On 12/02/2010 09:45 PM, Rik van Riel wrote:
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.
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;
You may well want to wake up a spinner. Suppose
A takes a lock
B preempts A
B grabs a ticket, starts spinning, yields to A
A releases lock
A grabs ticket, starts spinning
at this point, we want A to yield to B, but it won't because of this check.
+ if (!task)
+ continue;
+ if (waitqueue_active(&vcpu->wq))
+ continue;
+ if (task->flags& PF_VCPU)
+ continue;
+ kvm->last_boosted_vcpu = i;
+ yield_to(task);
+ break;
+ }
I think a random selection algorithm will be a better fit against
special guest behaviour.
- /* 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;
Need to guarantee termination.
--
error compiling committee.c: too many arguments to function
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