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

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Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote:
>> lantianyu1986@xxxxxxxxx writes:
>> 
>> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
>> >
>> > 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



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