linux-kernel - Re: [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

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Message-Id: <2cf23cb7-05c5-0a2d-2ed5-aa90d582f802@de.ibm.com>
Date:   Fri, 21 Oct 2016 17:08:54 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     David Miller <davem@...emloft.net>
Cc:     peterz@...radead.org, npiggin@...il.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        heiko.carstens@...ibm.com, schwidefsky@...ibm.com,
        noamc@...hip.com, virtualization@...ts.linux-foundation.org,
        xen-devel@...ts.xenproject.org, KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

On 10/21/2016 04:57 PM, David Miller wrote:
> From: Christian Borntraeger <borntraeger@...ibm.com>
> Date: Fri, 21 Oct 2016 13:58:53 +0200
> 
>> For spinning loops people did 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 s390
>> cpu_relax gives up the time slice to the hypervisor. On power cpu_relax
>> tries to give some of the CPU to the neighbor threads. To reduce the
>> latency another variant cpu_relax_lowlatency was introduced. Before this
>> is used in more and more places, lets revert the logic of provide a new
>> function cpu_relax_yield that can spend some time and for s390 yields
>> the guest CPU.
> 
> Sparc64, fwiw, behaves similarly to powerpc.

As sparc currently defines cpu_relax_lowlatency to cpu_relax, this patch set
should be a no-op then for sparc, correct?

My intend was that cpu_relax should not add a huge latency but can certainly
push some cpu power to hardware threads of the same core. This seems to be
the case for sparc/power and some arc variants. 

Christian