Messages in this thread | | | Date | Tue, 17 Jul 2012 14:39:09 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directed yield |
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On 07/17/2012 01:59 PM, Avi Kivity wrote: > On 07/16/2012 07:10 PM, Rik van Riel wrote: >> On 07/16/2012 06:07 AM, Avi Kivity wrote: >> >>>> +{ >>>> + bool eligible; >>>> + >>>> + eligible = !vcpu->ple.cpu_relax_intercepted || >>>> + (vcpu->ple.cpu_relax_intercepted&& >>>> + vcpu->ple.dy_eligible); >>>> + >>>> + if (vcpu->ple.cpu_relax_intercepted) >>>> + vcpu->ple.dy_eligible = !vcpu->ple.dy_eligible; >>> >>> Probably should assign 'true', since the previous value is essentially >>> random. >> >> I suspect the intended purpose of this conditional is to >> flip the eligibility of a vcpu for being selected as a >> direct yield target. >> >> In other words, that bit of the code is correct. > > If vcpu A is in a long spin loop and is preempted away, and vcpu B dips > several times in kvm_vcpu_on_spin(), then it will act as intended.
Yes, true.
But > if vcpu A is spinning for x% of its time and processing on the other, > then vcpu B will flip its dy_eligible for those x%, and not flip it when > it's processing. I don't understand how this is useful.
Suppose A is doing really good job and and has not done pause loop exit, we will not touch it's dy_eligible flag. Also dy_eligible flag will not prevent B doing yield_to to A.
Suppose A has started spinning in the beginning itself, it will do pause loop exit if it crosses threshold, and we will now start toggling dy_eligible.
Was that you were referring?
And it seems we may still have to set dy_eligible flag to false at the beginning of vcpu_on_spin along with cpu_relax_intercepted = true, like below, so that we do not have spill-over status from previous PL exits.
vcpu_on_spin() { cpu_relax_intercepted = true; dy_eligible = false; . . .
cpu_relax_intercepted = false; }
Let me know if that addresses your concern.
> > I guess this is an attempt to impose fairness on yielding, and it makes > sense to do this, but I don't know if this is the best way to achieve it. >
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