Status: Under development
From the ChibiOS project:
ChibiOS/RT is designed for deeply embedded real time applications where execution efficiency and compact code are important requirements. This RTOS is characterized by its high portability, compact size and, mainly, by its architecture optimized for extremely efficient context switching.
Having ported ChibiOS/RT to the Marvell Armada / Kirkwood chips, I've started a port of ChibiOS/RT to XMOS Xcore.