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:

quoted

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.