Re: [PATCH] kvm: lapic: add module parameters for LAPIC_TIMER_ADVANCE_ADJUST_MAX/MIN

[Haiwei Li]

On Wed, Mar 10, 2021 at 7:42 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Mar 03, 2021, Haiwei Li wrote:
> > On 21/3/3 10:09, lihaiwei.kernel@xxxxxxxxx wrote:
> > > From: Haiwei Li <lihaiwei@xxxxxxxxxxx>
> > >
> > > In my test environment, advance_expire_delta is frequently greater than
> > > the fixed LAPIC_TIMER_ADVANCE_ADJUST_MAX. And this will hinder the
> > > adjustment.
> >
> > Supplementary details:
> >
> > I have tried to backport timer related features to our production
> > kernel.
> >
> > After completed, i found that advance_expire_delta is frequently greater
> > than the fixed value. It's necessary to trun the fixed to dynamically
> > values.
>
> Does this reproduce on an upstream kernel?  If so...
>
>   1. How much over the 10k cycle limit is the delta?
>   2. Any idea what causes the large delta?  E.g. is there something that can
>      and/or should be fixed elsewhere?
>   3. Is it platform/CPU specific?

Hi, Sean

I have traced the flow on our production kernel and it frequently consumes more
than 10K cycles from sched_out to sched_in.
So two scenarios tested on Cascade lake Server(96 pcpu), v5.11 kernel.

1. only cyclictest in guest(88 vcpu and bound with isolated pcpus, w/o mwait
exposed, adaptive advance lapic timer is default -1). The ratio of occurrences:

greater_than_10k/total: 29/2060, 1.41%

2. cyclictest in guest(88 vcpu and not bound, w/o mwait exposed, adaptive
advance lapic timer is default -1) and stress in host(no isolate). The ratio of
occurrences:

greater_than_10k/total: 122381/1017363, 12.03%

-- 
Haiwei Li

>
> Ideally, KVM would play nice with "all" environments by default without forcing
> the admin to hand-tune things.