Abstract: Based on the fault analysis of real-time tasks, an improved version of EDF algorithm, named FR-EDF, is proposed, the necessary and sufficient condition of which is given. It is thus proved that the current research result on fault tolerant EDF is a nature consequence of the necessary and sufficient condition. The proposed algorithm is targeted at periodic tasks primarily and applicable to aperiodic tasks at the same time. It can also rectify a single fault which occurs beyond a fixed interval.

Key words: earliest deadline first scheduling, real-time system, fault rectify, periodic tasks, aperiodic tasks, hard real-time, soft real-time