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In-Reply-To: <e1d07b55-1539-ed33-911c-713403d776b3@redhat.com>
From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Tue, 20 Apr 2021 18:27:39 +0800
Message-ID: <CANRm+Cy-78UnrkX8nh5WdHut2WW5NU=UL84FRJnUNjsAPK+Uww@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>
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On Tue, 20 Apr 2021 at 18:23, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 20/04/21 10:48, Wanpeng Li wrote:
> >> 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=
 yield_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, boo=
l yield_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=
 VCPU.
> >>           */
> >> -       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, boo=
l yield_to_kernel_mode)
> >>                          if (rcuwait_active(&vcpu->wait) &&
> >>                              !vcpu_dy_runnable(vcpu))
> >>                                  continue;
> >> -                       if (READ_ONCE(vcpu->preempted) && yield_to_ker=
nel_mode &&
> >> -                               !kvm_arch_vcpu_in_kernel(vcpu))
> >> -                               continue;
> >>                          if (!kvm_vcpu_eligible_for_directed_yield(vcp=
u))
> >>                                  continue;
> >>
> >> +                       if (READ_ONCE(vcpu->preempted) && yield_to_ker=
nel_mode &&
> >> +                           !kvm_arch_vcpu_in_kernel(vcpu)) {
> >> +                           /*
> >> +                            * A vCPU running in userspace can get to =
kernel mode via
> >> +                            * an interrupt.  That's a worse choice th=
an a CPU already
> >> +                            * in kernel mode so only do it on a secon=
d pass.
> >> +                            */
> >> +                           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.
>
> Fair enough.  Of the two patches you posted I prefer the original, so
> I'll go with that one.

Great! Thanks. :)

    Wanpeng