If you need to combine real-time performance with the open-source benefits of Linux, then consider the Bluecat RT from LynuxWorks.
BlueCat RT ensures time-critical handling of interrupts and other low-level hardware operations by implementing a full Linux kernel as a thread of a smaller and highly responsive real-time operating system. This miniature RTOS traps interrupts and other low-level functions for preliminary processing before passing them on to the thread running Linux.
The two operating systems form a hybrid solution that runs on one processor. Applications operate mostly within Linux, except for a subset which makes use of the real-time performance of its RTLinux/Pro kernel (a distribution licensed from FSM Labs).
This makes BlueCat RT an ideal solution for applications that can benefit from a measure of real-time performance, but which do not require real-time for most of their tasks and features.