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


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