Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

From: Paolo Bonzini
Date: Thu May 19 2016 - 09:57:50 EST




On 19/05/2016 15:27, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> If an emulated lapic timer will fire soon(in the scope of 10us the
> base of dynamic halt-polling, lower-end of message passing workload
> latency TCP_RR's poll time < 10us) we can treat it as a short halt,
> and poll to wait it fire, the fire callback apic_timer_fn() will set
> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
> This can avoid context switch overhead and the latency which we wake
> up vCPU.

Would this work too and be simpler?

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4fd482fb9260..8d42f5304d94 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1964,16 +1964,12 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)

old = val = vcpu->halt_poll_ns;
grow = READ_ONCE(halt_poll_ns_grow);
- /* 10us base */
- if (val == 0 && grow)
- val = 10000;
- else
- val *= grow;
+ val *= grow;

if (val > halt_poll_ns)
val = halt_poll_ns;

- vcpu->halt_poll_ns = val;
+ vcpu->halt_poll_ns = max(10000u, val);
trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
}

@@ -1988,7 +1984,7 @@ static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
else
val /= shrink;

- vcpu->halt_poll_ns = val;
+ vcpu->halt_poll_ns = max(10000u, val);
trace_kvm_halt_poll_ns_shrink(vcpu->vcpu_id, val, old);
}


(Plus moving 10000 into a module parameter?) Can you measure higher CPU
utilization than with your patch? David, what do you think?

Thanks,

Paolo