Re: [RFC] Enabling CONFIG_NTP_PPS for NOHZ by adding ntp_error to system_time_snapshot

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2026-06-19 at 22:21 +0200, Thomas Gleixner wrote:
> On Fri, Jun 19 2026 at 16:34, David Woodhouse wrote:
> > On Fri, 2026-06-19 at 15:34 +0200, Thomas Gleixner wrote:
> > > 
> > > This formatting makes my brain hurt. Can you please split that out into
> > > a separate function?
> > 
> > Yep. There's also a potential error there — an *additional* discrepancy
> > comes from the enforced monotonicity that timekeeping_cycles_to_ns()
> > applies (the case where it just returns tkr->xtime_nsec >> tkr_shift).
> > 
> > I couldn't work out if I cared about the clocksource-is-non-monotonic
> > casse, and even if I did, what I should do about it.
> 
> I think the right thing is just to ignore it.

Yeah, that was basically my conclusion; I had just meant to *mention*
it when posting the RFC.

> The problem is very narrow and mostly related to the historically badly
> synchronized TSC between sockets. The TSC_ADJUST fixup is obviously
> error prone as it adjusts only to the point where the error is not
> longer observable. But in the update transition phase it can result in
> time going backwards because the readout on the other CPU is slightly
> behind tk::tkr_mono::cycles_last. That happens only once in a while and
> we talk about a very low single digit number of TSC cycles.
> 
> > I also wasn't sure if this should be a new CLOCK_REALTIME_NONMONOTONIC
> > or something like that, such that e.g. PTP clients could *ask* for it.
> 
> Hell no!

That was not about the above clocksource nonsense; that was the
question of what the caller (in my example case, the vmclock PTP
snapshot) should *do* with the reported error value.

If I just unconditionally "correct" the CLOCK_REALTIME values then
that's arguably an ABI change. We're silently reporting something
*different* to what we did before.

Maybe that's OK... as I said, in the PPS case we can justify it and
just call it a bug fix?

Or maybe we want a way for callers (not of ktime_get_snapshot_id()
itself, but *their* callers) to *ask* for the "corrected" value
instead. I happened to call that CLOCK_REALTIME_NONMONOTONIC as a straw
man, just because monotonicity is *one* of the reasons why we present
the xtime values that we do, not always the raw "corrected" values.

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