Re: [PATCH v4 2/4] pps: Drop the !NO_HZ_COMMON dependency from NTP_PPS

[Rodolfo Giometti <giometti@enneenne.com>]

Hi David,

On 30/09/2026 03:28, David Woodhouse wrote:
> Ok, I have the series in the shape I want to test it now. I got caught
> up in more side quests around minimising ntp_error, which aren't
> *strictly* around hardpps or snapshots at all but it was annoying me.
>
> https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/timekeeping
> now has:
>
>  • timekeeping: Allow tick_length changes to apply mid-tick
>    (reduce a fairly gratuitous cause of ntp_error accumulation, when
>     we *account* for a rate change before it actually takes effect)
>
>  • timekeeping: Apply extrapolated ntp_error to clock snapshots
>    (you know this one; as discussed maybe it'll shift to later)
>
>  • timekeeping: Bound idle sleep while a phase slew is in flight
>    (even standard adjtime() was hosed for tickless and would keep
>     applying 500µs/s skew the whole time the system slept)
>
>  • timekeeping: Reinstate proportional correction of ntp_error
>    (because once I fix the above, we *can*)
>
>  • ntp: Recalculate skew_delta when the phase offset changes
>    (another gratuitous cause of ntp_error accumulation. If time_offset
>     goes away *while* we're skewing towards it, the continued skew
>     for the rest of the second is unwanted and goes to ntp_error.
>     Just... stop skewing!)
>
>  • arm64: Support inlined clocksource reads for the arch counter
>    (this seems like an oversight)
>
>  • timekeeping: Read the counter as early as possible in ktime_get_snapshot_id()
>    (as I said, those 20ns are in the noise... but you can have them
>     back anyway)
>
>  • pps: Drop the !NO_HZ_COMMON dependency from NTP_PPS
>    (Really, it isn't needed. This is working)
>
>  • pps: Always use ktime_get_snapshot_id() for pps_get_ts()
>    (I think I mostly eliminated ntp_error as a significant source of
>     discrepancies now, but this is still the right thing to do for
>     precision)

As you suggested on the 28th, wouldn't it be better to move "pps: Always
use ktime_get_snapshot_id() for pps_get_ts()" before "timekeeping: Apply
extrapolated ntp_error to clock snapshots"? So the PPS patch has no
visible effect on ts_real, and the change stays in the timekeeping
patch, where the chrony and ntpd people can judge it.

Ciao,

Rodolfo