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

From: Christian Borntraeger
Date: Thu May 19 2016 - 10:52:16 EST


On 05/19/2016 03:57 PM, Paolo Bonzini wrote:
>
>
> 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?

Hmm, your patch does only fiddle with the grow/shrink logic (which might
be a good idea independently of this change), but the original patch
actually takes into account that we have a guaranteed maximum time by a
wakeup timer - IOW we know exactly what the maximum poll time is.

>
> 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);

That would prevent halt_poll_ns from going 0, no?


> 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
>