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

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Message-ID: <87ily73i0x.fsf@vitty.brq.redhat.com>
Date:   Fri, 08 Oct 2021 12:52:46 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 3/3] KVM: LAPIC: Optimize PMI delivering overhead

Wanpeng Li <kernellwp@...il.com> writes:

> From: Wanpeng Li <wanpengli@...cent.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@...cent.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