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

Radim Krčmář
2017-08-21T20:35:27Z

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.)

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);

--
2.13.3