Re: [PATCH] kvm: handle last_boosted_vcpu = 0 case

[Raghavendra K T]

On 06/21/2012 12:13 PM, Gleb Natapov wrote:
> On Tue, Jun 19, 2012 at 04:51:04PM -0400, Rik van Riel wrote:
> > On Wed, 20 Jun 2012 01:50:50 +0530
> > Raghavendra K T<raghavendra.kt@xxxxxxxxxxxxxxxxxx>  wrote:
> >
> > > In ple handler code, last_boosted_vcpu (lbv) variable is
> > > serving as reference point to start when we enter.
> >
> > > Also statistical analysis (below) is showing lbv is not very well
> > > distributed with current approach.
> >
> > You are the second person to spot this bug today (yes, today).
> >
> > Due to time zones, the first person has not had a chance yet to
> > test the patch below, which might fix the issue...
> >
> > Please let me know how it goes.
> >
> > ====8<====
> >
> > If last_boosted_vcpu == 0, then we fall through all test cases and
> > may end up with all VCPUs pouncing on vcpu 0.  With a large enough
> > guest, this can result in enormous runqueue lock contention, which
> > can prevent vcpu0 from running, leading to a livelock.
> >
> > Changing<  to<= makes sure we properly handle that case.
> >
> > Signed-off-by: Rik van Riel<riel@xxxxxxxxxx>
> > ---
> >   virt/kvm/kvm_main.c |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 7e14068..1da542b 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1586,7 +1586,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> >   	 */
> >   	for (pass = 0; pass<  2&&  !yielded; pass++) {
> >   		kvm_for_each_vcpu(i, vcpu, kvm) {
> > -			if (!pass&&  i<  last_boosted_vcpu) {
> > +			if (!pass&&  i<= last_boosted_vcpu) {
> >   				i = last_boosted_vcpu;
> >   				continue;
> >   			} else if (pass&&  i>  last_boosted_vcpu)
> Looks correct. We can simplify this by introducing something like:
>
> #define kvm_for_each_vcpu_from(idx, n, vcpup, kvm) \
>          for (n = atomic_read(&kvm->online_vcpus); \
>               n&&  (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
>               n--, idx = (idx+1) % atomic_read(&kvm->online_vcpus))

Thumbs up for this simplification. This really helps in all the places
where we want to start iterating from middle.

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