Have you heard about the ITRON (aka microITRON) project in Japan? ITRON is a Japanese open standard for real-time operating systems. The project started back in 1984 and was initiated by Ken Sakamura.
TOPPERS (Toyohashi Open Platform for Embedded Real-time Systems) is one flavour of ITRON RTOSes. It is open-source, and the documentaiton is released as open educational content.
Atollic TrueSTUDIO supports kernel-aware debugging of TOPPERS. The Atollic appointed distributor in Japan, AI Corporation, has written guides and created example projects, simplifying the getting started process with TOPPERS.