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

Tue Apr 16 15:04:24 EDT 2019

Dear Mathieu,

Thank you so much for your reply Mathieu! I have actually asked about this
to the raspberry pi github forum, but unfortunately it seems that I have
not gotten any reply from them on this matter yet. I'll be asking about how
to turn off the CONFIG_OPTPROBES from them again if that's the case.

If this still fails then I'll be trying out Tom Zanussi's upstream version
of the kernel as you suggested then. Thank you so much for your help once
again

On Mon, Apr 15, 2019 at 10:11 PM Mathieu Desnoyers <
mathieu.desnoyers at efficios.com> wrote:

> ----- On Apr 15, 2019, at 7:07 AM, Nicholas Sadjoli <
> nick.sadjoli at gmail.com> wrote:
>
> 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.
>
> Hi Nicholas,
>
> It looks like our lttng-modules kernel version blacklist is acting exactly
> as it should. Looking at this
> git tree:
>
> git clone https://github.com/raspberrypi/linux.git -b rpi-4.14.y-rt
>
> I notice that in the git log, we find this bogus commit:
>
> commit f9e0bc710347fadad55910846ee36f0681d0dca8
> Author: Kees Cook <keescook at chromium.org>
> Date:   Tue Oct 30 22:12:56 2018 +0100
>
>     ARM: 8806/1: kprobes: Fix false positive with FORTIFY_SOURCE
>
>     commit e46daee53bb50bde38805f1823a182979724c229 upstream.
>
> But not the fix for it (upstream):
>
> commit 0ac569bf6a7983c0c5747d6df8db9dc05bc92b6c
> Author: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Date:   Tue Feb 5 16:37:40 2019 +0100
>
>     ARM: 8834/1: Fix: kprobes: optimized kprobes illegal instruction
>
> So lttng-modules refuses to build against your kernel configuration with
> CONFIG_OPTPROBES=y
> to save you the trouble of having kprobes trigger illegal instruction OOPS
> at runtime.
>
> I'm unusure who maintains that raspberrypi rt kernel tree. However, the
> upstream rt tree
> for 4.14-rt maintained by Tom Zanussi seems to contain the fix:
>
>
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/rt/linux-stable-rt/+/refs/heads/v4.14-rt
>
> (currently at 4.14.109-rt57).
>
> So you should contact your raspberrypi linux rt tree clone maintainer and
> ask him
> to upgrade, or, as stated in the #error generated by lttng-modules, simply
> ensure
> CONFIG_OPTPROBES is unset when you build your kernel. You should ask the
> raspberrypi community advice on how to ensure this configuration option is
> not
> overridden by the raspberrypi build system.
>
> Thanks,
>
> Mathieu
>
>
>
> Thank you so much for your attention and looking forward to your reply
> soon.
>
>
>
> Best Regards,
>
>
>
> Nicholas Sadjoli
>
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> lttng-dev at lists.lttng.org
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>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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