!standard D.7(10.8/3)                               14-11-17    AI12-0117-1/03
!class binding interpretation 14-10-19
!status Corrigendum 2015 14-11-17
!status WG9 Approved 15-06-26
!status ARG Approved 7-0-1  14-10-19
!status promising 9-0-0  14-06-28
!status work item 14-06-19
!status received 14-06-19
!priority Low
!difficulty Easy
!subject Restriction No_Tasks_Unassigned_To_CPU 
!summary

Add restriction No_Tasks_Unassigned_To_CPU to provide safe use of Ravenscar.

!question

The Ravenscar Profile on multiprocessors requires that all tasks are assigned
to CPUs. In order to do this safely, we need a new restriction to be defined
to ensure that every task is assigned to a known, particular CPU.

We need to know the CPU of each task so that it is possible to analyze the
scheduling of each task, since each CPU has a separate ready queue. This means
that two tasks assigned to the same CPU will behave differently than the same
two tasks assigned to different CPUs.

Should we define such a restriction? (Yes.)

!recommendation

(See Summary.)

!wording

This AI assumes that AI12-0055-1 has been adopted.

Add a new restriction after D.7(10.8/3):

No_Tasks_Unassigned_To_CPU
   The CPU aspect is specified for the environment task. No CPU aspect is
   specified to be statically equal to Not_A_Specific_CPU. If aspect CPU
   is specified (dynamically) to the value Not_A_Specific_CPU, then
   Program_Error is raised. If Set_CPU or Delay_Until_And_Set_CPU are called
   with the CPU parameter equal to Not_A_Specific_CPU, then Program_Error is
   raised.

AARM Ramification: If this restriction is used in a context for which
restriction No_Dynamic_CPU_Assignment is in effect, then no runtime check
is needed when specifying the CPU aspect. If the restriction is used with
the Ravenscar profile, no runtime checks are needed.

!discussion

We add a new restriction:

    No_Tasks_Unassigned_To_CPU

to signify that (1) no task is assigned a CPU value of Not_A_Specific_CPU,
and (2) the environment task has a CPU specified.

This ensures that all tasks are assigned to a known, particular CPU. (If a
task does not have a specified CPU, it will run on the CPU of the task that
activated it -- that is the standard semantics for CPU assignment. Thus
we only have to ensure that the environment task has a specified CPU and
that no task is explicitly assigned Not_A_Specific_CPU.)

The purpose of this restriction is to ensure that there is no
implementation-defined assignment of CPUs; that ensures that the scheduling
of tasks can be analyzed.

!corrigendum D.7(10.8/3)

@dinsa
@xhang<@xterm<No_Specific_Termination_Handlers>
There is no use of a @fa<name> denoting the Set_Specific_Handler
and Specific_Handler subprograms in Task_Termination.>
@dinst
@xhang<@xterm<No_Tasks_Unassigned_To_CPU>
The CPU aspect is specified for the environment task. No CPU aspect is
specified to be statically equal to Not_A_Specific_CPU. If aspect CPU
is specified (dynamically) to the value Not_A_Specific_CPU, then
Program_Error is raised. If Set_CPU or Delay_Until_And_Set_CPU are called
with the CPU parameter equal to Not_A_Specific_CPU, then Program_Error is
raised.>

!ASIS

No ASIS impact.

!ACATS test

An ACATS B-Test and C-Test is needed to verify that the restriction is
implemented as specified.

!appendix

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