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I need to start a certain service after system clock was correctly stepped by crony.

System time is maintained by chrony (chronyd (chrony) version 3.5 (+CMDMON +NTP +REFCLOCK +RTC -PRIVDROP -SCFILTER -SIGND +ASYNCDNS -SECHASH +IPV6 -DEBUG)).

Chrony setup, if relevant, is:

server 192.168.100.1 trust minpoll 2 maxpoll 4 polltarget 30
refclock PPS /dev/pps0 refid KPPS trust lock GNSS maxdispersion 3 poll 2
refclock SOCK /var/run/chrony.sock refid GNSS maxdispersion 0.2 noselect
makestep 0.1 -1
driftfile /var/lib/chrony/drift
rtcsync

example of a "normal, tracking status" is:

/ # chronyc tracking
Reference ID    : C0A86401 (192.168.100.1)
Stratum         : 2
Ref time (UTC)  : Wed Dec 01 11:52:08 2021
System time     : 0.000004254 seconds fast of NTP time
Last offset     : +0.000000371 seconds
RMS offset      : 0.000011254 seconds
Frequency       : 17.761 ppm fast
Residual freq   : +0.001 ppm
Skew            : 0.185 ppm
Root delay      : 0.000536977 seconds
Root dispersion : 0.000051758 seconds
Update interval : 16.2 seconds
Leap status     : Normal

while "unsynchronized" (initial) status is:

/ # chronyc tracking
Reference ID    : 00000000 ()
Stratum         : 0
Ref time (UTC)  : Thu Jan 01 00:00:00 1970
System time     : 0.000000000 seconds fast of NTP time
Last offset     : +0.000000000 seconds
RMS offset      : 0.000000000 seconds
Frequency       : 0.000 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.000 ppm
Root delay      : 1.000000000 seconds
Root dispersion : 1.000000000 seconds
Update interval : 0.0 seconds
Leap status     : Not synchronised

I seem to remember crony can call a script whenever stratus level changes, but I was unable to find references.

In any case: Is there any way to instruct crony to run a script/program or otherwise send some signal whenever acquires/loses tracking with a valid server?

I am currently relying on a rather ugly: while chronyc tracking | grep -q "Not synchronised"; do sleep 1; done but a proactive signalling by chronyd would be preferred.

Details:

  • System is a (relatively) small IoT device running Linux (Yocto)
  • It has no RTC (it always starts with clock set to Epoch).
  • System has no connection to the Internet (initially).
  • System has connection to a device having a GNSS receiver and correct time is derived from there.
  • There may be a (sometimes 'very') long time before GNSS acquires a fix and thus can propagate time.
  • At a certain point chrony finally gets the right time and steps system clock. After this is done I need to start a service (or run a script or whatever).
  • I am currently polling chronyc tracking and parsing status, but that is not really nice.
Dudi Boy
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ZioByte
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2 Answers2

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Suggesting to investigate systemd-time-wait-sync.service in here.

The suggested technique is to use systemd service unit that waits for systemd-time-wait-sync.service to synchronize kernel clock.

Using after command in the service unit file or pipe. These technique are described here and here.

Dudi Boy
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I was looking to do the same and came up empty-handed.

I did, however, find chronyc waitsync, which appears to be a built-in way to do the polling, without the need to parse and sleep explicitly. This works well enough for my case, since I only need to delay a single start-up action.

The existence of this command also hints (albeit by no means proves) that direct triggering may not be supported. If triggering is a hard requirement, rsyslogd can help.

BTW, one can only admire the enthusiasm of systemd fans, spreading the love even when their purported answer is obviously and completely irrelevant.

Clearly, the target system does NOT use systemd. The question is about chronyd, not about systemd-timesyncd, while systemd-time-wait-sync.service applies only to the latter.

bezzerwizzer
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