linux-kernel - Re: [GIT PULL] KVM: x86: MMU changes for 6.9

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Message-ID: <CABgObfb7cc+q3qwr_zKk3SXec_3VtbJ5yWAkVwYXdsFQAB1X_Q@mail.gmail.com>
Date: Thu, 14 Mar 2024 19:43:17 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] KVM: x86: MMU changes for 6.9

On Thu, Mar 14, 2024 at 7:38 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Mar 14, 2024, Paolo Bonzini wrote:
> > On Fri, Mar 8, 2024 at 11:37 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > >  - Zap TDP MMU roots at 4KiB granularity to minimize the delay in yielding if
> > >    a reschedule is needed, e.g. if a high priority task needs to run.  Because
> > >    KVM doesn't support yielding in the middle of processing a zapped non-leaf
> > >    SPTE, zapping at 1GiB granularity can result in multi-millisecond lag when
> > >    attempting to schedule in a high priority.
> > >
> >
> > Would 2 MiB provide a nice middle ground?
>
> Not really?
>
> Zapping at 2MiB definitely fixes the worst of the tail latencies, but there is
> still a measurable difference between 2MiB and 4KiB.

Yeah, but you said multi millisecond so I guessed 5/512 is a 10
microsecond latency, which should be pretty acceptable (for PREEMPT_RT
tests at Red Hat we shoot at 10-15 worst case, so for CONFIG_PREEMPT
it would be more than enough).

> And on the other side of the
> coing, I was unable to observe a meaningful difference in total runtime by zapping
> at 2MiB, or even 1GiB, versus 4KiB.

Ok, that's the answer.

Paolo

> In other words, AFAICT, there's no need to shoot for a middle ground because trying
> to zap at larger granularities doesn't buy us anything.
>