linux-kernel - Re: [PATCH] kvm: lapic: add module parameters for LAPIC_TIMER_ADVANCE_ADJUST

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Date:   Tue, 9 Mar 2021 15:42:15 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Haiwei Li <lihaiwei.kernel@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH] kvm: lapic: add module parameters for
 LAPIC_TIMER_ADVANCE_ADJUST_MAX/MIN

On Wed, Mar 03, 2021, Haiwei Li wrote:
> On 21/3/3 10:09, lihaiwei.kernel@...il.com wrote:
> > From: Haiwei Li <lihaiwei@...cent.com>
> > 
> > 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?

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