Setting up the Real-Time Kernel

Kernel Compatibility

Different kernel versions are compatible with specific Ubuntu releases. The table below provides an overview of the recommended kernel versions for each supported Ubuntu distribution.

To control your robot using libfranka, the controller program on the workstation PC must run with real-time priority under a PREEMPT_RT kernel. This section describes how to patch a kernel and create an installation package.

First, install the necessary dependencies:

sudo apt-get install build-essential bc curl debhelper dpkg-dev devscripts \
fakeroot libssl-dev libelf-dev bison flex cpio kmod rsync libncurses-dev

Then decide which kernel version to use. Check your current version with uname -r. Real-time patches are only available for select versions: RT Kernel Patches. Choose the version closest to your current kernel. Use curl to download the sources.

curl -LO https://www.kernel.org/pub/linux/kernel/v5.x/linux-5.9.1.tar.xz
curl -LO https://www.kernel.org/pub/linux/kernel/v5.x/linux-5.9.1.tar.sign
curl -LO https://www.kernel.org/pub/linux/kernel/projects/rt/5.9/patch-5.9.1-rt20.patch.xz
curl -LO https://www.kernel.org/pub/linux/kernel/projects/rt/5.9/patch-5.9.1-rt20.patch.sign

curl -LO https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.8.tar.xz
curl -LO https://www.kernel.org/pub/linux/kernel/v6.x/linux-6.8.tar.sign
curl -LO https://www.kernel.org/pub/linux/kernel/projects/rt/6.8/older/patch-6.8-rt8.patch.xz
curl -LO https://www.kernel.org/pub/linux/kernel/projects/rt/6.8/older/patch-6.8-rt8.patch.sign

Decompress the files:

xz -d *.xz

Verifying File Integrity

Use the .sign files to verify downloads. Adapted from the Linux Kernel Archive.

Verify archives:

gpg2 --verify linux-*.tar.sign
gpg2 --verify patch-*.patch.sign

If you get a “No public key” warning, download the key (example for kernel 4.14.12):

gpg2 –keyserver hkp://keyserver.ubuntu.com:80 –recv-keys 6092693E gpg2 –keyserver hkp://keyserver.ubuntu.com:80 –recv-keys 2872E4CC

Then verify again. A correct output looks like:

gpg: Good signature from "Greg Kroah-Hartman <gregkh@linuxfoundation.org>"

For details, see the Linux Kernel Archive.

Compiling the Kernel

Extract the source and apply the patch:

tar xf linux-*.tar
cd linux-*/
patch -p1 < ../patch-*.patch

Copy your current kernel config:

cp -v /boot/config-$(uname -r) .config

Disable debug info:

scripts/config --disable DEBUG_INFO
scripts/config --disable DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
scripts/config --disable DEBUG_KERNEL

Disable system-wide trusted keys:

scripts/config --disable SYSTEM_TRUSTED_KEYS
scripts/config --disable SYSTEM_REVOCATION_LIST

Enable the Fully Preemptible Kernel:

scripts/config --disable PREEMPT_NONE
scripts/config --disable PREEMPT_VOLUNTARY
scripts/config --disable PREEMPT
scripts/config --enable PREEMPT_RT

Configure the build:

make olddefconfig

Compile the kernel (use multithreading based on CPU cores):

make -j$(nproc) bindeb-pkg

Install the generated packages:

sudo IGNORE_PREEMPT_RT_PRESENCE=1 dpkg -i ../linux-headers-*.deb ../linux-image-*.deb

Verifying the New Kernel

Reboot and select the new kernel in GRUB. Check with uname -a — it should include PREEMPT RT. Also, /sys/kernel/realtime should exist and contain 1.

Allowing Real-Time Permissions

After the kernel is running, create a realtime group and add the user controlling the robot:

sudo addgroup realtime
sudo usermod -a -G realtime $(whoami)

Add limits in /etc/security/limits.conf:

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

Log out and back in to apply the limits.