  In this paper we consider the scheduling problem of hard real-time systems
composed of periodic constrained-deadline tasks upon identical multiprocessor
platforms. We assume that tasks are scheduled by using the global-EDF
scheduler. We establish an exact schedulability test for this scheduler by
exploiting on the one hand its predictability property and by providing on the
other hand a feasibility interval so that if it is possible to find a valid
schedule for all the jobs contained in this interval, then the whole system
will be stamped feasible. In addition, we show by means of a counterexample
that the feasibility interval, and thus the schedulability test, proposed by
Leung [Leung 1989] is incorrect and we show which arguments are actually
incorrect.
