linux-kernel - Re: [PATCH] KVM: LAPIC: Loose fluctuation filter for auto tune lapic_timer_advance

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Date:   Thu, 26 Sep 2019 08:19:28 +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>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Loose fluctuation filter for auto tune lapic_timer_advance_ns

On Thu, 26 Sep 2019 at 03:29, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Wed, Sep 25, 2019 at 01:47:04PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > 5000 guest cycles delta is easy to encounter on desktop, per-vCPU
> > lapic_timer_advance_ns always keeps at 1000ns initial value, lets
> > loose fluctuation filter a bit to make auto tune can make some
> > progress.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/lapic.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 3a3a685..258407e 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -67,7 +67,7 @@
> >
> >  static bool lapic_timer_advance_dynamic __read_mostly;
> >  #define LAPIC_TIMER_ADVANCE_ADJUST_MIN 100
> > -#define LAPIC_TIMER_ADVANCE_ADJUST_MAX 5000
> > +#define LAPIC_TIMER_ADVANCE_ADJUST_MAX 10000
> >  #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
> >  /* step-by-step approximation to mitigate fluctuation */
> >  #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> > @@ -1504,7 +1504,7 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> >               timer_advance_ns += ns/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> >       }
> >
> > -     if (unlikely(timer_advance_ns > LAPIC_TIMER_ADVANCE_ADJUST_MAX))
> > +     if (unlikely(timer_advance_ns > LAPIC_TIMER_ADVANCE_ADJUST_MAX/2))
>
> Doh, missed that these are two different time domains in the original
> review, i.e. ns vs. cycles.

I try to save one #define in this patch, will fold below in next version.

    Wanpeng

>
> We should use separate defines for the filter since that check is done
> in cycles.  Not sure what names to use to keep things somewhat clear.
>
> Maybe s/ADJUST/EXPIRE for the cycles, and s/ADJUST/NS for the ns ones?
> E.g.:
>
> #define LAPIC_TIMER_ADVANCE_EXPIRE_MIN  100
> #define LAPIC_TIMER_ADVANCE_EXPIRE_MAX  10000
> #define LAPIC_TIMER_ADVANCE_NS_MAX      5000
> #define LAPIC_TIMER_ADVANCE_NS_INIT     1000
>
> >               timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> >       apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> >  }
> > --
> > 2.7.4
> >