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rpi_setup.md

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Installing PREEMPT_RT realtime Linux on a Raspberry Pi 3

To control the bean ejectors in the optical coffee sorter, the Raspberry Pi 3 must be able to react fast enough. Its CPU speed is sufficient for that, but interrupt handling can introduce significant delays. A realtime patch for Linux offers a solution. We chose the most well-known one, PREEMPT_RT. Here is how to get it to run on the Raspberry Pi 3.

Note that the instructions below work only with the software versions referenced, following the instructions by markusr. If you want to use other versions, make sure that the kernel version provided by the Pi Foundation and the PREEMPT_RT patch refer to the same version of Linux.

1. Steps on your computer

(1) Download and unpack the 2017-11-29 release image of Raspbian Lite or Raspbian:

wget https://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2017-12-01/2017-11-29-raspbian-stretch-lite.zip
unzip 2017-11-29-raspbian-stretch-lite.zip

(2) Insert a microSD card and find out its device file:

lsblk

(3) Copy your image file to the SD card, supplying your device file instead of /dev/sdx (details):

sudo dd bs=4M if=2017-11-29-raspbian-stretch-lite.img of=/dev/sdx conv=fsync

(4) Create an empty file /boot/ssh to start the Raspberry Pi with SSH enabled (background):

sudo mount /dev/sdb1 /media/username/boot
touch /media/username/boot/ssh
sudo umount /dev/sdb1

2. Steps on the Raspberry Pi 3

(1) Start the Raspberry Pi 3 and connect it with an Ethernet cable to your local network.

(2) Find out the Pi's IP address from the DHCP client list of your router (details).

(3) Log in to the Raspberry Pi 3 with the default account (username pi, password raspberry), using the IP address it has in your case of course:

ssh pi@192.168.0.100

(4) Change the SSH password of the Raspberry Pi, and note it down:

passwd

(5) Install vim-nox to prevent issues with navigation keys creating characters in vi:

sudo apt install vim-nox

(6) Download and extract the sources of the Linux kernel version 4.14.14 (195 MiB):

wget https://github.com/raspberrypi/linux/archive/23816e63fcc5efa8400f248304db13d5df69792f.zip
mv 23816e63fcc5efa8400f248304db13d5df69792f.zip linux-23816e63fcc5efa8400f248304db13d5df69792f.zip
unzip linux-23816e63fcc5efa8400f248304db13d5df69792f.zip

(7) Download and apply the PREEMPT_RT realtime kernel patch version 4.14.12 (which still works with the kernel we downloaded):

wget https://www.kernel.org/pub/linux/kernel/projects/rt/4.14/older/patch-4.14.12-rt10.patch.gz
cd linux-23816e63fcc5efa8400f248304db13d5df69792f
zcat ../patch-4.14.12-rt10.patch.gz | patch -p1
cd ..

(8) Prepare compilation:

export KERNEL=kernel7
export ARCH=arm
export CROSS_COMPILE=
export CONCURRENCY_LEVEL=$(nproc)
cd linux-23816e63fcc5efa8400f248304db13d5df69792f/
make bcm2709_defconfig

(9) Download and apply the RPi FQI patch, needed to prevent system freezes on the RPi with PREEMPT_RT kernels:

cd ..
wget https://raw.githubusercontent.com/fedberry/kernel/master/usb-dwc_otg-fix-system-lockup-when-interrupts-are-threaded.patch
cd linux-23816e63fcc5efa8400f248304db13d5df69792f/
patch -i ../usb-dwc_otg-fix-system-lockup-when-interrupts-are-threaded.patch -p1

We use a rebased version of the original patch for more recent kernel versions, but it is functional equivalent. You can also apply this patch to a wide range of other kernel versions, but then you should use patch with --dry-run first until you can make the patch apply all its pieces ("hunks"), with or without offsets.

(10) Configure the kernel to make it a realtime kernel:

sudo apt-get install libncurses5-dev bc 
make menuconfig
  • Set "Kernel Features → Preemption Model" to "Fully Preemptible Kernel (RT)".
  • Set "Kernel Features → Timer frequency" to "1000 Hz".
  • Save and exit menuconfig.

(11) Compile the kernel:

make clean
./scripts/config --disable DEBUG_INFO
make -j$(nproc) deb-pkg

Will take 1.5 - 2 hours on the Raspberry Pi 3.

(12) Install your brand new realtime kernel:

sudo dpkg -i ../*rt10*.deb

Open up /boot/config.txt and add the following line to it:

kernel=vmlinuz-4.14.14-rt10-v7

If you work with a different kernel version, the value to use behind "kernel=" is the filename that is printed by this command: (cd /boot/ && ls -1 vmlinuz*rt10*).

(13) Reboot and make sure your realtime kernel is running:

sudo reboot

ssh pi@192.168.0.100
uname -a

The output should start with Linux raspberrypi 4.14.14-rt10-v7, indicating it is a realtime kernel.

(14) Ensure your system is running stable with the realtime kernel, by running tests like the following for a few minutes (while letting the Raspberry Pi 3 do other tasks in parallel):

cd ~
git clone git://git.kernel.org/pub/scm/linux/kernel/git/clrkwllms/rt-tests.git
cd rt-tests/
make
sudo ./cyclictest -m -t1 -p 80 -n -i 500 -l 100000
sudo ./cyclictest -m -t4 -p 80 -n -i 500 -l 100000

For details about cyclictest, see:

3. References

The following contains background reading material about realtime Linux, beyond what is an immediate source linked in the instructions above: