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

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Message-ID: <87wog1kpib.fsf@vitty.brq.redhat.com>
Date:   Mon, 29 Jul 2019 14:13:16 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     lantianyu1986@...il.com, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-arch@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        sashal@...nel.org, daniel.lezcano@...aro.org, 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

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).

-- 
Vitaly