On Tue, 22 Aug 2017 16:06:57 +0200 David Hildenbrand <[email protected]> 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 <[email protected]> > > --- > > 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); > > > > > >
