Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

[Raghavendra K T]

On 04/01/2012 06:48 PM, Avi Kivity wrote:
> On 03/30/2012 01:07 PM, Raghavendra K T wrote:
> > On 03/29/2012 11:33 PM, Raghavendra K T wrote:
> > > On 03/29/2012 03:28 PM, Avi Kivity wrote:
> > > > On 03/28/2012 08:21 PM, Raghavendra K T wrote:
> >
> > > I really like below ideas. Thanks for that!.
> > >
> > > > - from the PLE handler, don't wake up a vcpu that is sleeping
> > > > because it
> > > > is waiting for a kick
> > >
> > > How about, adding another pass in the beginning of kvm_vcpu_on_spin()
> > > to check if any vcpu is already kicked. This would almost result in
> > > yield_to(kicked_vcpu). IMO this is also worth trying.
> > >
> > > will try above ideas soon.
> >
> > I have patch something like below in mind to try:
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index d3b98b1..5127668 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1608,15 +1608,18 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> >        * else and called schedule in __vcpu_run.  Hopefully that
> >        * VCPU is holding the lock that we need and will release it.
> >        * We approximate round-robin by starting at the last boosted VCPU.
> > +     * Priority is given to vcpu that are unhalted.
> >        */
> > -    for (pass = 0; pass<  2&&  !yielded; pass++) {
> > +    for (pass = 0; pass<  3&&  !yielded; pass++) {
> >           kvm_for_each_vcpu(i, vcpu, kvm) {
> >               struct task_struct *task = NULL;
> >               struct pid *pid;
> > -            if (!pass&&  i<  last_boosted_vcpu) {
> > +            if (!pass&&  !vcpu->pv_unhalted)
> > +                continue;
> > +            else if (pass == 1&&  i<  last_boosted_vcpu) {
> >                   i = last_boosted_vcpu;
> >                   continue;
> > -            } else if (pass&&  i>  last_boosted_vcpu)
> > +            } else if (pass == 2&&  i>  last_boosted_vcpu)
> >                   break;
> >               if (vcpu == me)
> >                   continue;
>
> Actually I think this is unneeded.  The loops tries to find vcpus that
> are runnable but not running (vcpu_active(vcpu->wq)), and halted vcpus
> don't match this condition.

I almost agree. But at corner of my thought,

Suppose there are 8 vcpus runnable out of which 4 of them are kicked
but not running, making yield_to those 4 vcpus would result in better
lock progress. no?

 I still have little problem getting PLE setup, here (instead rebasing 
patches).
Once I get PLE to get that running, and numbers prove no improvement, I 
will drop this idea.

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