I can easily get a large number of messages in the system log similar to this:
# dmesg | grep backwards
calcru: runtime went backwards from 5188 usec to 2727 usec for pid 770 (cron)
calcru: runtime went backwards from 2944 usec to 1488 usec for pid 766 (sendmail)
calcru: runtime went backwards from 2497 usec to 1262 usec for pid 766 (sendmail)
It looks this is caused by the wrongly-detected TSC frequency and I made this patch to address this:
use hyper-v time counter (rather than i8254) to calibrate TSC
The i8254 PIT counter emulated by Hyper-V is not reliable.
In probe_tsc_freq() -> DELAY -> init_ops.early_delay() -> i8254_delay(),
getit() can return these values when it is invoked 10 times:
1 pit count0 = 228
2 pit count0 = 131
3 pit count0 = 34
4 pit count0 = 65473 <-- this is a normal wrap-around.
5 pit count0 = 65375
6 pit count0 = 65278
7 pit count0 = 65180
8 pit count0 = 65388 <-- this is bad!
9 pit count0 = 65290
10 pit count0 = 65193
For the 8th time, the 'delta' in i8254_delay() is < 0 while
it shouldn't, so the later "delta += i8254_max_count;" and
"ticks_left -= delta" will cause i8254_delay() to wait shorter than
expected, and finally probe_tsc_freq() get a smaller 'tsc_freq'.
A smaller 'tsc_freq' can cause time inaccuracy in dtrace.
It can also cause warnings like
calcru: runtime went backwards from 50 usec to 25 usec for pid 0 (kernel)
calcru: runtime went backwards from 1471 usec to 743 usec for pid 0 (kernel)
calcru: runtime went backwards from 40 usec to 20 usec for pid 0 (kernel)
calcru: runtime went backwards from 18 usec to 9 usec for pid 0 (kernel)
calcru: runtime went backwards from 46204978 usec to 23362331 usec for pid 0 (kernel)
We use Hyper-V time counter, which is much more reliable than i8254,
to calibrate TSC.
Fixed in 11-CURRENT by:
The tsc-calibration fix was temporarily taken back last Friday because we found an interesting thing: with some kind of config, Xen can pretend to be Hyper-V by mimicking the hypervisor CPUID signature (this is usually to make Windows VM run better on Xen), but Xen doesn’t support Hyper-V timecounter, so the fix causes an unexpected panic when FreeBSD VM runs on Xen in this case.
We're making another patch to better detect if the underlying hypervisor supports Hyper-V timecounter or not:
The further patches on 11-CURRENT should fix the bug safely now: