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

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Date:   Tue, 15 Nov 2016 14:52:03 +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 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