Re: [PATCH RFC v3 6/9] KVM: rework kvm_vcpu_on_spin loop

Cornelia Huck
2017-08-29T15:24:39Z

On Tue, 22 Aug 2017 16:06:57 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 21.08.2017 22:35, Radim Krčmář wrote:
> > The original code managed to obfuscate a straightforward idea:
> > start iterating from the selected index and reset the index to 0 when
> > reaching the end of online vcpus, then iterate until reaching the index
> > that we started at.
> >
> > The resulting code is a bit better, IMO. (Still horrible, though.)
>
> I think I prefer dropping this patch and maybe _after_ we have the list
> implementation in place, simply start walking the list from
> last_boosted_vcpu? (store a pointer instead of an index then, of course)
>
> If I understand correctly, this would then be simply, one walk from
> last_boosted_vcpu until we hit last_boosted_vcpu again.

Yes, doing this change at this point in the series trades an ugly piece
of code for a slightly less ugly one.

> >
> > Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> > ---
> > include/linux/kvm_host.h | 13 +++++++++++++
> > virt/kvm/kvm_main.c | 47 ++++++++++++++++++-----------------------------
> > 2 files changed, 31 insertions(+), 29 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index abd5cb1feb9e..cfb3c0efdd51 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -498,6 +498,19 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> > (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
> > idx++)
> >
> > +#define kvm_for_each_vcpu_from(idx, vcpup, from, kvm) \
> > + for (idx = from, vcpup = kvm_get_vcpu(kvm, idx); \
> > + vcpup; \
> > + ({ \
> > + idx++; \
> > + if (idx >= atomic_read(&kvm->online_vcpus)) \
> > + idx = 0; \
> > + if (idx == from) \
> > + vcpup = NULL; \
> > + else \
> > + vcpup = kvm_get_vcpu(kvm, idx); \
> > + }))

The loop below is better after the change, but this macro... it gets at
least a bit better if you push this behind patch 8.

> > +
> > static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
> > {
> > struct kvm_vcpu *vcpu = NULL;
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index d89261d0d8c6..33a15e176927 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2333,8 +2333,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
> > struct kvm_vcpu *vcpu;
> > int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
> > int yielded = 0;
> > - int try = 3;
> > - int pass;
> > + int try = 2;
> > int i;
> >
> > kvm_vcpu_set_in_spin_loop(me, true);
> > @@ -2345,34 +2344,24 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
> > * VCPU is holding the lock that we need and will release it.
> > * We approximate round-robin by starting at the last boosted VCPU.
> > */
> > - for (pass = 0; pass < 2 && !yielded && try; pass++) {
> > - kvm_for_each_vcpu(i, vcpu, kvm) {
> > - if (!pass && i <= last_boosted_vcpu) {
> > - i = last_boosted_vcpu;
> > - continue;
> > - } else if (pass && i > last_boosted_vcpu)
> > - break;
> > - if (!ACCESS_ONCE(vcpu->preempted))
> > - continue;
> > - if (vcpu == me)
> > - continue;
> > - if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
> > - continue;
> > - if (yield_to_kernel_mode && !kvm_arch_vcpu_in_kernel(vcpu))
> > - continue;
> > - if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> > - continue;
> > + kvm_for_each_vcpu_from(i, vcpu, last_boosted_vcpu, kvm) {
> > + if (!ACCESS_ONCE(vcpu->preempted))
> > + continue;
> > + if (vcpu == me)
> > + continue;
> > + if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
> > + continue;
> > + if (yield_to_kernel_mode && !kvm_arch_vcpu_in_kernel(vcpu))
> > + continue;
> > + if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> > + continue;
> >
> > - yielded = kvm_vcpu_yield_to(vcpu);
> > - if (yielded > 0) {
> > - kvm->last_boosted_vcpu = i;
> > - break;
> > - } else if (yielded < 0) {
> > - try--;
> > - if (!try)
> > - break;
> > - }
> > - }
> > + yielded = kvm_vcpu_yield_to(vcpu);
> > + if (yielded > 0) {
> > + kvm->last_boosted_vcpu = i;
> > + break;
> > + } else if (yielded < 0 && !try--)
> > + break;
> > }
> > kvm_vcpu_set_in_spin_loop(me, false);
> >
> >
>
>