Re: [PATCH 3/3] KVM: LAPIC: Optimize PMI delivering overhead

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Wanpeng Li <kernellwp@gmail.com> writes:

> From: Wanpeng Li <wanpengli@tencent.com>
>
> The overhead of kvm_vcpu_kick() is huge since expensive rcu/memory
> barrier etc operations in rcuwait_wake_up(). It is worse when local 
> delivery since the vCPU is scheduled and we still suffer from this. 
> We can observe 12us+ for kvm_vcpu_kick() in kvm_pmu_deliver_pmi() 
> path by ftrace before the patch and 6us+ after the optimization. 
>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/lapic.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 76fb00921203..ec6997187c6d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1120,7 +1120,8 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
>  	case APIC_DM_NMI:
>  		result = 1;
>  		kvm_inject_nmi(vcpu);
> -		kvm_vcpu_kick(vcpu);
> +		if (vcpu != kvm_get_running_vcpu())
> +			kvm_vcpu_kick(vcpu);

Out of curiosity,

can this be converted into a generic optimization for kvm_vcpu_kick()
instead? I.e. if kvm_vcpu_kick() is called for the currently running
vCPU, there's almost nothing to do, especially when we already have a
request pending, right? (I didn't put too much though to it)

>  		break;
>  
>  	case APIC_DM_INIT:

-- 
Vitaly