Messages in this thread |  | | | Date | Sun, 05 Dec 2010 14:56:26 +0200 | | From | Avi Kivity <> | | Subject | Re: [RFC PATCH 3/3] kvm: use yield_to instead of sleep in kvm_vcpu_on_spin |
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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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