Linux Virtualization - Oct 2016 - [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

On 10/21/2016 04:57 PM, David Miller wrote:
> From: Christian Borntraeger <borntraeger at de.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

From: Christian Borntraeger <borntraeger at de.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 at de.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.