linux-kernel - Re: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance

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Message-ID: <CANRm+Cy-u4AJ8kBMy44JAv-7er9YmwgY0gbq1QR=Pt4MV9JvJA@mail.gmail.com>
Date:   Mon, 16 Sep 2019 16:09:55 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <wanpeng.li@...mail.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

On Mon, 16 Sep 2019 at 15:49, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 16/09/19 06:02, Wanpeng Li wrote:
> > Hi Paolo,
> >
> > Something like below? It still fluctuate when running complex guest os
> > like linux. Removing timer_advance_adjust_done will hinder introduce
> > patch v3 2/2 since there is no adjust done flag in each round
> > evaluation.
>
> That's not important, since the adjustment would be continuous.
>
> How much fluctuation can you see?

After I add a trace_printk to observe more closely, the adjustment is
continuous as expected.

>
> > I have two questions here, best-effort tune always as
> > below or periodically revaluate to get conservative value and get
> > best-effort value in each round evaluation as patch v3 2/2, which one
> > do you prefer? The former one can wast time to wait sometimes and the
> > later one can not get the best latency. In addition, can the adaptive
> > tune algorithm be using in all the scenarios contain
> > RT/over-subscribe?
>
> I prefer the former, like the patch below, mostly because of the extra
> complexity of the periodic reevaluation.

How about question two?

    Wanpeng