5_Realtime

Setting up the real-time patch

Prerequisites:

• at least 30GB free disk space.

Downloading kernel source

1. check your current kernel version

   uname -r
   

2. Find and download the real-time patch with matching major and minor versions from here (patch-<version>-rt<nr>.patch.gz)

3. Download the kernel with identical version as the real-time patch from here (linux-<version>.tar.gz)

4. Create directory to build kernel

   mkdir ~/kernel
   cd ~/kernel
   

   • copy the downloaded archives to the ~/kernel directory

5. Unpack archives

   tar -xzf linux-<version>.tar.gz
   gunzip patch-<version>-rt<nr>.patch.gz
   

6. Patch the kernel with the realtime patch:

   cd linux-<version>
   patch -p1 < ../patch-<version>-rt<nr>.patch
   

7. Install dependencies needed for building the kernel

   sudo apt install libncurses-dev flex bison openssl libssl-dev dkms libelf-dev libudev-dev libpci-dev libiberty-dev autoconf fakeroot debhelper
   

Kernel configuration

Create configuration for building the kernel

• The easiest is to use the configuration of the current installation:

  cp /boot/config-<version>-generic .config
  

• Modify certificates for signature checking

  scripts/config --disable SYSTEM_REVOCATION_KEYS
  scripts/config --disable SYSTEM_REVOCATION_LIST
  scripts/config --set-str SYSTEM_TRUSTED_KEYS ""
  

• Enable all new Ubuntu configurations:

  yes '' | make oldconfig
  

Real-time adaptations

The kernel configuration must be modified to make it real-time-capable. Open GUI for modifications:

make menuconfig

Enable fully preemptible kernel

-> General Setup
   -> Preemption Model
      (X) Fully Preemptible Kernel (Real-Time)

Enable high resolution timer support

-> General setup
-> Timers subsystem
   [*] High Resolution Timer Support

Omit scheduling-clock ticks on isolated CPU-s

-> General setup
-> Timers subsystem
   -> Timer tick handling
   (X) Full dynticks system (tickless)

Enforce maximum CPU frequency

->  Power management and ACPI options
-> CPU Frequency scaling
   -> CPU Frequency scaling
   -> Default CPUFreq governor
   (X) performance

Save (without modifying the name) and exit menuconfig.

Build and install kernel

1. Build the kernel (which will take quite some time):

   • Below kernel version 6.3:

   make -j `getconf _NPROCESSORS_ONLN` deb-pkg
   

   • From kernel version 6.3:

   make -j `getconf _NPROCESSORS_ONLN` bindeb-pkg
   

   After successful completion, there should be 4 debian packages in the ~/kernel directory

2. Install all kernel debian packages:

   sudo dpkg -i ../*.deb
   

3. Reboot

   • Now the real time kernel should be installed. Reboot the system.
   • At startup, choose the built kernel from the boot menu (Advanced options), or configure it to be default before restart.
   • Check new kernel version with uname -a
     • you should see PREEMPT_RT in the kernel version

Configuration

After installing the real-time kernel, the setting of scheduling priorities must be enabled for your user:

• extend /etc/security/limits.conf with

username	 -	 rtprio		 98

• Restart the system

Possible issues of building the kernel

SSL error at signing

Error: SSL error:FFFFFFFF80000002:system library::No such file or directory

Solution: disable ZSTD module compression and rebuild

scripts/config --disable CONFIG_MODULE_COMPRESS_ZSTD
scripts/config --enable CONFIG_MODULE_COMPRESS_NONE

Segmentation fault while building a specific driver

Error example

CC [M]  drivers/net/wireless/mediatek/mt76/mt7921/mac.occ1:
internal compiler error: Segmentation fault

Solution: remove the problematic driver (mt7921 in example) from the kernel configuration file