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Setup for real-time scheduling

In order to run the universal_robot_driver, we highly recommend to setup a ubuntu system with real-time capabilities. Especially with a robot from the e-Series the higher control frequency might lead to non-smooth trajectory execution if not run using a real-time-enabled system.

You might still be able to control the robot using a non-real-time system. This is, however, not recommended.

While the best-performing strategy would be to use a real-time enabled kernel, using a lowlatency kernel has shown to be sufficient in many situations which is why this is also shown as an option here.

Installing a lowlatency-kernel

Installing a lowlatency kernel is pretty straightforward:

$ sudo apt install linux-lowlatency

Setting up Ubuntu with a PREEMPT_RT kernel

To get real-time support into a ubuntu system, the following steps have to be performed:

  1. Get the sources of a real-time kernel
  2. Compile the real-time kernel
  3. Setup user privileges to execute real-time tasks

This guide will help you setup your system with a real-time kernel.

Preparing

To build the kernel, you will need a couple of tools available on your system. You can install them using

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

Note

For different kernel versions the dependencies might be different than that. If you experience problems such as fatal error: liXYZ.h: No such file or directory during compilation, try to install the library's corresponding dev-package.

Before you download the sources of a real-time-enabled kernel, check the kernel version that is currently installed:

$ uname -r
5.15.0-107-generic

To continue with this tutorial, please create a temporary folder and navigate into it. You should have sufficient space (around 25GB) there, as the extracted kernel sources take much space. After the new kernel is installed, you can delete this folder again.

In this example we will use a temporary folder inside our home folder:

$ mkdir -p ${HOME}/rt_kernel_build
$ cd ${HOME}/rt_kernel_build

All future commands are expected to be run inside this folder. If the folder is different, the $ sign will be prefixed with a path relative to the above folder.

Getting the sources for a real-time kernel

To build a real-time kernel, we first need to get the kernel sources and the real-time patch.

First, we must decide on the kernel version that we want to use. Above, we determined that our system has a 5.15 kernel installed. However, real-time patches exist only for selected kernel versions. Those can be found on the linuxfoundation wiki.

In this example, we will select a 5.15.158 kernel with RT patch version 76. Select a kernel version close to the one installed on your system. For easier reference later on we will export version information to our shell environment. Make sure to execute all following commands in this shell.

$ export KERNEL_MAJOR_VERSION=5
$ export KERNEL_MINOR_VERSION=15
$ export KERNEL_PATCH_VERSION=158
$ export RT_PATCH_VERSION=76
$ export KERNEL_VERSION="$KERNEL_MAJOR_VERSION.$KERNEL_MINOR_VERSION.$KERNEL_PATCH_VERSION"

Go ahead and download the kernel sources, patch sources and their signature files:

$ wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/$KERNEL_MAJOR_VERSION.$KERNEL_MINOR_VERSION/patch-$KERNEL_VERSION-rt$RT_PATCH_VERSION.patch.xz
$ wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/$KERNEL_MAJOR_VERSION.$KERNEL_MINOR_VERSION/patch-$KERNEL_VERSION-rt$RT_PATCH_VERSION.patch.sign
$ wget https://www.kernel.org/pub/linux/kernel/v$KERNEL_MAJOR_VERSION.x/linux-$KERNEL_VERSION.tar.xz
$ wget https://www.kernel.org/pub/linux/kernel/v$KERNEL_MAJOR_VERSION.x/linux-$KERNEL_VERSION.tar.sign

To unzip the downloaded files do

$ xz -dk patch-$KERNEL_VERSION-rt$RT_PATCH_VERSION.patch.xz
$ xz -d linux-$KERNEL_VERSION.tar.xz

Verification

Technically, you can skip this section, it is however highly recommended to verify the file integrity of such a core component of your system!

To verify file integrity, you must first import public keys by the kernel developers and the patch author. For the kernel sources use (as suggested on kernel.org)

$ gpg2 --locate-keys torvalds@kernel.org gregkh@kernel.org

and for the patch view the gpg information

$ gpg2 --verify patch-$KERNEL_VERSION-rt$RT_PATCH_VERSION.patch.sign
gpg: assuming signed data in 'patch-5.15.158-rt76.patch'
gpg: Signature made Fri May  3 17:12:45 2024 UTC
gpg:                using RSA key AD85102A6BE1CDFE9BCA84F36CEF3D27CA5B141E
gpg: Can't check signature: No public key

So, we need to import the key using

gpg2 --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys AD85102A6BE1CDFE9BCA84F36CEF3D27CA5B141E

Now we can verify the downloaded sources:

$ gpg2 --verify linux-$KERNEL_VERSION.tar.sign
gpg: assuming signed data in 'linux-5.15.158.tar'
gpg: Signature made Thu May  2 14:28:07 2024 UTC
gpg:                using RSA key 647F28654894E3BD457199BE38DBBDC86092693E
gpg: Good signature from "Greg Kroah-Hartman <gregkh@kernel.org>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 647F 2865 4894 E3BD 4571  99BE 38DB BDC8 6092 693E

and

$ gpg2 --verify patch-$KERNEL_VERSION-rt$RT_PATCH_VERSION.patch.sign
gpg: assuming signed data in 'patch-5.15.158-rt76.patch'
gpg: Signature made Fri May  3 17:12:45 2024 UTC
gpg:                using RSA key AD85102A6BE1CDFE9BCA84F36CEF3D27CA5B141E
gpg: Good signature from "Joseph Salisbury <joseph.salisbury@canonical.com>" [unknown]
gpg:                 aka "Joseph Salisbury <josephtsalisbury@gmail.com>" [unknown]
gpg:                 aka "Joseph Salisbury <joseph.salisbury@ubuntu.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: AD85 102A 6BE1 CDFE 9BCA  84F3 6CEF 3D27 CA5B 141E

Compilation

Before we can compile the sources, we have to extract the tar archive and apply the patch

$ tar xf linux-$KERNEL_VERSION.tar
$ cd linux-$KERNEL_VERSION
$ xzcat ../patch-$KERNEL_VERSION-rt$RT_PATCH_VERSION.patch.xz | patch -p1

Now to configure your kernel, just type

$ make oldconfig

This will ask for kernel options. For everything else then the Preemption Model use the default value (just press Enter) or adapt to your preferences. For the preemption model select Fully Preemptible Kernel:

Preemption Model
  1. No Forced Preemption (Server) (PREEMPT_NONE)
> 2. Voluntary Kernel Preemption (Desktop) (PREEMPT_VOLUNTARY)
  3. Preemptible Kernel (Low-Latency Desktop) (PREEMPT)
  4. Fully Preemptible Kernel (Real-Time) (PREEMPT_RT) (NEW)
choice[1-4?]: 4

On newer kernels you need to disable some key checking:

$ scripts/config --disable SYSTEM_TRUSTED_KEYS
$ scripts/config --disable SYSTEM_REVOCATION_KEYS

Now you can build the kernel. This will take some time...

$ make -j `getconf _NPROCESSORS_ONLN` deb-pkg

After building, install the linux-headers and linux-image packages in the parent folder (only the ones without the -dbg in the name)

$ sudo apt install ../linux-headers-$KERNEL_VERSION-rt$RT_PATCH_VERSION*.deb \
                   ../linux-image-$KERNEL_VERSION-rt$RT_PATCH_VERSION*.deb

Setup user privileges to use real-time scheduling

To be able to schedule threads with user privileges (what the driver will do) you'll have to change the user's limits by changing /etc/security/limits.conf (See the manpage for details)

We recommend to setup a group for real-time users instead of writing a fixed username into the config file:

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

Then, make sure /etc/security/limits.conf contains

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

Note: You will have to log out and log back in (Not only close your terminal window) for these changes to take effect. No need to do this now, as we will reboot later on, anyway.

Setup GRUB to always boot the lowlatency / real-time kernel

To make the new kernel the default kernel that the system will boot into every time, you'll have to change the grub config file inside /etc/default/grub.

Note: This works for ubuntu, but might not be working for other linux systems. It might be necessary to use another menuentry name there.

But first, let's find out the name of the entry that we will want to make the default. You can list all available kernels using

$ awk -F\' '/menuentry |submenu / {print $1 $2}' /boot/grub/grub.cfg
menuentry Ubuntu
submenu Advanced options for Ubuntu
        menuentry Ubuntu, with Linux 5.15.158-rt76
        menuentry Ubuntu, with Linux 5.15.158-rt76 (recovery mode)
        menuentry Ubuntu, with Linux 5.15.0-107-lowlatency
        menuentry Ubuntu, with Linux 5.15.0-107-lowlatency (recovery mode)
        menuentry Ubuntu, with Linux 5.15.0-107-generic
        menuentry Ubuntu, with Linux 5.15.0-107-generic (recovery mode)

From the output above, we'll need to generate a string with the pattern "submenu_name>entry_name". In our case this would be

"Advanced options for Ubuntu>Ubuntu, with Linux 5.15.158-rt76"

The double quotes and no spaces around the > are important!

With this, we can setup the default grub entry and then update the grub menu entries. Don't forget this last step!

$ sudo sed -i "s/^GRUB_DEFAULT=.*/GRUB_DEFAULT=\"Advanced options for Ubuntu>Ubuntu, with Linux ${KERNEL_VERSION}-rt${RT_PATCH_VERSION}\"/" /etc/default/grub
$ sudo update-grub

Reboot the PC

After having performed the above mentioned steps, reboot the PC. It should boot into the correct kernel automatically.

Check for preemption capabilities

Make sure that the kernel does indeed support real-time scheduling:

$ uname -v | cut -d" " -f1-4
#1 SMP PREEMPT_RT Tue

Optional: Disable CPU speed scaling

Many modern CPUs support changing their clock frequency dynamically depending on the currently requested computation resources. In some cases this can lead to small interruptions in execution. While the real-time scheduled controller thread should be unaffected by this, any external components such as a visual servoing system might be interrupted for a short period on scaling changes.

To check and modify the power saving mode, install cpufrequtils:

$ sudo apt install cpufrequtils

Run cpufreq-info to check available "governors" and the current CPU Frequency (current CPU frequency is XXX MHZ). In the following we will set the governor to "performance".

$ sudo systemctl disable ondemand
$ sudo systemctl enable cpufrequtils
$ sudo sh -c 'echo "GOVERNOR=performance" > /etc/default/cpufrequtils'
$ sudo systemctl daemon-reload && sudo systemctl restart cpufrequtils

This disables the ondemand CPU scaling daemon, creates a cpufrequtils config file and restarts the cpufrequtils service. Check with cpufreq-info.

For further information about governors, please see the kernel documentation.