Re: [PATCH] KVM: Boost vCPU candidiate in user mode which is delivering interrupt

[Wanpeng Li <kernellwp@gmail.com>]

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> wrote:
> >>>
> >>> 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 full pass?
> >>>> I.e. give vCPUs that are already in kernel mode priority, and only yield 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 yield_to_kernel_mode)
>   {
>         struct kvm *kvm = me->kvm;
>         struct kvm_vcpu *vcpu;
> -       int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
>         int yielded = 0;
>         int try = 3;
> -       int pass;
> +       int pass, num_passes = 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 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 = 0; pass < 2 && !yielded && try; pass++) {
> -               kvm_for_each_vcpu(i, vcpu, kvm) {
> -                       if (!pass && i <= last_boosted_vcpu) {
> -                               i = last_boosted_vcpu;
> -                               continue;
> -                       } else if (pass && i > last_boosted_vcpu)
> -                               break;
> +       for (pass = 0; pass < num_passes; pass++) {
> +               int idx = me->kvm->last_boosted_vcpu;
> +               int n = atomic_read(&kvm->online_vcpus);
> +               for (i = 0; i < n; i++, idx++) {
> +                       if (idx == n)
> +                               idx = 0;
> +
> +                       vcpu = kvm_get_vcpu(kvm, idx);
>                         if (!READ_ONCE(vcpu->ready))
>                                 continue;
>                         if (vcpu == me)
> @@ -3226,23 +3226,36 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_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 kernel mode via
> +                            * an interrupt.  That's a worse choice than a CPU already
> +                            * in kernel mode so only do it on a second pass.
> +                            */
> +                           if (!vcpu_dy_runnable(vcpu))
> +                                   continue;
> +                           if (pass == 0) {
> +                                   num_passes = 2;
> +                                   continue;
> +                           }
> +                       }
> +
>                         yielded = kvm_vcpu_yield_to(vcpu);
>                         if (yielded > 0) {
>                                 kvm->last_boosted_vcpu = 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%，the 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