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Real Time Threads with PREEMPT_RT

Example implementaion of real time threads with wrapper for POSIX thread.

Prerequisite

⚠️ NOTE: Make sure you have already patched PREEMPT_RT kernel, follow instruction.md to patch and install. To patch Raspberry PI 4 kernel with PREEMPT_RT follow instructions from raspberry-pi-rt.md

Building real time periodic thread

Download the source code and compile

git clone https://github.com/siddharthdeore/preempt.git
cd preempt
mkdir build && cd build
cmake ..
make

Execute example program with PREEMPT_RT

cd ../bin
sudo ./rt_thread

Test CPU affinity (pin thread to core)

./cpu_affinity

To check CPU affinity use htop system monitor, cores which have thread pinned succussfully shall show 100% CPU consumption. Or use taskset to check on which CPU core a process running

taskset -c -p $(pidof cpu_affinity)

Tracing and diagnostic

install kernel tracer tools trace-cmd and KernelShark

sudo apt-get install -y trace-cmd
sudo apt-get install -y kernelshark

Trace CPU sched_switch event

sudo trace-cmd record -e sched_switch ./rt_thread

Analize CPU trace with KernelShark GUI

kernelshark trace.dat

Fig: Kernelshark CPU trace for sched_switch event

Fig: Kernelshark sched_switch event ticks (zoom).

Troubleshoot

Issue: Unexpected freeze issue on COM Express Type 6

Description:

COM Express running Ubuntu 20.04 LTS freezes unexpectedly, this recurrent behavior has been noticed when processor usage goes close to 100% similar issue arise on when program/threads uses all processor cores, and when real time thread is affine to specific core.

Solution:

This behaviour obsereved with Ubuntu 18.04 LTS and 22.04 LTS and expected cause of such freezes is related to "CPU Power Management". Disabling all CPU power managements seems to have solved problem. Steps to Disable

Press "F2" or "DEL" key to enter Aptio Bios Setup utility. Select Advanced Tab

> Advanced
  > Power & Performance
    > CPU - Power Management Control
        Set Boot mode                   [Turbo Performance]
        Intel(R) SpeedStep(tm)          [Disable]
        Race To Halt (RTH)              [Disable]
        Intel(R) Speed Shift Technology [Disable]
        HDC Control                     [Disable]

⚠️ Important (Only for Xenomai): Problems on Xenomai ipipe kernel CPU affinity

Current kernel `cmdline` flags can be checked with following command, 

cat /proc/cmdline

To disable DWC features add following flags to kernel cmdline

dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0 dwc_otg.nak_holdoff=0

CPU Isolation (Xenomai)

Remove the CPU 0 and CPU 1 from the general kernel SMP balance and scheduler algorithms.

isolcpus=0,1 xenomai.supported_cpus=0x3
# above line isolates cpu 0 and 1 from SMP balance, and set xenomai supported cpu mask to 0011

To set flags edit /etc/default/grub and add flags to GRUB_CMDLINE_LINUX_DEFAULT, finaly update grub with sudo update-grub

RPI : flags can be set by adding above lines to /boot/cmdline.txt,

TODO

  • realtime periodic thread Program

  • cpu affinity

  • thread priority

  • Some of the tools listed below to be tested and integrated

    • Linux Trace Toolkit
    • Dynamic Probes
    • kGDB (kernel debugger)
    • Linux Kernel Crash Dump
    • Linux Test Project
    • LMBench
    • Ballista
    • strace
    • System Call Tracker

Maintainers

This repository is maintained by:

Siddharth Deore