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SubjectRe: [PATCH RFC v3 1/9] KVM: s390: optimize detection of started vcpus
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On 21.08.2017 22:35, 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.

You could keep the started vcpus in a list. Then you might drop unsigned
started_vcpus;

No started vcpus: Start pointer NULL
Single started vcpu: Only one element in the list (easy to check)
> 1 started vcpus: More than one element int he list (easy to check)


>
> Suggested-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
> arch/s390/include/asm/kvm_host.h | 1 +
> arch/s390/kvm/kvm-s390.c | 39 +++++++++++++++------------------------
> 2 files changed, 16 insertions(+), 24 deletions(-)
>
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index a409d5991934..be0d0bdf585b 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -735,6 +735,7 @@ struct kvm_arch{
> struct mutex ipte_mutex;
> struct ratelimit_state sthyi_limit;
> spinlock_t start_stop_lock;
> + unsigned started_vcpus;
> struct sie_page2 *sie_page2;
> struct kvm_s390_cpu_model model;
> struct kvm_s390_crypto crypto;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 9f23a9e81a91..1534778a3c66 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3414,25 +3414,17 @@ static void __enable_ibs_on_vcpu(struct kvm_vcpu *vcpu)
>
> void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
> {
> - int i, online_vcpus, started_vcpus = 0;
> -
> if (!is_vcpu_stopped(vcpu))
> return;
>
> trace_kvm_s390_vcpu_start_stop(vcpu->vcpu_id, 1);
> /* 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);
>
> - for (i = 0; i < online_vcpus; i++) {
> - if (!is_vcpu_stopped(vcpu->kvm->vcpus[i]))
> - started_vcpus++;
> - }
> -
> - if (started_vcpus == 0) {
> + if (vcpu->kvm->arch.started_vcpus == 0) {
> /* we're the only active VCPU -> speed it up */
> __enable_ibs_on_vcpu(vcpu);
> - } else if (started_vcpus == 1) {
> + } else if (vcpu->kvm->arch.started_vcpus == 1) {
> /*
> * As we are starting a second VCPU, we have to disable
> * the IBS facility on all VCPUs to remove potentially
> @@ -3441,6 +3433,8 @@ void kvm_s390_vcpu_start(struct kvm_vcpu *vcpu)
> __disable_ibs_on_all_vcpus(vcpu->kvm);
> }
>
> + vcpu->kvm->arch.started_vcpus++;
> +
> atomic_andnot(CPUSTAT_STOPPED, &vcpu->arch.sie_block->cpuflags);
> /*
> * Another VCPU might have used IBS while we were offline.
> @@ -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;
> - 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)
> + if (!is_vcpu_stopped(started_vcpu)) {
> + __enable_ibs_on_vcpu(started_vcpu);
> + break;
> + }
> }
>
> spin_unlock(&vcpu->kvm->arch.start_stop_lock);
>


--

Thanks,

David

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