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

Sean Christopherson
2021-10-15T23:26:30Z

On Sat, Oct 09, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.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@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.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.

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

> + goto out;
> +
> + if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
> + WARN_ON_ONCE(vcpu->mode == IN_GUEST_MODE);
> + goto out;
> + }
>
> /*
> * Note, the vCPU could get migrated to a different pCPU at any point
> @@ -3324,12 +3331,12 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
> * IPI is to force the vCPU to leave IN_GUEST_MODE, and migrating the
> * vCPU also requires it to leave IN_GUEST_MODE.
> */
> - me = get_cpu();
> if (kvm_arch_vcpu_should_kick(vcpu)) {
> cpu = READ_ONCE(vcpu->cpu);
> if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
> smp_send_reschedule(cpu);
> }
> +out:
> put_cpu();
> }
> EXPORT_SYMBOL_GPL(kvm_vcpu_kick);
> --
> 2.25.1
>