Re: [PATCH v4] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

[Wanpeng Li]

On Tue, 17 Sep 2019 at 02:55, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Mon, Sep 16, 2019 at 04:39:59PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > Filter out drastic fluctuation and random fluctuation, remove
> > timer_advance_adjust_done altogether, the adjustment would be
> > continuous.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/lapic.c | 26 ++++++++++++--------------
> >  arch/x86/kvm/lapic.h |  1 -
> >  arch/x86/kvm/x86.c   |  2 +-
> >  arch/x86/kvm/x86.h   |  1 +
> >  4 files changed, 14 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index dbbe478..2585b86 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -69,6 +69,7 @@
> >  #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
> >  /* step-by-step approximation to mitigate fluctuation */
> >  #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> > +#define LAPIC_TIMER_ADVANCE_FILTER 5000
> >
> >  static inline int apic_test_vector(int vec, void *bitmap)
> >  {
> > @@ -1482,29 +1483,28 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> >                                             s64 advance_expire_delta)
> >  {
> >       struct kvm_lapic *apic = vcpu->arch.apic;
> > -     u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
> > -     u64 ns;
> > +     u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns, ns;
>
> Is changing 'ns' to a u32 intentionaly?  It's still cast to a u32 in the
> calculations, and set from @advance_expire_delta.
>
> > +
> > +     if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_FILTER ||
>
> Shouldn't LAPIC_TIMER_ADVANCE_FILTER be used in the other "if ... > 5000"
> check?
>
>         if (unlikely(timer_advance_ns > 5000))
>
> And maybe name it LAPIC_TIMER_ADVANCE_MAX or something?
>
> > +             abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE) {
>
> This should be aligned with the other 'abs', e.g.:
>
>         if (abs(...) ||
>             abs(...))
>                 return
>
> > +             /* filter out random fluctuations */
>
> If you put the comment above the if statement then you can drop the
> parentheses.  And if you're going to bother with a comment, maybe be a bit
> more explicit?  E.g.:
>
>         /* Do not adjust for tiny fluctuations or large random spikes. */
>         if (abs(...) ||
>             abs(...))
>                 return;
>
> > +             return;
> > +     }
> >
> >       /* too early */
> >       if (advance_expire_delta < 0) {
> >               ns = -advance_expire_delta * 1000000ULL;
> >               do_div(ns, vcpu->arch.virtual_tsc_khz);
> > -             timer_advance_ns -= min((u32)ns,
> > -                     timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> > +             timer_advance_ns -= ns/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> >       } else {
> >       /* too late */
> >               ns = advance_expire_delta * 1000000ULL;
> >               do_div(ns, vcpu->arch.virtual_tsc_khz);
> > -             timer_advance_ns += min((u32)ns,
> > -                     timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> > +             timer_advance_ns += ns/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> >       }
> >
> > -     if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
> > -             apic->lapic_timer.timer_advance_adjust_done = true;
> > -     if (unlikely(timer_advance_ns > 5000)) {
> > +     if (unlikely(timer_advance_ns > 5000))
> >               timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> > -             apic->lapic_timer.timer_advance_adjust_done = false;
> > -     }
> >       apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> >  }
> >
> > @@ -1524,7 +1524,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> >       if (guest_tsc < tsc_deadline)
> >               __wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
> >
> > -     if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
> > +     if (lapic_timer_advance_ns == -1)
>
> Rather than expose 'lapic_timer_advance_ns' from x86.c, what if we add a
> 'static bool dynamically_adjust_timer_advance __read_mostly;' in lapic.c,
> and have that be set in kvm_create_lapic() and checked here?  That'd make
> this code a little more readable, would make this patch more obvious (it
> wasn't immediately clear why lapic_timer_advance_ns was being exposed),
> and would reduce the probability of unintentionally writing/corrupting the
> module param.
>
> >               adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
> >  }
> >
> > @@ -2302,10 +2302,8 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
> >       apic->lapic_timer.timer.function = apic_timer_fn;
> >       if (timer_advance_ns == -1) {
> >               apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> > -             apic->lapic_timer.timer_advance_adjust_done = false;
> >       } else {
> >               apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> > -             apic->lapic_timer.timer_advance_adjust_done = true;
> >       }
>
> Parentheses can be dropped (unless this is converted to a local global, as
> above).

I just handle all the comments in the new version.

    Wanpeng