mod-memset

This is a small test tool to verify a CPU scheduling bug on the MOD Dwarf.

By simply allocating a big pool of memory and non-stop calling memset(mem, 0, size) we can generate audio xruns.
The same bug is also seen by running cyclictest -S -p 80 and then starting this tool.
It does not matter that cycletest is running with elevated/realtime privileges like SCHED_FIFO, or the priority of this tool, there are always xruns and CPU scheduling issues regardless of priority levels.

In order to have better control over the memset call, we copied it from the glibc repository in here, allowing us to tweak what it does and its optimization level too.

Building

Project uses a simple regular Makefile, cross-compiling for Dwarf is as easy as:

make CC=aarch64-linux-gnu-gcc

Notes

By forcing the tool to run on a specific CPU, and have the Dwarf IRQ 62 (ff300000.usb) also running on that CPU, and USB Audio is activated, we weirdly no longer have xruns.
This can be verified with:

# Enable USB Audio, only needed once
touch /data/enable-usb-multi-gadget
touch /data/enable-usb-audio-gadget
sync && hmi-reset && reboot

# Test with core #1
echo 1 > /proc/irq/62/smp_affinity
./mod-memset-cpu1

and using the 2nd CPU core, for good measure:

# Test with core #2
echo 2 > /proc/irq/62/smp_affinity
./mod-memset-cpu2