Re: [patch 2/3] KVM: x86: add option to advance tscdeadline hrtimer expiration

Radim Krcmar
2014-12-17T14:58:17Z

2014-12-16 09:08-0500, Marcelo Tosatti:
> For the hrtimer which emulates the tscdeadline timer in the guest,
> add an option to advance expiration, and busy spin on VM-entry waiting
> for the actual expiration time to elapse.
>
> This allows achieving low latencies in cyclictest (or any scenario
> which requires strict timing regarding timer expiration).
>
> Reduces average cyclictest latency from 12us to 8us
> on Core i5 desktop.
>
> Note: this option requires tuning to find the appropriate value
> for a particular hardware/guest combination. One method is to measure the
> average delay between apic_timer_fn and VM-entry.
> Another method is to start with 1000ns, and increase the value
> in say 500ns increments until avg cyclictest numbers stop decreasing.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>

> +++ kvm/arch/x86/kvm/lapic.c
> @@ -1087,11 +1089,64 @@ static void apic_timer_expired(struct kv
[...]
> +/*
> + * On APICv, this test will cause a busy wait
> + * during a higher-priority task.
> + */

(A bit confusing ... this test doesn't busy wait.)

> +
> +static bool lapic_timer_int_injected(struct kvm_vcpu *vcpu)
[...]
> +void wait_lapic_expire(struct kvm_vcpu *vcpu)
> +{
[...]
> + tsc_deadline = apic->lapic_timer.expired_tscdeadline;
> + apic->lapic_timer.expired_tscdeadline = 0;
> + guest_tsc = kvm_x86_ops->read_l1_tsc(vcpu, native_read_tsc());
> +
> + while (guest_tsc < tsc_deadline) {
> + int delay = min(tsc_deadline - guest_tsc, 1000ULL);

Why break the __delay() loop into smaller parts?

> +
> + __delay(delay);

(Does not have to call delay_tsc, but I guess it won't change.)

> + guest_tsc = kvm_x86_ops->read_l1_tsc(vcpu, native_read_tsc());
> + }
> }
>

Btw. simple automatic delta tuning had worse results?