Real-time(PREEMPT-RT) Kernel for Bubblegum-96


#1

Decided to run some of my motion control applications on the board (I bought around 30 of the board :frowning:). For that I need an RT Kernel which allows preemption in the kernel code to minimize latency. Despite lacking any support from the vendor, I managed to patch the latest 3.10.99 kernel to 3.10.100-rt110 and tested the latency. The figures are not really good but still usable:

Without background stress:

➜  rt-tests git:(master) βœ— sudo ./cyclictest -a -t -n -p99
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.38 0.64 0.81 1/145 19377

T: 0 (17893) P:99 I:1000 C: 125668 Min:      0 Act:   19 Avg:   14 Max:     158
T: 1 (17894) P:99 I:1500 C:  83778 Min:      0 Act:   19 Avg:   14 Max:     150
T: 2 (17895) P:99 I:2000 C:  62834 Min:      0 Act:   19 Avg:   14 Max:     117
T: 3 (17896) P:99 I:2500 C:  50267 Min:      0 Act:   19 Avg:   14 Max:      86

System under high load by stress -c 4 -i 1 -m 1 --vm-bytes 128M -t 100s

➜  rt-tests git:(master) βœ— sudo ./cyclictest -p 99 -t 4 -n -a
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 7.77 3.95 2.01 9/160 23279

T: 0 (21233) P:99 I:1000 C: 285193 Min:      0 Act:   16 Avg:   15 Max:     342
T: 1 (21234) P:99 I:1500 C: 190122 Min:      0 Act:   16 Avg:   16 Max:     320
T: 2 (21235) P:99 I:2000 C: 142596 Min:      0 Act:   16 Avg:   17 Max:     254
T: 3 (21236) P:99 I:2500 C: 114077 Min:      0 Act:   16 Avg:   15 Max:     190

You can see the latency is well below 400us, which is well suitable for normal control purposes.

The code lies here: https://github.com/ProfFan/bubblegum96-linux-rt

Lazy man’s precompiled kernel :slight_smile: : https://mega.nz/#!0GpngAzC!lca4clCYwe8XqMPL7Yfatso7RnDjXALx23HtTZR9POc

Build the kernel by:

export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-gnu-
make bubblegum_linux_defconfig
make oldconfig

Now choose 4 - Basic Preempt at the prompt. Quickly skip all others. Then just

make -j8 Image
make modules

For folks who wants to manually patch the kernel, here are my notes.

Happy hacking!