linux-kernel - Re: [PATCH] arch,locking: Ciao arch_mutex_cpu

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Date:	Mon, 23 Jun 2014 08:58:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch,locking: Ciao arch_mutex_cpu_relax()

On Fri, Jun 20, 2014 at 11:21:13AM -0700, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <davidlohr@...com>
> 
> The arch_mutex_cpu_relax() function, introduced by 34b133f, is
> hacky and ugly. It was added a few years ago to address the fact
> that common cpu_relax() calls include yielding on s390, and thus
> impact the optimistic spinning functionality of mutexes. Nowadays
> we use this function well beyond mutexes: rwsem, qrwlock, mcs and
> lockref. Since the macro that defines the call is in the mutex header,
> any users must include mutex.h and the naming is misleading as well.
> 
> This patch (i) renames the call to arch_cpu_relax (for lack of a better
> name), and (ii) defines it in each arch's asm/processor.h local header,
> just like for regular cpu_relax() functions. On all archs, except s390,
> arch_cpu_relax is simply cpu_relax, and thus we can take it out of
> mutex.h. While this can seem redundant or weird, I believe it is a
> good choice as it allows us to move out arch specific logic from generic
> locking primitives and enables future(?) archs to transparently define
> it, similarly to System Z.
> 
> Please note that these changes are only tested on x86-64.

While I like the general idea; does anyone have a better name for this?
So in particular, the difference is that on s390:

 cpu_relax()                - yields the vcpu
 arch_{,mutex_}cpu_relax()  - will actually spin-wait


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