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

Marcelo Tosatti
2014-12-11T03:08:10Z

On Thu, Dec 11, 2014 at 12:37:57AM +0100, Paolo Bonzini wrote:
>
>
> On 10/12/2014 21:57, Marcelo Tosatti wrote:
> > 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 cyclictest avg latency by 50%.
> >
> > 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.
>
> What values are you using in practice for the parameter?

7us.

> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >
> > Index: kvm/arch/x86/kvm/lapic.c
> > ===================================================================
> > --- kvm.orig/arch/x86/kvm/lapic.c
> > +++ kvm/arch/x86/kvm/lapic.c
> > @@ -33,6 +33,7 @@
> > #include <asm/page.h>
> > #include <asm/current.h>
> > #include <asm/apicdef.h>
> > +#include <asm/delay.h>
> > #include <linux/atomic.h>
> > #include <linux/jump_label.h>
> > #include "kvm_cache_regs.h"
> > @@ -1073,6 +1074,7 @@ static void apic_timer_expired(struct kv
> > {
> > struct kvm_vcpu *vcpu = apic->vcpu;
> > wait_queue_head_t *q = &vcpu->wq;
> > + struct kvm_timer *ktimer = &apic->lapic_timer;
> >
> > /*
> > * Note: KVM_REQ_PENDING_TIMER is implicitly checked in
> > @@ -1087,11 +1089,58 @@ static void apic_timer_expired(struct kv
> >
> > if (waitqueue_active(q))
> > wake_up_interruptible(q);
> > +
> > + if (ktimer->timer_mode_mask == APIC_LVT_TIMER_TSCDEADLINE)
> > + ktimer->expired_tscdeadline = ktimer->tscdeadline;
> > +}
> > +
> > +static bool lapic_timer_int_injected(struct kvm_vcpu *vcpu)
> > +{
> > + struct kvm_lapic *apic = vcpu->arch.apic;
> > + u32 reg = kvm_apic_get_reg(apic, APIC_LVTT);
> > +
> > + if (kvm_apic_hw_enabled(apic)) {
> > + int vec = reg & APIC_VECTOR_MASK;
> > +
> > + if (kvm_x86_ops->test_posted_interrupt)
> > + return kvm_x86_ops->test_posted_interrupt(vcpu, vec);
> > + else {
> > + if (apic_test_vector(vec, apic->regs + APIC_ISR))
> > + return true;
> > + }
>
> One branch here is testing IRR, the other is testing ISR. I think
> testing ISR is right; on APICv, the above test will cause a busy wait
> during a higher-priority task (or during an interrupt service routine
> for the timer itself), just because the timer interrupt was delivered.

Yes.

> So, on APICv, if the interrupt is in PIR but it has bits 7:4 <=
> PPR[7:4], you have a problem. :( There is no APICv hook that lets you
> get a vmexit when the PPR becomes low enough.

Well, you simply exit earlier and busy spin for VM-exit
time.

For Linux guests, there is no problem.

> > + }
> > + return false;
> > +}
> > +
> > +void wait_lapic_expire(struct kvm_vcpu *vcpu)
> > +{
> > + struct kvm_lapic *apic = vcpu->arch.apic;
> > + u64 guest_tsc, tsc_deadline;
> > +
> > + if (!kvm_vcpu_has_lapic(vcpu))
> > + return;
> > +
> > + if (!apic_lvtt_tscdeadline(apic))
> > + return;
>
> This test is wrong, I think. You need to check whether the timer
> interrupt was a TSC deadline interrupt. Instead, you are checking
> whether the current mode is TSC-deadline. This can be different if the
> interrupt could not be delivered immediately after it was received.
> This is easy to fix: replace the first two tests with
> "apic->lapic_timer.expired_tscdeadline != 0" and...

Yes.

> > + if (!lapic_timer_int_injected(vcpu))
> > + return;
> > + tsc_deadline = apic->lapic_timer.expired_tscdeadline;
>
> ... set apic->lapic_timer.expired_tscdeadline to 0 here.
>
> But I'm not sure how to solve the above problem with APICv. That's a
> pity. Knowing what values you use in practice for the parameter, would
> also make it easier to understand the problem. Please report that
> together with the graphs produced by the unit test you added.