[Linuxptp-users] question regarding clockcheck_sample() @clockcheck.c

[Ruei, E. <e-...@ti...>]

To whom it may concern:

We observed that the PTP reported "clockcheck: clock jumped" events when we  added some CPU/Memory load by using stress-ng.
It seemed to us that the problem was triggered by the uncertainty of the mono_interval = (int64_t)mono_ts - cc->last_mono_ts at this function where mono_ts is the local time when this routine is called.
There is some uncertainty when the function is called in particular when the system is busy and therefore there will be a large variance of mono_interval.

In our test case, the PTP will receive the sync event every 120/130ms,  the difference of receive timestamp is very stable, but there could be large difference at the mono_interval and it triggers false clock jump event.

// light load
ptp4l[1984.369]: interval  130021595 mono_interval  130022231
ptp4l[1984.489]: interval  119977963 mono_interval  119997411
ptp4l[1984.619]: interval  130005312 mono_interval  129992626
ptp4l[1984.739]: interval  120002293 mono_interval  119996434
ptp4l[1984.870]: interval  130287464 mono_interval  130400107
ptp4l[1984.989]: interval  119719964 mono_interval  119621813
ptp4l[1985.119]: interval  129993071 mono_interval  129975709

// two bad intervals
ptp4l[1985.262]: interval  119997966 mono_interval  142649231
ptp4l[1985.369]: interval  130029848 mono_interval  107389768
ptp4l[1985.369]: clockcheck: clock jumped forward or running faster than expected!

//heavy load
ptp4l[1090.977]: interval  119990608 mono_interval  109828955
ptp4l[1091.110]: interval  130000707 mono_interval  133090775
ptp4l[1091.226]: interval  120002438 mono_interval  116894219
ptp4l[1091.373]: interval  130025626 mono_interval  146662543
ptp4l[1091.500]: interval  120022987 mono_interval  126504891
ptp4l[1091.607]: interval  129964127 mono_interval  106861426
ptp4l[1091.608]: clockcheck: clock jumped forward or running faster than expected!

Is it expected behavior?

Best regards,

Eric