linux-kernel - Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function

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Date:   Tue, 30 Jul 2019 21:41:18 +0800
From:   Tianyu Lan <lantianyu1986@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-arch@...r.kernel.org, linux-hyperv@...r.kernel.org,
        "linux-kernel@...r kernel org" <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, michael.h.kelley@...rosoft.com,
        ashal@...nel.org
Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function

Hi Vitaly & Peter:
    Thanks for your review.

On Mon, Jul 29, 2019 at 8:13 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Peter Zijlstra <peterz@...radead.org> writes:
>
> > On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote:
> >> lantianyu1986@...il.com writes:
> >>
> >> > From: Tianyu Lan <Tianyu.Lan@...rosoft.com>
> >> >
> >> > Hyper-V guests use the default native_sched_clock() in pv_ops.time.sched_clock
> >> > on x86.  But native_sched_clock() directly uses the raw TSC value, which
> >> > can be discontinuous in a Hyper-V VM.   Add the generic hv_setup_sched_clock()
> >> > to set the sched clock function appropriately.  On x86, this sets
> >> > pv_ops.time.sched_clock to read the Hyper-V reference TSC value that is
> >> > scaled and adjusted to be continuous.
> >>
> >> Hypervisor can, in theory, disable TSC page and then we're forced to use
> >> MSR-based clocksource but using it as sched_clock() can be very slow,
> >> I'm afraid.
> >>
> >> On the other hand, what we have now is probably worse: TSC can,
> >> actually, jump backwards (e.g. on migration) and we're breaking the
> >> requirements for sched_clock().
> >
> > That (obviously) also breaks the requirements for using TSC as
> > clocksource.
> >
> > IOW, it breaks the entire purpose of having TSC in the first place.
>
> Currently, we mark raw TSC as unstable when running on Hyper-V (see
> 88c9281a9fba6), 'TSC page' (which is TSC * scale + offset) is being used
> instead. The problem is that 'TSC page' can be disabled by the
> hypervisor and in that case the only remaining clocksource is MSR-based
> (slow).
>

Yes, that will be slow if Hyper-V doesn't expose hv tsc page and
kernel uses MSR based
clocksource. Each MSR read will trigger one VM-EXIT. This also happens on other
hypervisors (e,g, KVM doesn't expose KVM clock). Hypervisor should
take this into
account and determine which clocksource should be exposed or not.

-- 
Best regards
Tianyu Lan