Messages in this thread | | | Date | Wed, 11 Jul 2012 17:21:06 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler |
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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 >
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