Re: [PATCH RFC v3 1/9] KVM: s390: optimize detection of started vcpus

Christian Borntraeger
2017-08-22T07:26:50Z

On 08/21/2017 10:35 PM, Radim Krčmář wrote:
> We can add a variable instead of scanning all online VCPUs to know how
> many are started. We can't trivially tell which VCPU is the last one,
> though.
>
> Suggested-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

> @@ -3453,16 +3447,12 @@ void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
>
> void kvm_s390_vcpu_stop(struct kvm_vcpu *vcpu)
> {
> - int i, online_vcpus, started_vcpus = 0;

here you remove i.

> - struct kvm_vcpu *started_vcpu = NULL;
> -
> if (is_vcpu_stopped(vcpu))
> return;
>
> trace_kvm_s390_vcpu_start_stop(vcpu->vcpu_id, 0);
> /* Only one cpu at a time may enter/leave the STOPPED state. */
> spin_lock(&vcpu->kvm->arch.start_stop_lock);
> - online_vcpus = atomic_read(&vcpu->kvm->online_vcpus);
>
> /* SIGP STOP and SIGP STOP AND STORE STATUS has been fully processed */
> kvm_s390_clear_stop_irq(vcpu);
> @@ -3470,19 +3460,20 @@ void kvm_s390_vcpu_stop(struct kvm_vcpu *vcpu)
> atomic_or(CPUSTAT_STOPPED, &vcpu->arch.sie_block->cpuflags);
> __disable_ibs_on_vcpu(vcpu);
>
> - for (i = 0; i < online_vcpus; i++) {
> - if (!is_vcpu_stopped(vcpu->kvm->vcpus[i])) {
> - started_vcpus++;
> - started_vcpu = vcpu->kvm->vcpus[i];
> - }
> - }
> + vcpu->kvm->arch.started_vcpus--;
> +
> + if (vcpu->kvm->arch.started_vcpus == 1) {
> + struct kvm_vcpu *started_vcpu;
>
> - if (started_vcpus == 1) {
> /*
> - * As we only have one VCPU left, we want to enable the
> - * IBS facility for that VCPU to speed it up.
> + * As we only have one VCPU left, we want to enable the IBS
> + * facility for that VCPU to speed it up.
> */
> - __enable_ibs_on_vcpu(started_vcpu);
> + kvm_for_each_vcpu(i, started_vcpu, vcpu->kvm)

here you need i.
> + if (!is_vcpu_stopped(started_vcpu)) {
> + __enable_ibs_on_vcpu(started_vcpu);
> + break;
> + }
> }
>
> spin_unlock(&vcpu->kvm->arch.start_stop_lock);
>