linux-kernel - Re: [GIT PULL v2 1/5] processor.h: introduce cpu_relax

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Date:   Tue, 15 Nov 2016 14:19:53 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-kernel@...r.kernel.org,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Noam Camus <noamc@...hip.com>, sparclinux@...r.kernel.org,
        x86@...nel.org, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        virtualization@...ts.linux-foundation.org,
        xen-devel@...ts.xenproject.org, kvm@...r.kernel.org
Subject: Re: [GIT PULL v2 1/5] processor.h: introduce cpu_relax_yield

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.