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

[Raghavendra K T]

On 07/09/2012 01:25 PM, 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.
> 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.

Thanks for the review. Nice to know that, patch has potential to help 
s390 also.


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