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 D.2.2 Task Dispatching Pragmas The Standard Task Dispatching Policy

0.1/2
  {AI95-00355-01} [This clause allows a single task dispatching policy to be defined for all priorities, or the range of priorities to be split into subranges that are assigned individual dispatching policies.]

Syntax

1
The form of a pragma Task_Dispatching_Policy is as follows: 
2
  pragma Task_Dispatching_Policy(policy_identifier);
2.1/2
{AI95-00355-01} The form of a pragma Priority_Specific_Dispatching is as follows: 
2.2/2
  pragma Priority_Specific_Dispatching (
     policy_identifier, first_priority_expression, last_priority_expression);

Name Resolution Rules

2.3/2
  {AI95-00355-01} The expected type for first_priority_expression and last_priority_expression is Integer. 

Legality Rules

3/2
{AI95-00321-01} {AI95-00355-01} The policy_identifier used in a pragma Task_Dispatching_Policy shall be the name of a task dispatching policy shall either be FIFO_Within_Priorities or an implementation-defined identifier.
3.a/2
This paragraph was deleted.Implementation defined: Implementation-defined policy_identifiers allowed in a pragma Task_Dispatching_Policy.
3.1/2
  {AI95-00355-01} The policy_identifier used in a pragma Priority_Specific_Dispatching shall be the name of a task dispatching policy.
3.2/2
  {AI95-00355-01} Both first_priority_expression and last_priority_expression shall be static expressions in the range of System.Any_Priority; last_priority_expression shall have a value greater than or equal to first_priority_expression.

Static Semantics

3.3/2
  {AI95-00355-01} Pragma Task_Dispatching_Policy specifies the single task dispatching policy.
3.4/2
  {AI95-00355-01} Pragma Priority_Specific_Dispatching specifies the task dispatching policy for the specified range of priorities. Tasks with base priorities within the range of priorities specified in a Priority_Specific_Dispatching pragma have their active priorities determined according to the specified dispatching policy. Tasks with active priorities within the range of priorities specified in a Priority_Specific_Dispatching pragma are dispatched according to the specified dispatching policy. 
3.b/2
Reason: {AI95-00355-01} Each ready queue is managed by exactly one policy. Anything else would be chaos. The ready queue is determined by the active priority. However, how the active priority is calculated is determined by the policy; in order to break out of this circle, we have to say that the active priority is calculated by the method determined by the policy of the base priority. 
3.5/2
  {AI95-00355-01} If a partition contains one or more Priority_Specific_Dispatching pragmas the dispatching policy for priorities not covered by any Priority_Specific_Dispatching pragmas is FIFO_Within_Priorities.

Post-Compilation Rules

4/2
{AI95-00355-01} {configuration pragma (Task_Dispatching_Policy) [partial]} {pragma, configuration (Task_Dispatching_Policy) [partial]} A Task_Dispatching_Policy pragma is a configuration pragma. A Priority_Specific_Dispatching pragma is a configuration pragma. {configuration pragma (Priority_Specific_Dispatching) [partial]} {pragma, configuration (Priority_Specific_Dispatching) [partial]}
4.1/2
  {AI95-00355-01} The priority ranges specified in more than one Priority_Specific_Dispatching pragma within the same partition shall not be overlapping.
4.2/2
  {AI95-00355-01} If a partition contains one or more Priority_Specific_Dispatching pragmas it shall not contain a Task_Dispatching_Policy pragma.
5/2
This paragraph was deleted.{AI95-00333-01} If the FIFO_Within_Priorities policy is specified for a partition, then the Ceiling_Locking policy (see D.3) shall also be specified for the partition.

Dynamic Semantics

6/2
{AI95-00355-01} {task dispatching policy} [A task dispatching policy specifies the details of task dispatching that are not covered by the basic task dispatching model. These rules govern when tasks are inserted into and deleted from the ready queues, and whether a task is inserted at the head or the tail of the queue for its active priority.] A single The task dispatching policy is specified by a Task_Dispatching_Policy configuration pragma. Pragma Priority_Specific_Dispatching assigns distinct dispatching policies to subranges of System.Any_Priority. {unspecified [partial]} If no such pragma appears in any of the program units comprising a partition, the task dispatching policy for that partition is unspecified.
6.1/2
  {AI95-00355-01} {unspecified [partial]} If neither pragma applies to any of the program units comprising a partition, the task dispatching policy for that partition is unspecified.
6.2/2
  {AI95-00355-01} If a partition contains one or more Priority_Specific_Dispatching pragmas a task dispatching point occurs for the currently running task of a processor whenever there is a non-empty ready queue for that processor with a higher priority than the priority of the running task.
6.a/2
Discussion: If we have priority specific dispatching then we want preemption across the entire range of priorities. That prevents higher priority tasks from being blocked by lower priority tasks that have a different policy. On the other hand, if we have a single policy for the entire partition, we want the characteristics of that policy to apply for preemption; specifically, we may not require any preemption. Note that policy Non_Preemptive_FIFO_Within_Priorities is not allowed in a priority specific dispatching pragma. 
6.3/2
  {AI95-00355-01} A task that has its base priority changed may move from one dispatching policy to another. It is immediately subject to the new dispatching policy.
6.b/2
Ramification: Once subject to the new dispatching policy, it may be immediately preempted or dispatched, according the rules of the new policy. 
Paragraphs 7 through 13 were moved to D.2.3. 
7/2
{AI95-00321-01} The language defines only one task dispatching policy, FIFO_Within_Priorities; when this policy is in effect, modifications to the ready queues occur only as follows:
8/2
9/2
10/2
11/2
11.a/2
Ramification: If the delay does result in blocking, the task moves to the “delay queue”, not to the ready queue. 
12/2
 {AI95-00321-01} {task dispatching point [partial]} {dispatching point [partial]} Each of the events specified above is a task dispatching point (see D.2.1).
13/2
 {AI95-00321-01} In addition, when a task is preempted, it is added at the head of the ready queue for its active priority.

Implementation Requirements

13.1/2
   {AI95-00333-01} {AI95-00355-01} An implementation shall allow, for a single partition, both the locking policy (see D.3) to be specified as Ceiling_Locking and also one or more Priority_Specific_Dispatching pragmas to be given. 

Documentation Requirements

Paragraphs 14 through 16 were moved to D.2.3. 
14/2
 {AI95-00321-01} {priority inversion} Priority inversion is the duration for which a task remains at the head of the highest priority ready queue while the processor executes a lower priority task. The implementation shall document: 
15/2
16/2
16.a/2
This paragraph was deleted.Implementation defined: Implementation-defined aspects of priority inversion.

Implementation Permissions

17/2
 {AI95-00256-01} Implementations are allowed to define other task dispatching policies, but need not support more than one task dispatching such policy per partition.
18/2
 {AI95-00355-01} An implementation need not support pragma Priority_Specific_Dispatching if it is infeasible to support it in the target environment. [For optimization purposes,] an implementation may alter the points at which task dispatching occurs, in an implementation defined manner. However, a delay_statement always corresponds to at least one task dispatching point.
18.a/2
Implementation defined: Implementation defined task dispatching policies.
NOTES
Paragraphs 19 through 21 were deleted. 
19/2
14  {AI95-00321-01} 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).
20/2
15  {AI95-00321-01} The setting of a task's base priority as a result of a call to Set_Priority does not always take effect immediately when Set_Priority is called. The effect of setting the task's base priority is deferred while the affected task performs a protected action.
21/2
16  {AI95-00321-01} Setting the base priority of a ready task causes the task to move to the end of the queue for its active priority, regardless of whether the active priority of the task actually changes.

Extensions to Ada 95

21.a/2
{AI95-00333-01} {extensions to Ada 95} Amendment Correction: It is no longer required to specify Ceiling_Locking with the language-defined task dispatching policies; we only require that implementations allow them to be used together.
21.b/2
{AI95-00355-01} Pragma Priority_Specific_Dispatching is new; it allows specifying different policies for different priorities.

Wording Changes from Ada 95

21.c/2
{AI95-00256-01} Clarified that an implementation need support only one task dispatching policy (of any kind, language-defined or otherwise) per partition.
21.d/2
{AI95-00321-01} This description is simplified to describe only the rules for the Task_Dispatching_Policy pragma that are common to all policies. In particular, rules about preemption are moved elsewhere. This makes it easier to add other policies (which may not involve preemption).

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