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

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Message-ID: <87zhkxksxd.fsf@vitty.brq.redhat.com>
Date:   Mon, 29 Jul 2019 12:59:26 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     lantianyu1986@...il.com
Cc:     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

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().

-- 
Vitaly