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D.2.3 Preemptive Dispatching

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This clause defines a preemptive task dispatching policy. 

Static Semantics

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The policy_identifier FIFO_Within_Priorities is a task dispatching policy.

Dynamic Semantics

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When FIFO_Within_Priorities is in effect, modifications to the ready queues occur only as follows:
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Each of the events specified above is a task dispatching point (see D.2.1).
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A task dispatching point occurs for the currently running task of a processor whenever there is a nonempty ready queue for that processor with a higher priority than the priority of the running task. The currently running task is said to be preempted and it is added at the head of the ready queue for its active priority.

Implementation Requirements

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 An implementation shall allow, for a single partition, both the task dispatching policy to be specified as FIFO_Within_Priorities and also the locking policy (see D.3) to be specified as Ceiling_Locking. 

Documentation Requirements

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 Priority inversion is the duration for which a task remains at the head of the highest priority nonempty ready queue while the processor executes a lower priority task. The implementation shall document:
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NOTES
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14  If the active priority of a running task is lowered due to loss of inherited priority (as it is on completion of a protected operation) and there is a ready task of the same active priority that is not running, the running task continues to run (provided that there is no higher priority task).
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15  Setting the base priority of a ready task causes the task to move to the tail of the queue for its active priority, regardless of whether the active priority of the task actually changes.

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