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!standard D.2.4(1)          09-10-22 AI05-0166-1/01
!class Amendment 09-10-22
!status work item 09-10-22
!status received 09-10-22
!priority Medium
!difficulty Easy
!subject Yield for non-preemptive dispatching
A support package for non-preemptive dispatching is proposed that allows a task to offer to be preempted by a higher priority task, but not by one of equal priority. A further 'yield' procedure is included to improve the readability and maintainability of programs. It replaces the current need to include 'delay 0.0' to force a preemption.
With non-preemptive dispatching the only current non-blocking dispatching point is a non-blocking delay_statement. We need another kind so that a task in a system scheduled by non-preemptive dispatching can indicate that it is prepared to be preempted by tasks of higher priority but not by those of equal priority.
Add the following library package:
package Ada.Dispatching.Non_Preemptive is
procedure Yield_To_Higher;
procedure Yield; -- Bounded error if executed within a -- protected operation
end Ada.Dispatching.Non_Preemptive;
With non-preemptive dispatching the only current non-blocking dispatching point is a non-blocking delay_statement - D.2.4(9/2). With this new package a call to Yield_To_Higher will cause a task switch if there is a runnable task with an active priority higher than the calling task's active priority. Note this can be called from within a protected object.
To program cooperative scheduling a task explicitly includes the points at which it is able to be preempted. It does this by using either 'delay 0.0' or 'delay until Ada.Real_Time.Time_First' (the latter being the one available with Ravenscar). As these statements are not actually concerned with delaying the task they are potentially misleading (to the human reader/maintainer of the code). The procedure Yield is equivalent to these delay statements; and would therefore cause the same bounded error if called from within a protected action.
** TBD **
This AI was raised by an industrial user of non-preemptive scheduling, and was discussed and endorsed by the 14th IRTAW. Details are contained in the workshop paper: Providing Additional Real-Time Capability and Flexibility for Ada 2005, by Rod White.
** TBD **
--!corrigendum D.2.4(1)
!ACATS test
Add an ACATS C-Test of this package.


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