Re: [RFC v2] Enabling CONFIG_NTP_PPS for NOHZ by adding ntp_error to system_time_snapshot

[David Woodhouse]

On Mon, 2026-06-22 at 10:04 +0100, David Woodhouse wrote:
> Hm, I'm leaning towards adding it unconditionally in
> ktime_get_snapshot_id() and get_device_system_crosststamp(), and not
> adding the extra field to the system_time_snapshot at all...

That works.

https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/pps

David Woodhouse (4):
      timekeeping: Apply extrapolated ntp_error to clock snapshots
      pps: Drop the !NO_HZ_COMMON dependency from NTP_PPS
      pps: Always use ktime_get_snapshot_id() for pps_get_ts()
      ptp: ptp_vmclock: Add simulated 1PPS support

Booted with NOHZ_FULL in QEMU with a test program as /init that sets
the clock coarsely from vmclock using clock_settime(), then binds the
simulated 1PPS signal to the kernel's hardpps support.

Then uses PTP_SYS_OFFSET_PRECISE() every second to see the difference
between the kernel's timekeeping, and the vmclock that it's supposed to
be converging to.

The only "problem" is that if there's occasionally a nanosecond of
jitter after ntpdata->pps_jitter has literally reached zero,
hardpps_update_phase() complains:
   [   33.917103] hardpps: PPSJITTER: jitter=1, limit=0

That's *probably* only an issue for the vmclock hack; even with
upcoming PTM-based PPS devices that literally latch the counter value
on the pulse, I don't think it'll literally get to *zero*.

When chronyd on the host adjusts the time, we see it propagate to the
guest and the guest's timekeeping quickly converge from +36ns to +0ns
again...

[    0.653414] Run /init as init process
PPS: coarse-set CLOCK_REALTIME from vmclock
[    1.655824] pps pps0: bound kernel consumer: edge=0x1
PPS: enabled, bound to hardpps, STA_PPSTIME|STA_PPSFREQ set
PRECISE: dev-sys(adj)=+5609ns OK
EXT[ 0] diff=+5608ns
EXT[ 1] diff=+5262ns
[    2.917076] hardpps: PPSJITTER: jitter=5173, limit=0
EXT[ 2] diff=+4914ns
EXT[ 3] diff=+1197ns
EXT[ 4] diff=-297ns
EXT[ 5] diff=-103ns
EXT[ 6] diff=+0ns
EXT[ 7] diff=+0ns
EXT[ 8] diff=-1ns
EXT[ 9] diff=+0ns
EXT[10] diff=+0ns
EXT[11] diff=+0ns
EXT[12] diff=+0ns
EXT[13] diff=+0ns
EXT[14] diff=+0ns
EXT[15] diff=+0ns
EXT[16] diff=+1ns
EXT[17] diff=+0ns
EXT[18] diff=+0ns
EXT[19] diff=-1ns
EXT[20] diff=-1ns
EXT[21] diff=+0ns
EXT[22] diff=+0ns
EXT[23] diff=-1ns
EXT[24] diff=-1ns
EXT[25] diff=-1ns
EXT[26] diff=+0ns
EXT[27] diff=+0ns
EXT[28] diff=+0ns
EXT[29] diff=+0ns
EXT[30] diff=+0ns
EXT[31] diff=+0ns
EXT[32] diff=+1ns
[   33.917103] hardpps: PPSJITTER: jitter=1, limit=0
EXT[33] diff=+1ns
[   34.917102] hardpps: PPSJITTER: jitter=1, limit=0
EXT[34] diff=+0ns
EXT[35] diff=+0ns
EXT[36] diff=+0ns
[   37.917105] hardpps: PPSJITTER: jitter=1, limit=0
EXT[37] diff=+0ns
[   38.917010] hardpps: PPSJITTER: jitter=1, limit=0
EXT[38] diff=-1ns
EXT[39] diff=+0ns
EXT[40] diff=+0ns
EXT[41] diff=-1ns
EXT[42] diff=-1ns
EXT[43] diff=+0ns
[   44.917107] hardpps: PPSJITTER: jitter=1, limit=0
EXT[44] diff=+0ns
[   45.917105] hardpps: PPSJITTER: jitter=1, limit=0
EXT[45] diff=-1ns
EXT[46] diff=+0ns
EXT[47] diff=-1ns
EXT[48] diff=+0ns
EXT[49] diff=-1ns
EXT[50] diff=-1ns
EXT[51] diff=+0ns
EXT[52] diff=-1ns
[   53.917104] hardpps: PPSJITTER: jitter=1, limit=0
EXT[53] diff=+0ns
[   54.917099] hardpps: PPSJITTER: jitter=1, limit=0
EXT[54] diff=+0ns
EXT[55] diff=+0ns
EXT[56] diff=+0ns
vmclock: host_cv=0x998433b116699 offset=0xfff667dd86c7d42d guest_cv=0x20c1d93ac6 tsc_khz=2400000
EXT[57] diff=+0ns
[   58.917101] hardpps: PPSJITTER: jitter=18, limit=0
EXT[58] diff=+36ns
EXT[59] diff=+26ns
EXT[60] diff=+41ns
EXT[61] diff=+44ns
EXT[62] diff=+26ns
EXT[63] diff=+40ns
EXT[64] diff=+31ns
EXT[65] diff=+33ns
EXT[66] diff=+17ns
EXT[67] diff=+21ns
EXT[68] diff=+23ns
EXT[69] diff=+14ns
EXT[70] diff=+10ns
EXT[71] diff=+21ns
EXT[72] diff=+23ns
EXT[73] diff=+24ns
EXT[74] diff=+14ns
EXT[75] diff=+21ns
EXT[76] diff=+23ns
EXT[77] diff=+14ns
EXT[78] diff=+20ns
EXT[79] diff=+23ns
EXT[80] diff=+24ns
EXT[81] diff=+14ns
EXT[82] diff=+2ns
EXT[83] diff=+0ns
EXT[84] diff=+0ns

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