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!standard D.13.1(4/2)          10-06-04 AI05-0171-1/04
!standard D.16
!class Amendment 09-10-22
!status work item 09-10-22
!status received 09-10-22
!priority Medium
!difficulty Easy
!subject Pragma CPU and Ravenscar Profile
!summary
The Ravenscar Profile, originally designed for single processors, has proven remarkably useful for modelling verifiable real-time single-processor systems. It can be extended to support multiprocessor systems using a fully partitioned approach. The implementation of this scheme is simple, and it can be used to develop applications amenable to schedulability analysis.
To specify the required Ravenscar behaviour on a multiprocessor platform requires the definition of a pragma that will statically set the affinity of a task. This pragma has utility outside its use with the Ravenscar profile.
!problem
The Ravenscar Profile defines a deterministic and analysable tasking model for single processors which can be supported with a run-time system of reduced size and complexity. For multiprocessor systems, the Ravenscar Profile could be extended using a fully partitioned approach, with each task allocated statically to a physical processor. This scheme could be supported by a streamlined run-time system, and applications built following this approach can apply timing analysis techniques on each processor separately.
!proposal
We propose to allow the implementation of the Ravenscar Profile on multiprocessor systems following a fully partitioned approach.
The first part of the proposal contains the definition of a pragma for controlling task affinity, a general package is then defined that will be used by the Ravenscar profile.
!wording
Add a new clause:
D.16 Multiprocessor Implementation
This clause allows implementations on multiprocessor platforms to be configured.
Syntax
The form of a pragma CPU is as follows:
pragma CPU (expression);
Name Resolution Rules
The expected type for the expression is System.Multiprocessors.CPU_Range.
A CPU pragma is allowed only immediately within a task_definition, or the declarative_part of a subprogram_body. At most one such pragma shall appear within a given construct.
For a CPU pragma that appears in the declarative_part of a subprogram_body, the expression shall be static.
Static Semantics
The following language-defined library package exists:
package System.Multiprocessors is pragma Preelaborate(Multiprocessors); type CPU_Range is range 0 .. <implementation-defined>; Not_A_Specific_CPU : constant CPU_Range := 0; subtype CPU is CPU_Range range 1 .. CPU_Range'last; function Number_Of_CPUs return CPU; end System.Multiprocessors;
Dynamic Semantics
A CPU pragma has no effect if it occurs in the declarative part of the subprogram_body of a subprogram other than the main subprogram.
The CPU value determines the processor on which the task will activate and execute; the task is said to be assigned to that processor. If the CPU value is Not_A_Specific_CPU then the task is not assigned to a processor. A task without a CPU pragma will activate and execute on the same processor as its activating task if the activating task is assigned a processor. If the CPU value is not in the range of System.Multiprocessors.CPU_Range or is greater than Number_Of_CPUs the task is defined to have failed, and it becomes a completed task (see 9.2(1)).
The CPU value is associated with the environment task if the pragma appears in the declarative_part of the main subprogram. If a pragma CPU does not apply to the main subprogram it is implementation defined where the environment task executes.
A call of Number_Of_CPUs returns the number of processors available to the program. Within a given partition, each call on Number_Of_CPUs will return the same value.
---
Add to D.13.1(4/2) a new restriction:
No_Dependence => System.Multiprocessors.Dispatching_Domains;
[Should there be a new restrictions_parameter Max_Dispatching_Domains set to 1 for Ravenscar]
--- Add after D.13.1(4/2):
Implementation Requirements
A task shall only be on the ready queues of one processor, and the processor to which a task belongs shall be defined statically. Whenever a task running on a processor reaches a task dispatching point, it goes back to the ready queues of the same processor. A task with a CPU value of Not_A_Specific_CPU will execute on an implementation defined processor. A task without a CPU pragma will activate and execute on the same processor as its activating task.
Implementation Advice
On a multiprocessor system, an implementation should support a fully partitioned approach. Each processor should have separate and disjoint ready queues.
!discussion
Although there are many dispatching approaches that make use of task migration this significantly complicates the run-time and adds overheads. For Ravenscar and its focus on static properties it is in keeping to disallow migration. The scheduling problem for partitioned allocation is a combination of bin packing followed by single processor dispatching. These are known techniques and do not add complications to the Ravenscar model.
!example
A task type that defines worker tasks to be allocated to different CPUs would have the form:
task type Worker (W : CPU_Range) is pragma CPU(W); end Worker;
Alan : Worker(1);
--!corrigendum D.13.1(4/2)
!ACATS test
Add an ACATS C-Test of this package.
!appendix

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