lkml.org 
[lkml]   [2019]   [Aug]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function
Date
From: Tianyu Lan <lantianyu1986@gmail.com> Sent: Tuesday, July 30, 2019 6:41 AM
>
> 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.
>

We've confirmed with the Hyper-V team that the TSC page is always available
on Hyper-V 2016 and later, and on Hyper-V 2012 R2 when the physical
hardware presents an InvariantTSC. But the Linux Kconfig's are set up so
the TSC page is not used for 32-bit guests -- all clock reads are synthetic MSR
reads. For 32-bit, this set of changes will add more overhead because the
sched clock reads will now be MSR reads.

I would be inclined to fix the problem, even with the perf hit on 32-bit Linux.
I don’t have any data on 32-bit Linux being used in a Hyper-V guest, but it's not
supported in Azure so usage is pretty small. The alternative would be to continue
to use the raw TSC value on 32-bit, even with the risk of a discontinuity in case of
live migration or similar scenarios.

Michael
\
 
 \ /
  Last update: 2019-08-12 21:23    [W:0.079 / U:0.020 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site