Re: [PATCH 1/2] KVM: LAPIC: Optimize timer latency consider world switch time

[Wanpeng Li]

On Fri, 31 May 2019 at 03:36, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Thu, May 23, 2019 at 12:18:50PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > Advance lapic timer tries to hidden the hypervisor overhead between the
> > host emulated timer fires and the guest awares the timer is fired. However,
> > even though after more sustaining optimizations, kvm-unit-tests/tscdeadline_latency
> > still awares ~1000 cycles latency since we lost the time between the end of
> > wait_lapic_expire and the guest awares the timer is fired. There are
> > codes between the end of wait_lapic_expire and the world switch, futhermore,
> > the world switch itself also has overhead. Actually the guest_tsc is equal
> > to the target deadline time in wait_lapic_expire is too late, guest will
> > aware the latency between the end of wait_lapic_expire() and after vmentry
> > to the guest. This patch takes this time into consideration.
> >
> > The vmentry_lapic_timer_advance_ns module parameter should be well tuned by
> > host admin, it can reduce average cyclictest latency from 3us to 2us on
> > Skylake server. (guest w/ nohz=off, idle=poll, host w/ preemption_timer=N,
> > the cyclictest latency is not too sensitive when preemption_timer=Y for this
> > optimization in my testing), kvm-unit-tests/tscdeadline_latency can reach 0.
> >
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/lapic.c   | 17 +++++++++++++++--
> >  arch/x86/kvm/lapic.h   |  1 +
> >  arch/x86/kvm/vmx/vmx.c |  2 +-
> >  arch/x86/kvm/x86.c     |  3 +++
> >  arch/x86/kvm/x86.h     |  2 ++
> >  5 files changed, 22 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index fcf42a3..6f85221 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1531,6 +1531,19 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> >       apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> >  }
> >
> > +u64 get_vmentry_advance_delta(struct kvm_vcpu *vcpu)
>
> Hmm, this isn't a delta, I think get_vmentry_advance_cycles would be more
> appropriate.
>
> > +{
> > +     u64 vmentry_lapic_timer_advance_cycles = 0;
> > +
> > +     if (vmentry_lapic_timer_advance_ns) {
> > +             vmentry_lapic_timer_advance_cycles = vmentry_lapic_timer_advance_ns *
> > +                     vcpu->arch.virtual_tsc_khz;
> > +             do_div(vmentry_lapic_timer_advance_cycles, 1000000);
> > +     }
> > +     return vmentry_lapic_timer_advance_cycles;
> > +}
> > +EXPORT_SYMBOL_GPL(get_vmentry_advance_delta);
> > +
> >  void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> >  {
> >       struct kvm_lapic *apic = vcpu->arch.apic;
> > @@ -1544,7 +1557,7 @@ void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> >
> >       tsc_deadline = apic->lapic_timer.expired_tscdeadline;
> >       apic->lapic_timer.expired_tscdeadline = 0;
> > -     guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> > +     guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) + get_vmentry_advance_delta(vcpu);
> >       apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
> >
> >       if (guest_tsc < tsc_deadline)
> > @@ -1572,7 +1585,7 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
> >       local_irq_save(flags);
> >
> >       now = ktime_get();
> > -     guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> > +     guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) + get_vmentry_advance_delta(vcpu);
> >
> >       ns = (tscdeadline - guest_tsc) * 1000000ULL;
> >       do_div(ns, this_tsc_khz);
> > diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> > index f974a3d..df2fe17 100644
> > --- a/arch/x86/kvm/lapic.h
> > +++ b/arch/x86/kvm/lapic.h
> > @@ -221,6 +221,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
> >  bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
> >
> >  void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
> > +u64 get_vmentry_advance_delta(struct kvm_vcpu *vcpu);
> >
> >  bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
> >                       struct kvm_vcpu **dest_vcpu);
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index da24f18..0199ac3 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7047,7 +7047,7 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc,
> >
> >       vmx = to_vmx(vcpu);
> >       tscl = rdtsc();
> > -     guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> > +     guest_tscl = kvm_read_l1_tsc(vcpu, tscl) + get_vmentry_advance_delta(vcpu);
> >       delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> >       lapic_timer_advance_cycles = nsec_to_cycles(vcpu,
> >                                                   ktimer->timer_advance_ns);
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index a4eb711..a02e2c3 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -145,6 +145,9 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
> >  static int __read_mostly lapic_timer_advance_ns = -1;
> >  module_param(lapic_timer_advance_ns, int, S_IRUGO | S_IWUSR);
> >
> > +u32 __read_mostly vmentry_lapic_timer_advance_ns = 0;
> > +module_param(vmentry_lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
>
> Hmm, an interesting idea would be to have some way to "lock" this param,
> e.g. setting bit 0 locks the param.  That would allow KVM to calculate the
> cycles value to avoid the function call and the MUL+DIV.  If I'm not
> mistaken, vcpu->arch.virtual_tsc_khz is set only in kvm_set_tsc_khz().
>
> For example, if get_vmentry_advance_cycles() sees the value is locked, it
> caches the value in struct kvm_lapic.  The cached value would also need to
> be updated in kvm_set_tsc_khz() if it has been set.

Good point, handle it in v2.

Regards,
Wanpeng Li

>
> static inline u64 get_vmentry_advance_cycles(struct kvm_lapic *lapic)
> {
>         if (lapic->vmentry_advance_cycles)
>                 return lapic->vmentry_advance_cycles;
>
>         return compute_vmentry_advance_cycles(lapic);
> }
>
> u64 compute_vmentry_advance_cycles(struct kvm_lapic *lapic)
> {
>         u64 val = vmentry_lapic_timer_advance_ns;
>         u64 cycles = (val & ~1ULL) * lapic->vcpu->arch.virtual_tsc_khz;
>
>         do_div(cycles, 1000000);
>
>         if (val & 1)
>                 lapic->vmentry_advance_cycles = cycles;
>         return cycles;
> }
>
> > +
> >  static bool __read_mostly vector_hashing = true;
> >  module_param(vector_hashing, bool, S_IRUGO);
> >
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index 275b3b6..b0a3b84 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -294,6 +294,8 @@ extern u64 kvm_supported_xcr0(void);
> >
> >  extern unsigned int min_timer_period_us;
> >
> > +extern unsigned int vmentry_lapic_timer_advance_ns;
> > +
> >  extern bool enable_vmware_backdoor;
> >
> >  extern struct static_key kvm_no_apic_vcpu;
> > --
> > 2.7.4
> >