Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 07/11/2012 03:47 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

Unfortunately we do not have information on lock-holder.

> - 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, yes the data structure itself seems based on the algorithm
> and not on arch specific things. That should work.

Ok. can you please elaborate, on the flow.

  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.
>
> So this would probably improve guests that uses cpu_relax, for example
> stop_machine_run. I have no measurements, though.
>
> Christian
>