Classical approaches based on preemption, such as RM (Rate Monotonic), DM (Deadline Monotonic), EDF (Earliest Deadline First), LLF (Least Laxity First), etc, give schedulability conditions in the case of a single processor, but assume the cost of the preemption to be negligible compared to the duration of each task. Clearly the global cost is difcult to determine accurately because, if the cost of one preemption is known for a given processor, it is not the same for the exact number of preemptions of each task. Because we are interested in hard real-time systems with precedence and strict periodicity constraints where it is mandatory to satisfy these constraints, we give a scheduling algorithm which counts the exact number of preemptions for each task, and thus leads to a new schedulability condition. This is currently done in the particular case where the periods of all the tasks constitute an harmonic sequence.