linux-kernel - Re: [PATCH v2 3/3] KVM: vCPU kick tax cut for running vCPU

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Date:   Sat, 16 Oct 2021 10:48:46 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.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 v2 3/3] KVM: vCPU kick tax cut for running vCPU

On Sat, 16 Oct 2021 at 07:26, Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Sat, Oct 09, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Sometimes a vCPU kick is following a pending request, even if @vcpu is
> > the running vCPU. It suffers from both rcuwait_wake_up() which has
> > rcu/memory barrier operations and cmpxchg(). Let's check vcpu->wait
> > before rcu_wait_wake_up() and whether @vcpu is the running vCPU before
> > cmpxchg() to tax cut this overhead.
> >
> > We evaluate the kvm-unit-test/vmexit.flat on an Intel ICX box, most of the
> > scores can improve ~600 cpu cycles especially when APICv is disabled.
> >
> > tscdeadline_immed
> > tscdeadline
> > self_ipi_sti_nop
> > ..............
> > x2apic_self_ipi_tpr_sti_hlt
> >
> > Suggested-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> > v1 -> v2:
> >  * move checking running vCPU logic to kvm_vcpu_kick
> >  * check rcuwait_active(&vcpu->wait) etc
> >
> >  virt/kvm/kvm_main.c | 13 ++++++++++---
> >  1 file changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 7851f3a1b5f7..18209d7b3711 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -3314,8 +3314,15 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
> >  {
> >       int me, cpu;
> >
> > -     if (kvm_vcpu_wake_up(vcpu))
> > -             return;
> > +     me = get_cpu();
> > +
> > +     if (rcuwait_active(&vcpu->wait) && kvm_vcpu_wake_up(vcpu))
>
> This needs to use kvm_arch_vcpu_get_wait(), not vcpu->wait, because PPC has some
> funky wait stuff.
>
> One potential issue I didn't think of before.  rcuwait_active() comes with the
> below warning, which means we might be at risk of a false negative that could
> result in a missed wakeup.  I'm not postive on that though.

There is only ever a single waiting vCPU, an event will be requested
before kick the sleeping vCPU and it will be checked after setting
vcpu->wait to task. I can't find scenario could result in a missed
wakeup.

    Wanpeng

>
> /*
>  * Note: this provides no serialization and, just as with waitqueues,
>  * requires care to estimate as to whether or not the wait is active.
>  */