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

Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx> · Wed, 18 Jul 2012 07:58:05 +0530

On 07/17/2012 02:39 PM, Raghavendra K T wrote:
[...]

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.

Thought you brought in is miraculous. taking care of not having 

spill-over dy_eligible status is needed for making algorithm technically 

more correct. will spin V5 with all these changes.

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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