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

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Date:   Fri, 21 Oct 2016 11:12:19 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     borntraeger@...ibm.com
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@...r.kernel.org
Subject: Re: [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

From: Christian Borntraeger <borntraeger@...ibm.com>
Date: Fri, 21 Oct 2016 17:08:54 +0200

> 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. 

Agreed.