Re: [GIT PULL v2 1/5] processor.h: introduce cpu_relax_yield

Christian Borntraeger
2016-11-15T13:19:53Z

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.