1. An apparatus for scheduling execution of a plurality of tasks by a processor, each of the plurality of tasks having an associated periodicity and an associated priority based upon the associated periodicity comprising: the processor to execute each of the plurality of tasks periodically according to the associated periodicity of the task; and a scheduler, at each of a series of scheduling time points, to: update the priorities of the plurality of tasks; determine whether a task needs to be executed in accordance with the periodicity associated with that task; and schedule the plurality of tasks that need to be executed in accordance with the priorities of the tasks, wherein the scheduler is to update the priorities of the plurality of tasks by: identifying an unexecuted task which, at a preceding scheduling time point, was scheduled for execution but which, since that preceding scheduling time point, has not been executed; and increasing the priority of the unexecuted task by setting the priority of the unexecuted task as greater than the priority of other tasks that have the same periodicity as the unexecuted task and that are not themselves unexecuted tasks.