NuttX review scheduling note (cpuload) performance

[dagar]

From profiling on STM32F7 we see that our NuttX scheduling note functions and hrt_absolute_time (used by nearly everything in PX4 including cpuload) are quite hot. https://github.com/PX4/Firmware/blob/1d5309060a7b26366f8aa0973fc078593c04a47e/src/lib/systemlib/cpuload.c#L107

We should consider using ram functions (CONFIG_ARCH_RAMFUNCS) as well as simpler cpuload calculations that aren't a burden on every single scheduler operation.

[dagar]

https://nuttx.org/doku.php?id=wiki:nxinternal:cpuload-oneshot

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dagar]

I'll come back to this once profiling on hardware. Either with https://github.com/orbcode/orbuculum or a J-Trace.