Linux Virtualization - Nov 2016 - [GIT PULL v2 1/5] processor.h: introduce cpu_relax_yield

On Tue, Nov 15, 2016 at 02:19:53PM +0100, Christian Borntraeger
wrote:
> 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.
It's three weeks since you posted them.  For one of those weeks (the
week you posted them) I was away, and missed them while catching up.
Sorry, but it sometimes takes a while to spot things amongst the
backlog, and normally takes some subsequent activity on the thread to
bring it back into view.

-- 
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On 11/15/2016 02:37 PM, Russell King - ARM Linux wrote:
> On Tue, Nov 15, 2016 at 02:19:53PM +0100, Christian Borntraeger wrote:
>> 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.
> 
> It's three weeks since you posted them.  For one of those weeks (the
> week you posted them) I was away, and missed them while catching up.
> Sorry, but it sometimes takes a while to spot things amongst the
> backlog, and normally takes some subsequent activity on the thread to
> bring it back into view.
Absolutely no need to apologize. Thank you for doing the review and the
proposal.
I will do whatever is consensus, but since this looks like tip/locking material
I will wait for Peter or Ingo to decide about the if and how.

Christian