!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

!summary

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.

!problem

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.

!proposal

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.

!wording

** TBD **

!discussion

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.

!example

** TBD **

--!corrigendum D.2.4(1)


!ACATS test

Add an ACATS C-Test of this package.


!appendix

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