install_real-time_kernel.md

The rt kernel I tested at this moment is 5.15.21-rt30 (Feb 2022).

Install real-time linux kernel

Follow a community contributed tutorial to install the latest stable RT_PREEMPT version https://docs.ros.org/en/foxy/Tutorials/Building-Realtime-rt_preempt-kernel-for-ROS-2.html

Open Software & Updates. in the Ubuntu Software menu tick the ‘Source code’ box.

Install dependencies:

sudo apt-get update
sudo apt-get build-dep linux
sudo apt-get install build-essential bc curl ca-certificates gnupg2 lsb-release libncurses-dev flex bison openssl libssl-dev dkms libelf-dev libudev-dev libpci-dev libiberty-dev autoconf fakeroot
sudo apt update
sudo apt install zstd

cd ~
mkdir rt_kernel
cd rt_kernel

wget https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.15.21.tar.gz
wget https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.15.21.tar.sign
wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patch-5.15.21-rt30.patch.gz
wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patch-5.15.21-rt30.patch.sign

gunzip linux-5.15.21.tar.gz
# tar -xzf linux-5.15.21.tar.gz

gunzip patch-5.15.21-rt30.patch.gz

Verify the kernel files integrity

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

tar xf linux-*.tar

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

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

yes '' | make oldconfig
make menuconfig

apply these modifications:

• general setup / preemption model
  • Fully preemptible Kernel (Real-time)
• general setup / Timers subsystem
  • High Resolution Timer Support
• general setup / Timers subsystem / Timer tick handling
  • Full dynticks system (tickless)
• Processor type and features / Timer frequency
  • 1000 Hz
• Power management and ACPI options
  • CPU Frequency scaling, CPU Frequency scaling (CPU_FREQ [=y]) -> Default CPUFreq governor ( [=y]) (X) performance
• Cryptographic API* > Certificates for signature checking (at the very bottom of the list) > Provide system-wide ring of trusted keys > Additional X.509 keys for default system keyring
  • Remove the “debian/canonical-certs.pem” from the prompt and press Ok.

save and exit.

make -j$(nproc) deb-pkg

or

make -j $(nproc)
sudo make bzImage
sudo make INSTALL_MOD_STRIP=1 modules_install -j $(nproc)
sudo make install

According to the post by cacao:

the INSTALL_MOD_STRIP is important - it shrinks the initial ramdisk size by ~90%. In some cases, when not used, the initial ramdisk would not load and the patched kernel would not boot.

If you see these errors:

• if you see this error: CONFIG_X86_X32 enabled but no binutils support, change config_x86_x32 to n
• sed: can't read modules.order: No such file or directory
  • set CONFIG_SYSTEM_TRUSTED_KEY=""
  • set CONFIG_SYSTEM_REVOCATION_KEYS=""
• Missing file: arch/x86/boot/bzImage , you need to run sudo make bzImage before modules_install
• if you see this error:bin/sh: 1: zstd: not found, you need to install Zstandard bysudo apt install zstd

Make sure realtime kernel is set to default

Edit /etc/default/grub file Change GRUB_DEFAULT=0 to GRUB_DEFAULT=saved. Add GRUB_SAVEDEFAULT=true

Allow a user to set real-time permissions for its processes

According to https://frankaemika.github.io/docs/installation_linux.html#setting-up-the-real-time-kernel:

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

Afterwards, add the following limits to the realtime group 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