The uC/OS-II Realtime OS, by Jean Labrosse, is great for learning RTOS basics.
It comes with a 500 page book which describes it in detail.
This is not to say that the OS is a toy. It’s a full grown and very competent OS. It even has a DO178B certification for use in medical and aviation products.

For more info about uC/OS-II, see Jean Labrosse’s own pages at www.ucos-ii.com.

To get to the point….. I’ve made a port for the AVR processors, for version 2.95.2 of the AVR-GCC compiler.
Note that it is ONLY for version 2.92.2 !!!
You need to modify the likner file for later compiler versions. Please do NOT ask me about this.