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SubjectRe: [PATCH RFC v3 6/9] KVM: rework kvm_vcpu_on_spin loop
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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 <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); \
> + }))
> +
> 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

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