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SubjectRe: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
On 11/07/12 13:04, Avi Kivity wrote:
> On 07/11/2012 01:17 PM, Christian Borntraeger wrote:
>> On 11/07/12 11:06, Avi Kivity wrote:
>> [...]
>>>> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
>>>
>>> Perhaps x86 should copy this.
>>
>> See arch/s390/lib/spinlock.c
>> The basic idea is using several heuristics:
>> - loop for a given amount of loops
>> - check if the lock holder is currently scheduled by the hypervisor
>> (smp_vcpu_scheduled, which uses the sigp sense running instruction)
>> Dont know if such thing is available for x86. It must be a lot cheaper
>> than a guest exit to be useful
>
> We could make it available via shared memory, updated using preempt
> notifiers. Of course piling on more pv makes this less attractive.
>
>> - if lock holder is not running and we looped for a while do a directed
>> yield to that cpu.
>>
>>>
>>>> So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax.
>>>> I have to double check with others, if these cases are critical, but for now, it seems
>>>> that your dummy implementation for s390 is just fine. After all it is a no-op until
>>>> we implement something.
>>>
>>> Does the data structure make sense for you? If so we can move it to
>>> common code (and manage it in kvm_vcpu_on_spin()). We can guard it with
>>> CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
>>> have to pay anything.
>>
>> Ignoring the name,
>
> What name would you suggest?

maybe vcpu_no_progress instead of pause_loop_exited

>
>> yes the data structure itself seems based on the algorithm
>> and not on arch specific things. That should work. If we move that to common
>> code then s390 will use that scheme automatically for the cases were we call
>> kvm_vcpu_on_spin(). All others archs as well.
>
> ARM doesn't have an instruction for cpu_relax(), so it can't intercept
> it. Given ppc's dislike of overcommit, and the way it implements
> cpu_relax() by adjusting hw thread priority, I'm guessing it doesn't
> intercept those either, but I'm copying the ppc people in case I'm
> wrong. So it's s390 and x86.
>
>> So this would probably improve guests that uses cpu_relax, for example
>> stop_machine_run. I have no measurements, though.
>
> smp_call_function() too (though that can be converted to directed yield
> too). It seems worthwhile.

Indeed. For those places where is is possible I would like to see an
architecture primitive for directed yield. That could be useful for
other places as well (e.g. maybe lib/spinlock_debug.c, which has no
yielding at all)

Christian





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