linux-kernel - Re: The vcpu won't be wakened for a long time

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Date:   Fri, 17 Dec 2021 10:11:41 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>,
        Huangzhichao <huangzhichao@...wei.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: The vcpu won't be wakened for a long time

On Fri, 17 Dec 2021 at 07:48, Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Dec 16, 2021, Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
> > > What kernel version?  There have been a variety of fixes/changes in the
> > > area in recent kernels.
> >
> > The kernel version is 4.18, and it seems the latest kernel also has this problem.
> >
> > The following code can fixes this bug, I've tested it on 4.18.
> >
> > (4.18)
> >
> > @@ -3944,6 +3944,11 @@ static void vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
> >         if (pi_test_and_set_on(&vmx->pi_desc))
> >                 return;
> >
> > +       if (swq_has_sleeper(kvm_arch_vcpu_wq(vcpu))) {
> > +               kvm_vcpu_kick(vcpu);
> > +               return;
> > +       }
> > +
> >         if (vcpu != kvm_get_running_vcpu() &&
> >                 !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> >                 kvm_vcpu_kick(vcpu);
> >
> >
> > (latest)
> >
> > @@ -3959,6 +3959,11 @@ static int vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
> >         if (pi_test_and_set_on(&vmx->pi_desc))
> >                 return 0;
> >
> > +       if (rcuwait_active(&vcpu->wait)) {
> > +               kvm_vcpu_kick(vcpu);
> > +               return 0;
> > +       }
> > +
> >         if (vcpu != kvm_get_running_vcpu() &&
> >             !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> >                 kvm_vcpu_kick(vcpu);
> >
> > Do you have any suggestions ?
>
> Hmm, that strongly suggests the "vcpu != kvm_get_running_vcpu()" is at fault.

This was introduced in 5.8-rc1, however, his kernel version is 4.18.

> Can you try running with the below commit?  It's currently sitting in kvm/queue,
> but not marked for stable because I didn't think it was possible for the check
> to a cause a missed wake event in KVM's current code base.
>
> commit 6a8110fea2c1b19711ac1ef718680dfd940363c6
> Author: Sean Christopherson <seanjc@...gle.com>
> Date:   Wed Dec 8 01:52:27 2021 +0000
>
>     KVM: VMX: Wake vCPU when delivering posted IRQ even if vCPU == this vCPU
>
>     Drop a check that guards triggering a posted interrupt on the currently
>     running vCPU, and more importantly guards waking the target vCPU if
>     triggering a posted interrupt fails because the vCPU isn't IN_GUEST_MODE.
>     The "do nothing" logic when "vcpu == running_vcpu" works only because KVM
>     doesn't have a path to ->deliver_posted_interrupt() from asynchronous
>     context, e.g. if apic_timer_expired() were changed to always go down the
>     posted interrupt path for APICv, or if the IN_GUEST_MODE check in
>     kvm_use_posted_timer_interrupt() were dropped, and the hrtimer fired in
>     kvm_vcpu_block() after the final kvm_vcpu_check_block() check, the vCPU
>     would be scheduled() out without being awakened, i.e. would "miss" the
>     timer interrupt.
>
>     One could argue that invoking kvm_apic_local_deliver() from (soft) IRQ
>     context for the current running vCPU should be illegal, but nothing in
>     KVM actually enforces that rules.  There's also no strong obvious benefit
>     to making such behavior illegal, e.g. checking IN_GUEST_MODE and calling
>     kvm_vcpu_wake_up() is at worst marginally more costly than querying the
>     current running vCPU.
>
>     Lastly, this aligns the non-nested and nested usage of triggering posted
>     interrupts, and will allow for additional cleanups.
>
>     Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>     Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
>     Message-Id: <20211208015236.1616697-18-seanjc@...gle.com>
>     Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 38749063da0e..f61a6348cffd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3995,8 +3995,7 @@ static int vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
>          * guaranteed to see PID.ON=1 and sync the PIR to IRR if triggering a
>          * posted interrupt "fails" because vcpu->mode != IN_GUEST_MODE.
>          */
> -       if (vcpu != kvm_get_running_vcpu() &&
> -           !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> +       if (!kvm_vcpu_trigger_posted_interrupt(vcpu, false))
>                 kvm_vcpu_wake_up(vcpu);
>
>         return 0;