> To analyze the kernel behaviour I would at least need the information fed
> to the kernel, e.g. by tracing the ntp daemon via "TZ=utc strace -f -tt
> -e trace=adjtimex -o ntpd.trace ..." and the ntp peer logs of same time
> period. Kernel boot messages and config might help as well.
Actually, I just found it! It's kind of a combination bug.
I had hacked NTP_MINPOLL to 2 in the ntpd source, and was using that
value on the pps source. But ntpd would copy that value, minus 4,
into ntv.constant (and set ADJ_TIMECONST), and adjtimex() would complain
and return EINVAL.
It wouldn't accept the passed-in values, but would still fill in the
structure with the current values.
But ntpd didn't notice or log this error, and would log the *returned*
consstant. Which was oddly dissociated from the expected one.
The failure to check the return code is an ntpd bug.
Now, in the kernel, I wonder what the allowed range should properly be.
I notice that on the input side, you have
time_constant = min(txc->constant + 4, (long)MAXTC);
but on the output side, it's
txc->constant = time_constant;
This is clearly inconsistent and wrong, but which side should be fixed
is an interesting question I haven't got an answer to yet.
For now, I've just hacked the kernel to allow values of time_constant
down to 0 (txc->constant >= -4), as nothing will break with that
value. (The only complicated one is ntp.c:296, and SHIFT_PLL is 4,
while SHIFT_NSEC is 12, so it all works.)
(Also, is MAXTC high enough for ntpv4? I think it's in "-4" space,
so it should be time_constant = min(txc->constant, (long)MAXTC) + 4;)
That was an interesting piece of debugging... I was all over the map
before finding it. And your suggestion of "strace -e adjtimex" was
indeed exactly what found it in the end.
So thanks!
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