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

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Message-ID: <20170105094853.GE3093@worktop>
Date:   Thu, 5 Jan 2017 10:48:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pan Xinhui <xinhui@...ux.vnet.ibm.com>
Cc:     Waiman Long <longman@...hat.com>, 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 Thu, Jan 05, 2017 at 04:16:38PM +0800, Pan Xinhui wrote:
> 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.

Right, curious about that relaxed one. In any case, I was more wondering
about the performance impact on the larger construct of the pvlock
itself.