe6eb5e4a0@redhat.com>
Date:   Thu, 5 Jan 2017 10:17:46 -0500
From:   Waiman Long <longman@...hat.com>
To:     Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On 01/05/2017 03:16 AM, Pan Xinhui wrote:
>
>
> 在 2017/1/4 17:41, Peter Zijlstra 写道:
>> On Tue, Jan 03, 2017 at 05:07:54PM -0500, Waiman Long wrote:
>>> On 01/03/2017 11:18 AM, Peter Zijlstra wrote:
>>>> On Sun, Dec 25, 2016 at 03:26:01PM -0500, Waiman Long wrote:
>>>>> A number of cmpxchg calls in qspinlock_paravirt.h were replaced by
>>>>> more
>>>>> relaxed versions to improve performance on architectures that use
>>>>> LL/SC.
>>>> Claim without numbers ;-)
>>>
>>> Well it is hard to produce actual numbers here as I don't have the
>>> setup
>>> to gather data.
>>
>> Surely RHT has big PPC machines around? I know that getting to them is a
>> wee bit of a bother, but they should be available somewhere.
>>
> hi,
>
> I do some tests about cmpxchg and cmpxchg_acquire before on ppc.
>
> loops in 15s of each cmpxchg is below.
>
> cmpxchg_relaxed: 336663
> cmpxchg_release: 369054
> cmpxchg_acquire: 363364
> cmpxchg:     179435
>
> so cmpxchg is really expensive than others.
> but I also have doubt about the cmpxchg_relaxed, it should be the
> cheapest, but from the tests, release/acquire are faster than it.
>
> thanks
> xinhui
>
Thanks for doing the test. It looks like we should just focus on using
either cmpxchg_release or cmpxchg_acquire and forget about cmpxchg_relaxed.

Cheers,
Longman