linux-kernel - Re: [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time

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Message-ID: <CABCjUKCU_3q_VfzergdmRK4Tc2BxUcmCotf+E8MpWX6o+g0CTA@mail.gmail.com>
Date: Wed, 5 Mar 2025 13:17:04 +0900
From: Suleiman Souhlal <suleiman@...gle.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Chao Gao <chao.gao@...el.com>, 
	David Woodhouse <dwmw2@...radead.org>, Sergey Senozhatsky <senozhatsky@...omium.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, ssouhlal@...ebsd.org
Subject: Re: [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time

On Sun, Mar 2, 2025 at 2:22 AM Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
>
> On Fri, Feb 21, 2025 at 02:39:27PM +0900, Suleiman Souhlal wrote:
> > When the host resumes from a suspend, the guest thinks any task
> > that was running during the suspend ran for a long time, even though
> > the effective run time was much shorter, which can end up having
> > negative effects with scheduling.
> >
> > To mitigate this issue, the time that the host was suspended is included
> > in steal time, which lets the guest can subtract the duration from the
>
> s/can//
> > tasks' runtime.
> >
> > In order to implement this behavior, once the suspend notifier fires,
> > vCPUs trying to run block until the resume notifier finishes. This is
>
> s/run/run will/
> > because the freezing of userspace tasks happens between these two points,
> Full stop at the end of that                                              ^
> > which means that vCPUs could otherwise run and get their suspend steal
> > time misaccounted, particularly if a vCPU would run after resume before
> > the resume notifier.
>
> s/notifier/notifier fires/
>
> > Incidentally, doing this also addresses a potential race with the
> > suspend notifier setting PVCLOCK_GUEST_STOPPED, which could then get
> > cleared before the suspend actually happened.
> >
> > One potential caveat is that in the case of a suspend happening during
> > a VM migration, the suspend time might not be accounted.
>
> s/accounted/accounted for./
> > A workaround would be for the VMM to ensure that the guest is entered
> > with KVM_RUN after resuming from suspend.
>
> So ..does that mean there is a QEMU patch as well?

No, I am not planning on making a QEMU patch.
A QEMU patch would only be needed if you cared about the caveat mentioned there.

Thanks,
-- Suleiman