EBC Exercise 36 PREEMPT RT

Here are instructions on how to install a PREEMPT_RT Kernel and run some tests on it.

Installing a PREEMPT_RT Kernel
To install a real time kernel, first see what you are currently running.

bone$ uname -a Linux beaglebone 4.14.52-ti-r60 #1 SMP PREEMPT Tue Jun 26 23:02:46 UTC 2018 armv7l GNU/Linux

Also do the following. bone$ head /boot/uEnv.txt uname_r=4.14.52-ti-r60
 * 1) Docs: http://elinux.org/Beagleboard:U-boot_partitioning_layout_2.0
 * 1) uuid=
 * 2) dtb=

Note what uname is set to. You'll want to return to it when you are done.

Now, look for a kernel with similar number.

bone$ sudo apt update bone$ '''apt-cache search linux-image-4.14 > /tmp/search

Look through /tmp/search and pick an image with -rt- in it's name.

bone$ grep 14.52 /tmp/search linux-image-4.14.52-armv7-lpae-x3 - Linux kernel, version 4.14.52-armv7-lpae-x3 linux-image-4.14.52-armv7-rt-x4 - Linux kernel, version 4.14.52-armv7-rt-x4 linux-image-4.14.52-armv7-x4 - Linux kernel, version 4.14.52-armv7-x4 linux-image-4.14.52-bone-rt-r16 - Linux kernel, version 4.14.52-bone-rt-r16 linux-image-4.14.52-bone16 - Linux kernel, version 4.14.52-bone16 linux-image-4.14.52-ti-r59 - Linux kernel, version 4.14.52-ti-r59 linux-image-4.14.52-ti-r60 - Linux kernel, version 4.14.52-ti-r60 linux-image-4.14.52-ti-r61 - Linux kernel, version 4.14.52-ti-r61 linux-image-4.14.52-ti-rt-r59 - Linux kernel, version 4.14.52-ti-rt-r59 linux-image-4.14.52-ti-rt-r60 - Linux kernel, version 4.14.52-ti-rt-r60 linux-image-4.14.52-ti-rt-r61 - Linux kernel, version 4.14.52-ti-rt-r61

bone$ sudo apt install linux-image-4.14.52-ti-rt-r60 Reading package lists... Done Building dependency tree Reading state information... Done Suggested packages: linux-firmware-image-4.14.52-ti-rt-r60 The following NEW packages will be installed: linux-image-4.14.52-ti-rt-r60 ... Setting up linux-image-4.14.52-ti-rt-r60 (1stretch) ... update-initramfs: Generating /boot/initrd.img-4.14.52-ti-rt-r60 zz-uenv_txt: Updating /boot/uEnv.txt [uname_r=-4.14.52-ti-rt-r60]

Mine took some 2.5 minutes.

Measure the latency
Before rebooting let's measure the non-rt latency.

bone$ cd ~/exercises/linux/kernel/rt bone$ sudo apt install rt-tests (Check install.sh for latest instructions of what to install.) Now run the test: bone$ '''time sudo ./hist.gen > nort.hist

If you get an error: Unable to change scheduling policy! Probably missing capabilities, either run as root or increase RLIMIT_RTPRIO limits try running ./setup.sh. If that doesn't work try: bone$ sudo bash bone# ulimit -r unlimited bone# ./hist.gen > nort.hist bone# exit This sets the maximum real-time scheduling priority to unlimited.

While hist.gen is running, open another window on the bone and run some time intensive tasks. Here's what I did. bone$ cd ~/exercises/linux/modules bone$ make bone$ make clean Repeat the two make commands until hist.gen finishes. This will take a while. Here are the times I got for hist.gen. real	1m40.222s user	0m0.938s sys	0m7.767s

This creates a histogram data file 'nort.hist. Now you can reboot into the rt kernel. bone$ sudo reboot

After rebooting you will see. bone$ uname -a Linux beaglebone 4.14.52-ti-rt-r60 #1 SMP PREEMPT Tue Jun 26 23:28:55 UTC 2018 armv7l GNU/Linux

Now repeat the test. bone$ cd ~/exercises/linux/kernel/rt bone$ '''time sudo ./hist.gen > rt.hist Again, do the same time intensive things in another window.

Once the test is done, go to your host computer to generate the plots. (There's less to install than on the bone.

Let's mount the bone's files on the host so they are easy to get to.

host$ cd host$ mkdir bone host$ sudo apt update host$ sudo apt install sshfs host$ '''sshfs bone:. bone'''

This says to mount the home directory on the bone (bone:.) on the local directory, called bone, on the host. Now change to that directory.

host$ cd bone/exercises/linux/kernel/rt host$ ls hist.gen hist.plt  install.sh  nort.hist  rt.hist  setup.sh

You should now see the files you just generated on the bone. host$ sudo apt install gnuplot host$ gnuplot < hist.plt This will generate the file cyclictest.png which contains your plot. host$ xdg-open cyclictest.png This will display your cyclictest.png file. Mine looks something like:



Returning to the non-rt kernel
You can return to the non-rt kernel that you running before by editing /boot/uEnv.txt and returning uname_r to what it originally was and then rebooting.

To view the old kernel version you can look into the /boot directory.

bone$ ls /boot

bone$ sudo nano /boot/uEnv.txt uname_r=4.14.52-ti-r60

bone$ sudo reboot

You should now be back to where you started.