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From: Wanpeng Li <kernellwp@gmail.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 3/3] KVM: LAPIC: Optimize PMI delivering overhead
Date: Fri, 8 Oct 2021 19:06:34 +0800	[thread overview]
Message-ID: <CANRm+Cy=bb_iap6JKsux7ekmo6Td0FXqwpuVdgPSC8u8b2wFNA@mail.gmail.com> (raw)
In-Reply-To: <87ily73i0x.fsf@vitty.brq.redhat.com>

On Fri, 8 Oct 2021 at 18:52, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> 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)

I thought about it before, I will do it in the next version since you
also vote for it. :)

    Wanpeng

  reply	other threads:[~2021-10-08 11:06 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-08  9:57 [PATCH 1/3] KVM: emulate: #GP when emulating rdpmc if CR0.PE is 1 Wanpeng Li
2021-10-08  9:57 ` [PATCH 2/3] KVM: vPMU: Fill get_msr MSR_CORE_PERF_GLOBAL_OVF_CTRL w/ 0 Wanpeng Li
2021-10-08 11:02   ` Like Xu
2021-10-08 11:17     ` Wanpeng Li
2021-10-08  9:57 ` [PATCH 3/3] KVM: LAPIC: Optimize PMI delivering overhead Wanpeng Li
2021-10-08 10:52   ` Vitaly Kuznetsov
2021-10-08 11:06     ` Wanpeng Li [this message]
2021-10-08 15:59       ` Sean Christopherson
2021-10-09  9:14         ` Wanpeng Li
2021-10-08 15:20 ` [PATCH 1/3] KVM: emulate: #GP when emulating rdpmc if CR0.PE is 1 Sean Christopherson
2021-10-09  9:09   ` Wanpeng Li

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