Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directedyield

From: Raghavendra K T
Date: Tue Jul 17 2012 - 05:11:37 EST

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

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.

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

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.

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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