This paper describes a framework that provides a task programming model with optional components and the appropriate operating system mechanisms for supporting it. The approach provides 100% guarantees to hard real-time tasks using fixed priority pre-emptive scheduling. Optional components, which increase the utility of the guaranteed tasks, are executed when spare processor capacity is available. The framework has been developed extending the existing RT-Linux capabilities. Furthermore, the design of these kernel extensions have been proved to be predictable, in such a way that it is possible to perform a schedulability analysis of the entire system, including kernel overheads.