linux-kernel - Re: [PATCH] KVM: LAPIC: Don't kick vCPU which is injecting already-expired timer

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Date:   Mon, 24 Aug 2020 08:19:25 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Don't kick vCPU which is injecting
 already-expired timer

On Sat, 22 Aug 2020 at 12:01, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Fri, Aug 21, 2020 at 05:17:34PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > The kick after setting KVM_REQ_PENDING_TIMER is used to handle the timer
> > fires on a different pCPU which vCPU is running on, we don't need this
> > kick when injecting already-expired timer, this kick is expensive since
> > memory barrier, rcu, preemption disable/enable operations. This patch
> > reduces the overhead by don't kick vCPU which is injecting already-expired
> > timer.
>
> This should also call out the VMX preemption timer case, which also passes
> from_timer_fn=false but doesn't need a kick because kvm_lapic_expired_hv_timer()
> is called from the target vCPU.
>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/lapic.c | 2 +-
> >  arch/x86/kvm/x86.c   | 5 +++--
> >  arch/x86/kvm/x86.h   | 2 +-
> >  3 files changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 248095a..5b5ae66 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1642,7 +1642,7 @@ static void apic_timer_expired(struct kvm_lapic *apic, bool from_timer_fn)
> >       }
> >
> >       atomic_inc(&apic->lapic_timer.pending);
> > -     kvm_set_pending_timer(vcpu);
> > +     kvm_set_pending_timer(vcpu, from_timer_fn);
>
> My vote would be to open code kvm_set_pending_timer() here and drop the
> helper, i.e.
>
>         kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
>         if (from_timer_fn)
>                 kvm_vcpu_kick(vcpu);
>
> with that and an updated changelog:

Agreed.

>
> Reviewed-by: Sean Christopherson <sean.j.christopherson@...el.com>

Thanks.
    Wanpeng