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

From: Tianyu Lan <lantianyu1986@gmail.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Tianyu Lan <Tianyu.Lan@microsoft.com>,
	linux-arch@vger.kernel.org, linux-hyperv@vger.kernel.org,
	"linux-kernel@vger kernel org" <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	KY Srinivasan <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Sasha Levin <sashal@kernel.org>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	michael.h.kelley@microsoft.com, ashal@kernel.org
Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function
Date: Tue, 30 Jul 2019 21:41:18 +0800	[thread overview]
Message-ID: <CAOLK0py6ngy9kAnZcRMBK8U45s2L5Wo4X0NP_qPM0zv7WjeVQQ@mail.gmail.com> (raw)
In-Reply-To: <87wog1kpib.fsf@vitty.brq.redhat.com>

Hi Vitaly & Peter:
    Thanks for your review.

On Mon, Jul 29, 2019 at 8:13 PM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Peter Zijlstra <peterz@infradead.org> writes:
>
> > On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote:
> >> lantianyu1986@gmail.com writes:
> >>
> >> > From: Tianyu Lan <Tianyu.Lan@microsoft.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