From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [GIT PULL v2 1/5] processor.h: introduce cpu_relax_yield
Date: Tue, 15 Nov 2016 14:19:53 +0100
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To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: linux-arch@vger.kernel.org, linux-s390 <linux-s390@vger.kernel.org>, kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>, x86@kernel.org, Heiko Carstens <heiko.carstens@de.ibm.com>, linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>, virtualization@lists.linux-foundation.org, sparclinux@vger.kernel.org, Noam Camus <noamc@ezchip.com>, Catalin Marinas <catalin.marinas@arm.com>, Martin Schwidefsky <schwidefsky@de.ibm.com>, xen-devel@lists.xenproject.org, linuxppc-dev@lists.ozlabs.org, Ingo Molnar <mingo@kernel.org>
List-ID: <linux-s390.vger.kernel.org>

On 11/15/2016 01:30 PM, Russell King - ARM Linux wrote:
> On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote:
>> For spinning loops people do often use barrier() or cpu_relax().
>> For most architectures cpu_relax and barrier are the same, but on
>> some architectures cpu_relax can add some latency.
>> For example on power,sparc64 and arc, cpu_relax can shift the CPU
>> towards other hardware threads in an SMT environment.
>> On s390 cpu_relax does even more, it uses an hypercall to the
>> hypervisor to give up the timeslice.
>> In contrast to the SMT yielding this can result in larger latencies.
>> In some places this latency is unwanted, so another variant
>> "cpu_relax_lowlatency" was introduced. Before this is used in more
>> and more places, lets revert the logic and provide a cpu_relax_yield
>> that can be called in places where yielding is more important than
>> latency. By default this is the same as cpu_relax on all architectures.
> 
> Rather than having to update all these architectures in this way, can't
> we put in some linux/*.h header something like:
> 
> #ifndef cpu_relax_yield
> #define cpu_relax_yield() cpu_relax()
> #endif
> 
> so only those architectures that need to do something need to be
> modified?

These patches are part of linux-next since a month or so, changing that 
would invalidate all the next testing. If people want that, I can certainly
do that, though.