[Xenomai] tsc not monotonic

["Vincent Berenz" <vincent.berenz@tuebingen.mpg.de>]

Hi,

After using for years xenomai 2.5.6 on ubuntu 12.04, we decided to upgrade to ubuntu 14.04 and a newer machine.
I installed xenomai 2.6.4 and kernel 3.14.39.
The installation boots correctly, the latency is low and our software seems to work ok.

But the system has "frequency surge" (I could not find better wording). For example:

- sometime when typing on the keyboard, the pressed key is printed many times ('aaaaaaaa' instead of 'a')

- 'glxgears' has change in frame rates, the gears can be seen as sometime changing speed. For example:

---
1141 frames in 5.0 seconds = 228.186 FPS
1024 frames in 5.0 seconds = 204.787 FPS
506 frames in 5.0 seconds = 101.194 FPS
482 frames in 5.0 seconds = 96.317 FPS
1416 frames in 5.0 seconds = 283.182 FPS
2614 frames in 5.0 seconds = 521.100 FPS
2618 frames in 5.0 seconds = 522.314 FPS
3073 frames in 5.0 seconds = 614.562 FPS
---

All the tests run fine (as far as I could tell) with the notable exception of tsc which sometimes (not always) terminates with something like:

---
tsc not monotonic after 7430687798 ticks, jumped back 49567650 tick
---

I could find this in the syslog:

-------
[    0.092932] TSC deadline timer enabled
[    0.092941] Performance Events: PEBS fmt2+, 16-deep LBR, Haswell events, full-width counters, Intel PMU driver.
[    0.092961] ... version:                3
[    0.092962] ... bit width:              48
[    0.092963] ... generic registers:      4
[    0.092964] ... value mask:             0000ffffffffffff
[    0.092965] ... max period:             0000ffffffffffff
[    0.092965] ... fixed-purpose events:   3
[    0.092966] ... event mask:             000000070000000f
[    0.094914] x86: Booting SMP configuration:
[    0.094916] .... node  #0, CPUs:        #1
[    0.109150] TSC synchronization [CPU#0 -> CPU#1]:
[    0.109157] Measured 25802382 cycles TSC warp between CPUs, turning off TSC clock.
[    0.109161] tsc: Marking TSC unstable due to check_tsc_sync_source failed
---------

Best

Vincent
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