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

[Avi Kivity <avi@redhat.com>]

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.  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.

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.

-- 
error compiling committee.c: too many arguments to function