Preamble

I have recently been working with robotic systems that have a greater requirement for deterministic operation. While Ubuntu will likely never be an RTOS the PREEMPT RT patch does allow for soft real-time constraints on specified threads which can be enough of a step up for many research applications.

While there are other guides on how to do this online (some linked below) this post will present as a record for the steps I needed to follow recently for my system.

Dependencies

sudo apt-get install build-essential bc ca-certificates gnupg2 libssl-dev wget gawk flex bison dwarves

Download

Supported Linux RT kernel patches found here. Check your kernel version and find an appropriate patch.

uname -r

For my system this returns 5.15.0-76-generic so I’ll need Joseph Salisbury’s patch located here.

wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/patch-5.15.119-rt65.patch.xz

wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/patch-5.15.119-rt65.patch.sign

wget https://www.kernel.org/pub/linux/kernel/v5.x/linux-5.15.119.tar.xz

wget https://www.kernel.org/pub/linux/kernel/v5.x/linux-5.15.119.tar.sign

xz -dk patch-5.15.119-rt65.patch.xz
xz -d linux-5.15.119.tar.xz

Verification

You will need to get the appropriate keys for your patch maintainer.

# Main Kernel
gpg2 --locate-keys torvalds@kernel.org gregkh@kernel.org 

# Patch Maintainer
gpg2 --locate-keys joseph.salisbury@canonical.com
gpg2 --keyserver hkp://keyserver.ubuntu.com --search-keys "Joseph Salisbury"

gpg2 --verify linux-5.15.119.tar.sign linux-5.15.119.tar
gpg2 --verify patch-5.15.119-rt65.patch.sign patch-5.15.119-rt65.patch

Apply the Patch to the Kernel

tar xf linux-5.15.119.tar
cd linux-5.15.119/
xzcat ../patch-5.15.119-rt65.patch.xz | patch -p1

Select fully pre-emptable kernel option and just enter through the rest

cp /boot/config-5.15.0-76-generic .config
make menuconfig

Activate “Fully Preemptible Kernel (Real-Time)” option from “General setup” / “Preemption Model” then SAVE and EXIT.

Apply this fix to the .config file.

Build and Install

sudo make

Wait a long time…

sudo make modules_install
sudo make install

Real-Time User Privileges

Description of the limits conf on manpage here.

sudo groupadd realtime
sudo usermod -aG realtime $(whoami)

Edit /etc/security/limits.conf to contain…

@realtime soft rtprio 99
@realtime soft priority 99
@realtime soft memlock 102400
@realtime hard rtprio 99
@realtime hard priority 99
@realtime hard memlock 102400

Grub Settings

Edit /etc/default/grub to contain…

 GRUB_DEFAULT=saved
 GRUB_SAVEDEFAULT=true

sudo update-grub

Then reboot and choose the RT kernel from the Advanced Options for Ubuntu in Grub

sudo reboot

Trouble Shooting

I have ran into several issues when installing on other machines that have shown up straight after selecting the rt build from GRUB’s menu.

One was…

error: bad shim signature

…which was fixed my turning off secure boot in the bios.

Another was…

error: out of memory

…this was rectified by updating the compression method for initramfs as detailed here and here.

Assigning Priorities to System Services

You can leverage the RT kernel by setting the priority of the threads spawned by your system services (see my previous post on running ROS nodes on boot). Use the CPUSchedulingPolicy and CPUSchedulingPriority directives under the Service section of a system service. Take a look at this guide from RedHat for a run down on the options, but for robotics I’d tend to use CPUSchedulingPolicy=rr and either CPUSchedulingPriority=2-49, for CPU-bound tasks and CPUSchedulingPriority=51-98 for I/O bound tasks, see the example usage below.

[Unit]
Requires=roscore.service
After=network-online.target roscore.service

[Service]
Type=simple
User=your_robots_user
ExecStart=/usr/sbin/ros_package_auto_launch
CPUSchedulingPolicy=rr
CPUSchedulingPriority=90

[Install]
WantedBy=multi-user.target

Resources and Further Reading

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