Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <1618542490-14756-1-git-send-email-wanpengli@tencent.com>
 <9c49c6ff-d896-e6a5-c051-b6707f6ec58a@redhat.com> <CANRm+Cy-xmDRQoUfOYm+GGvWiS+qC_sBjyZmcLykbKqTF2YDxQ@mail.gmail.com>
 <YH2wnl05UBqVhcHr@google.com> <c1909fa3-61f3-de6b-1aa1-8bc36285e1e4@redhat.com>
 <CANRm+CwQ266j6wTxqFZtGhp_HfQZ7Y_e843hzROqNUxf9BcaFA@mail.gmail.com>
 <CANRm+CyHX-_vQLck1a9wpCv8a-YnnemEWm+zVv4eWYby5gdAeg@mail.gmail.com> <b2fca9a5-9b2b-b8f2-0d1e-fc8b9d9b5659@redhat.com>
In-Reply-To: <b2fca9a5-9b2b-b8f2-0d1e-fc8b9d9b5659@redhat.com>
From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Tue, 20 Apr 2021 16:48:34 +0800
Message-ID: <CANRm+Czysw6z1u+fbsRF3JUyiJc0jErVATusar_Vj8CcSBy5LQ@mail.gmail.com>
Subject: Re: [PATCH] KVM: Boost vCPU candidiate in user mode which is
 delivering interrupt
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, 20 Apr 2021 at 15:23, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 20/04/21 08:08, Wanpeng Li wrote:
> > On Tue, 20 Apr 2021 at 14:02, Wanpeng Li <kernellwp@gmail.com> wrote:
> >>
> >> On Tue, 20 Apr 2021 at 00:59, Paolo Bonzini <pbonzini@redhat.com> wrot=
e:
> >>>
> >>> On 19/04/21 18:32, Sean Christopherson wrote:
> >>>> If false positives are a big concern, what about adding another pass=
 to the loop
> >>>> and only yielding to usermode vCPUs with interrupts in the second fu=
ll pass?
> >>>> I.e. give vCPUs that are already in kernel mode priority, and only y=
ield to
> >>>> handle an interrupt if there are no vCPUs in kernel mode.
> >>>>
> >>>> kvm_arch_dy_runnable() pulls in pv_unhalted, which seems like a good=
 thing.
> >>>
> >>> pv_unhalted won't help if you're waiting for a kernel spinlock though=
,
> >>> would it?  Doing two passes (or looking for a "best" candidate that
> >>> prefers kernel mode vCPUs to user mode vCPUs waiting for an interrupt=
)
> >>> seems like the best choice overall.
> >>
> >> How about something like this:
>
> I was thinking of something simpler:
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 9b8e30dd5b9b..455c648f9adc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3198,10 +3198,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yi=
eld_to_kernel_mode)
>   {
>         struct kvm *kvm =3D me->kvm;
>         struct kvm_vcpu *vcpu;
> -       int last_boosted_vcpu =3D me->kvm->last_boosted_vcpu;
>         int yielded =3D 0;
>         int try =3D 3;
> -       int pass;
> +       int pass, num_passes =3D 1;
>         int i;
>
>         kvm_vcpu_set_in_spin_loop(me, true);
> @@ -3212,13 +3211,14 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool y=
ield_to_kernel_mode)
>          * VCPU is holding the lock that we need and will release it.
>          * We approximate round-robin by starting at the last boosted VCP=
U.
>          */
> -       for (pass =3D 0; pass < 2 && !yielded && try; pass++) {
> -               kvm_for_each_vcpu(i, vcpu, kvm) {
> -                       if (!pass && i <=3D last_boosted_vcpu) {
> -                               i =3D last_boosted_vcpu;
> -                               continue;
> -                       } else if (pass && i > last_boosted_vcpu)
> -                               break;
> +       for (pass =3D 0; pass < num_passes; pass++) {
> +               int idx =3D me->kvm->last_boosted_vcpu;
> +               int n =3D atomic_read(&kvm->online_vcpus);
> +               for (i =3D 0; i < n; i++, idx++) {
> +                       if (idx =3D=3D n)
> +                               idx =3D 0;
> +
> +                       vcpu =3D kvm_get_vcpu(kvm, idx);
>                         if (!READ_ONCE(vcpu->ready))
>                                 continue;
>                         if (vcpu =3D=3D me)
> @@ -3226,23 +3226,36 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool y=
ield_to_kernel_mode)
>                         if (rcuwait_active(&vcpu->wait) &&
>                             !vcpu_dy_runnable(vcpu))
>                                 continue;
> -                       if (READ_ONCE(vcpu->preempted) && yield_to_kernel=
_mode &&
> -                               !kvm_arch_vcpu_in_kernel(vcpu))
> -                               continue;
>                         if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
>                                 continue;
>
> +                       if (READ_ONCE(vcpu->preempted) && yield_to_kernel=
_mode &&
> +                           !kvm_arch_vcpu_in_kernel(vcpu)) {
> +                           /*
> +                            * A vCPU running in userspace can get to ker=
nel mode via
> +                            * an interrupt.  That's a worse choice than =
a CPU already
> +                            * in kernel mode so only do it on a second p=
ass.
> +                            */
> +                           if (!vcpu_dy_runnable(vcpu))
> +                                   continue;
> +                           if (pass =3D=3D 0) {
> +                                   num_passes =3D 2;
> +                                   continue;
> +                           }
> +                       }
> +
>                         yielded =3D kvm_vcpu_yield_to(vcpu);
>                         if (yielded > 0) {
>                                 kvm->last_boosted_vcpu =3D i;
> -                               break;
> +                               goto done;
>                         } else if (yielded < 0) {
>                                 try--;
>                                 if (!try)
> -                                       break;
> +                                       goto done;
>                         }
>                 }
>         }
> +done:

We just tested the above post against 96 vCPUs VM in an over-subscribe
scenario, the score of pbzip2 fluctuated drastically. Sometimes it is
worse than vanilla, but the average improvement is around 2.2%. The
new version of my post is around 9.3%=EF=BC=8Cthe origial posted patch is
around 10% which is totally as expected since now both IPI receivers
in user-mode and lock-waiters are second class citizens. Big VM
increases the probability multiple vCPUs may enter PLE handler, the
previous vCPU who starts searching earlier can mark IPI receivers in
user-mode as dy_eligible, the vCPU who starts searching a little later
can select it directly. However, after the above posting, the
PLE-caused vCPU should search the second full pass by himself.

    Wanpeng