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!standard D.03 (00)          03-09-27 AI95-00356/01
!class amendment 03-09-27
!status work item 03-09-27
!status received 03-09-27
!priority Medium
!difficulty Medium
!subject Support for Preemption Level Locking Policy
!summary
A new locking policy is defined that is more general than ceiling locking (but which is equivalent to ceiling locking when priority based dispatching is used).
!problem
For many scheduling schemes, including for example EDF (Earliest Deadline First), the most effective locking policy for protected objects is one known as the Stack Resource Policy. This was defined by Baker in 1991 [1] and has the advantage that is does not enforce the same policy on task dispatching and PO locking; but it does lead to effective implementation of POs. Moreover when priority dispatching is used the SRP policy is the same as ceiling locking.
!proposal
Preemption-Level locking (which is the more general term for this protocol) requires all tasks and POs (that use the policy) to define a preemption-level. A POs preemption-level must not be lower than the preemption-level of any tasks that uses the PO.
To integrate preemption-level locking with ceiling locking we propose that preemption levels are only used with Priority_Specific dispatching and hence only apply to tasks and POs that have identical priority/ ceiling-priority. Hence the scheme is one of preemption-levels within ceilings.
Add to System:
subtype Preemption_Level is Integer range implementation-defined -- must be at least 16 bit integer
Default_Preemption_Level is constant Preemption_Level :=
(Premption_Level'First + Preemption_Level'Last)/2;
Define new policy_identifier for Locking_Policy: Preemption_Level_Within_Ceiling_Locking
If Locking_Policy is Preemption_Level_Within_Ceiling_Locking then Dispatching_Policy must be Priority_Specific.
Add new pragma pragma Preemption_Level(expression); The expected type of the expression is a Preemption_Level.
A new static rule: Anywhere that Ceiling_Locking is required, Preemption_Level_Within_ Ceiling_Locking is acceptable.
Dynamic Semantics All the rules for Ceiling_Locking apply, with the addition:
When a task calls a protected operation with a priority equal to the ceiling of the corresponding protected object a check is made that the preemption_level of the task is not higher than that of the corresponding protected object; Program_Error is raised if this check fails.
!wording
The NOTE, D.3(18) is wrong. This is only true if FIFO_Within_Priority is the dispatching policy; but Ceiling_Locking does not require FIFO_Within_Priority. Although FIFO_Within_Priority does require Ceiling_Locking.
!example
!discussion
This locking policy enables other dispatching policies to be defined. See for example the AI-00357 on EDF scheduling. Only static preemption levels are defined although a more dynamic scheme that allows these levels to be set dynamically would be possible.
!ACATS test
Tests should be created to check on the implementation of this feature.
!appendix

Reference
[1] Baker, T.P., Stack-Based Scheduling of Real-Time Processes,
Journal of Real-Time, Vol 3, No 1, pp67-99, March 1991.

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