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. > > 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); \ > + })) > + > 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); > > -- Thanks, David
