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Subject[RFC PATCH 3/3] kvm: use yield_to instead of sleep in kvm_vcpu_on_spin
Instead of sleeping in kvm_vcpu_on_spin, which can cause gigantic
slowdowns of certain workloads, we instead use yield_to to hand
the rest of our timeslice to another vcpu in the same KVM guest.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index c011ba3..7637dd3 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -90,6 +90,7 @@ struct kvm_vcpu {
int fpu_active;
int guest_fpu_loaded;
wait_queue_head_t wq;
+ int spinning;
int sigset_active;
sigset_t sigset;
struct kvm_vcpu_stat stat;
@@ -185,6 +186,7 @@ struct kvm {
#endif
struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
atomic_t online_vcpus;
+ int last_boosted_vcpu;
struct list_head vm_list;
struct mutex lock;
struct kvm_io_bus *buses[KVM_NR_BUSES];
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 80f17db..a6eeafc 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1880,18 +1880,53 @@ void kvm_resched(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_resched);

-void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu)
+void kvm_vcpu_on_spin(struct kvm_vcpu *me)
{
- ktime_t expires;
- DEFINE_WAIT(wait);
+ struct kvm *kvm = me->kvm;
+ struct kvm_vcpu *vcpu;
+ int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
+ int first_round = 1;
+ int i;

- prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
+ me->spinning = 1;
+
+ /*
+ * We boost the priority of a VCPU that is runnable but not
+ * currently running, because it got preempted by something
+ * else and called schedule in __vcpu_run. Hopefully that
+ * VCPU is holding the lock that we need and will release it.
+ * We approximate round-robin by starting at the last boosted VCPU.
+ */
+ again:
+ kvm_for_each_vcpu(i, vcpu, kvm) {
+ struct task_struct *task = vcpu->task;
+ if (first_round && i < last_boosted_vcpu) {
+ i = last_boosted_vcpu;
+ continue;
+ } else if (!first_round && i > last_boosted_vcpu)
+ break;
+ if (vcpu == me)
+ continue;
+ if (vcpu->spinning)
+ continue;
+ if (!task)
+ continue;
+ if (waitqueue_active(&vcpu->wq))
+ continue;
+ if (task->flags & PF_VCPU)
+ continue;
+ kvm->last_boosted_vcpu = i;
+ yield_to(task);
+ break;
+ }

- /* Sleep for 100 us, and hope lock-holder got scheduled */
- expires = ktime_add_ns(ktime_get(), 100000UL);
- schedule_hrtimeout(&expires, HRTIMER_MODE_ABS);
+ if (first_round && last_boosted_vcpu == kvm->last_boosted_vcpu) {
+ /* We have not found anyone yet. */
+ first_round = 0;
+ goto again;
+ }

- finish_wait(&vcpu->wq, &wait);
+ me->spinning = 0;
}
EXPORT_SYMBOL_GPL(kvm_vcpu_on_spin);


--
All rights reversed.


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