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 > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
