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Date:   Tue, 14 May 2019 09:45:40 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH 3/3] KVM: LAPIC: Optimize timer latency further

On Tue, 14 May 2019 at 03:54, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Thu, May 09, 2019 at 07:29:21PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Advance lapic timer tries to hidden the hypervisor overhead between host
> > timer fires and the guest awares the timer is fired. However, it just hidden
> > the time between apic_timer_fn/handle_preemption_timer -> wait_lapic_expire,
> > instead of the real position of vmentry which is mentioned in the orignial
> > commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline hrtimer
> > expiration"). There is 700+ cpu cycles between the end of wait_lapic_expire
> > and before world switch on my haswell desktop, it will be 2400+ cycles if
> > vmentry_l1d_flush is tuned to always.
> >
> > This patch tries to narrow the last gap, it measures the time between
> > the end of wait_lapic_expire and before world switch, we take this
> > time into consideration when busy waiting, otherwise, the guest still
> > awares the latency between wait_lapic_expire and world switch, we also
> > consider this when adaptively tuning the timer advancement. The patch
> > can reduce 50% latency (~1600+ cycles to ~800+ cycles on a haswell
> > desktop) for kvm-unit-tests/tscdeadline_latency when testing busy waits.
> >
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> > Cc: Liran Alon <liran.alon@...cle.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/lapic.c   | 23 +++++++++++++++++++++--
> >  arch/x86/kvm/lapic.h   |  8 ++++++++
> >  arch/x86/kvm/vmx/vmx.c |  2 ++
> >  3 files changed, 31 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index e7a0660..01d3a87 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1545,13 +1545,19 @@ void 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, (apic->lapic_timer.measure_delay_done == 2) ?
> > +             rdtsc() + apic->lapic_timer.vmentry_delay : rdtsc());
> >       trace_kvm_wait_lapic_expire(vcpu->vcpu_id, guest_tsc - tsc_deadline);
> >
> >       if (guest_tsc < tsc_deadline)
> >               __wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
> >
> >       adaptive_tune_timer_advancement(vcpu, guest_tsc, tsc_deadline);
> > +
> > +     if (!apic->lapic_timer.measure_delay_done) {
> > +             apic->lapic_timer.measure_delay_done = 1;
> > +             apic->lapic_timer.vmentry_delay = rdtsc();
> > +     }
> >  }
> >
> >  static void start_sw_tscdeadline(struct kvm_lapic *apic)
> > @@ -1837,6 +1843,18 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
> >       }
> >  }
> >
> > +void kvm_lapic_measure_vmentry_delay(struct kvm_vcpu *vcpu)
> > +{
> > +     struct kvm_timer *ktimer = &vcpu->arch.apic->lapic_timer;
>
> This will #GP if the APIC is not in-kernel, i.e. @apic is NULL.
>
> > +
> > +     if (ktimer->measure_delay_done == 1) {
> > +             ktimer->vmentry_delay = rdtsc() -
> > +                     ktimer->vmentry_delay;
> > +             ktimer->measure_delay_done = 2;
>
> Measuring the delay a single time is bound to result in random outliers,
> e.g. if an NMI happens to occur after wait_lapic_expire().
>
> Rather than reinvent the wheel, can we simply move the call to
> wait_lapic_expire() into vmx.c and svm.c?  For VMX we'd probably want to
> support the advancement if enable_unrestricted_guest=true so that we avoid
> the emulation_required case, but other than that I don't see anything that
> requires wait_lapic_expire() to be called where it is.

I also considered to move wait_lapic_expire() into vmx.c and svm.c
before, what do you think, Paolo, Radim?

Regards,
Wanpeng Li

>
> > +     }
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_lapic_measure_vmentry_delay);
> > +
> >  int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
> >  {
> >       int ret = 0;
> > @@ -2318,7 +2336,8 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
> >               apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> >               apic->lapic_timer.timer_advance_adjust_done = true;
> >       }
> > -
> > +     apic->lapic_timer.vmentry_delay = 0;
> > +     apic->lapic_timer.measure_delay_done = 0;
> >
> >       /*
> >        * APIC is created enabled. This will prevent kvm_lapic_set_base from
> > diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> > index d6d049b..f1d037b 100644
> > --- a/arch/x86/kvm/lapic.h
> > +++ b/arch/x86/kvm/lapic.h
> > @@ -35,6 +35,13 @@ struct kvm_timer {
> >       atomic_t pending;                       /* accumulated triggered timers */
> >       bool hv_timer_in_use;
> >       bool timer_advance_adjust_done;
> > +     /**
> > +      * 0 unstart measure
> > +      * 1 start record
> > +      * 2 get delta
> > +      */
> > +     u32 measure_delay_done;
> > +     u64 vmentry_delay;
> >  };
> >
> >  struct kvm_lapic {
> > @@ -230,6 +237,7 @@ void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu);
> >  void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu);
> >  bool kvm_lapic_hv_timer_in_use(struct kvm_vcpu *vcpu);
> >  void kvm_lapic_restart_hv_timer(struct kvm_vcpu *vcpu);
> > +void kvm_lapic_measure_vmentry_delay(struct kvm_vcpu *vcpu);
> >
> >  static inline enum lapic_mode kvm_apic_mode(u64 apic_base)
> >  {
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 9663d41..a939bf5 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6437,6 +6437,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >       if (vcpu->arch.cr2 != read_cr2())
> >               write_cr2(vcpu->arch.cr2);
> >
> > +     kvm_lapic_measure_vmentry_delay(vcpu);
>
> This should be wrapped in an unlikely of some form given that it happens
> literally once out of thousands/millions runs.
>
> > +
> >       vmx->fail = __vmx_vcpu_run(vmx, (unsigned long *)&vcpu->arch.regs,
> >                                  vmx->loaded_vmcs->launched);
> >
> > --
> > 2.7.4
> >

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