Where can I find a simple RTOS for avr?

Question

Atmel AVR

Can't find a simple cooperative RTOS to learn.
mRTOS did not suit me for the project, it was very buggy when it became more complex.
I need only one scheduler, normal task switching: save context, select task, restore context
Everything else: timers, events, etc. I can write myself.
because right now, I don’t set myself the task of deep learning the assembler and the inner workings of rtos, and as time is running out, I thought that maybe someone wrote their OS and throw off the code for an example.
I will be very grateful