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

Sun Apr 28 06:11:26 EDT 2019

Hello again Mathieu,

I apologize that this is a rather late re-opening of the topic again.
However, after consulting with the RPi kernel side, it seems that even
after trying to fix for the mentioned issue (i.e. by upgrading to the
listed 'fixed' commit), I am still having issues installing the
lttng-modules. The following link is the thread that I am having with the
RPi kernel github: https://github.com/raspberrypi/linux/issues/2930

Even though it is listed that version 2.9 seems to be okay, recently I
tried to re-install the lttng-modules onto another RPi, but this time even
with the 'fixed' commit as suggested in the thread, lttng-modules still
give the same 'buggy kernel implementation' error. Please do note that it
seems CONFIG_OPTPROBES cannot be turned off at all during the kernel build,
as it will be overriden by default and seems to be integral to the build of
the RPi's RT kernel as well, hence this is not really an option either.

Is there a specific way on how to upgrade to the mentioned 'fixed' commit
without requiring a complete kernel overhaul? I am getting rather desparate
as I need this to work for an upcoming assignment deadline this week. So
I'm hoping that there is still potential solution to this.

Best Regards,

Nicholas Sadjoli

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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