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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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.
>>
>