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

[Avi Kivity <avi@redhat.com>]

On 07/09/2012 10:55 AM, Christian Borntraeger wrote:
> On 09/07/12 08:20, Raghavendra K T wrote:
>> Currently Pause Looop Exit (PLE) handler is doing directed yield to a
>> random VCPU on PL exit. Though we already have filtering while choosing
>> the candidate to yield_to, we can do better.
>> 
>> Problem is, for large vcpu guests, we have more probability of yielding
>> to a bad vcpu. We are not able to prevent directed yield to same guy who
>> has done PL exit recently, who perhaps spins again and wastes CPU.
>> 
>> Fix that by keeping track of who has done PL exit. So The Algorithm in series
>> give chance to a VCPU which has:
> 
> 
> We could do the same for s390. The appropriate exit would be diag44 (yield to hypervisor).
> 
> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.

Perhaps x86 should copy this.

> 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.

-- 
error compiling committee.c: too many arguments to function