[lttng-dev] Problems setting up lttng on PREEMPT_RT Raspberry Pi

[Nicholas Sadjoli]

Dear Lttng dev Team,

I am currently interested in using lttng to measure the interrupt or
hardware real-time latencies experienced within my user-space applications.
Unfortunately I have been having issues installing lttng on my Raspberry Pi
that is using the PREEMPT_RT patched kernel, version 4.14.91-rt49-v7+. The
problems so far can be seen documented in the RPi Linux github forum page
here: https://github.com/raspberrypi/linux/issues/2930

Initially I was about to just install the Debian package of the lttng onto
the RPi, and just proceeded with the commands 'sudo apt-get install
lttng-modules-dkms liblttng-ust-dev lttng-tools' However, I was unable to
use lttng package at all and found out that for the RPi, a build from
source is necessary for lttng, as described here:
https://stackoverflow.com/questions/52386448/linux-trace-tool-lttng-on-raspbian-kernel-tracer-not-available


Following their suggestion, I then tried to build the lttng packages from
source, and even re-building the RPi kernel locally to ensure the kernel
headers are in the RPi and linked up properly to the source packages.

Unfortunately when I tried to do this for the lttng-modules package, it
instead gave me the following warning in my latest attempt:

In file included from
/home/pi/BangEco_rvmscan/stagelight_project/lttng/lttng-modules/probes/lttng-kprobes.c:31:0:
/home/pi/BangEco_rvmscan/stagelight_project/lttng/lttng-modules/probes/../blacklist/kprobes.h:19:4:
error: #error "Your kernel is known to have buggy optimized kprobes
implementation. Fixed by commit 0ac569bf6a7983c0c5747d6df8db9dc05bc92b6c
\"ARM: 8834/1: Fix: kprobes: optimized kprobes illegal instruction\" in
Linux. Disable CONFIG_OPTPROBES or upgrade your kernel."
 #  error "Your kernel is known to have buggy optimized kprobes
implementation. Fixed by commit 0ac569bf6a7983c0c5747d6df8db9dc05bc92b6c
\"ARM: 8834/1: Fix: kprobes: optimized kprobes illegal instruction\" in
Linux. Disable CONFIG_OPTPROBES or upgrade your kernel."

I then tried to turn off the CONFIG_OPTPROBES kernel config, but it seemed
to be automatically overwritten during kernel building and is considered an
important part of the real-time RPi kernels. Has such problems been
reported with this version of the PREEMPT_RT kernel in the past (again, the
current kernel version that I am using is 4.14.91-rt49-v7+)? If there are I
would like to know how this issue was resolved so that I can properly build
and use the lttng package for my project.


Thank you so much for your attention and looking forward to your reply soon.



Best Regards,



Nicholas Sadjoli
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