D.5.1 Dynamic Priorities for Tasks
[This subclause describes how the base priority of a task can be modified
or queried at run time.]
The following language-defined
library package exists:
Ada.Task_Identification; -- See C.7.1
Set_Priority(Priority : in
T : in
Get_Priority (T : Ada.Task_Identification.Task_Id :=
The procedure Set_Priority sets the base priority
of the specified task to the specified Priority value. Set_Priority has
no effect if the task is terminated.
The function Get_Priority returns T's current base
Tasking_Error is raised if the task is
Reason: There is no harm in setting the
priority of a terminated task. A previous version of Ada 9X made this
a run-time error. However, there is little difference between setting
the priority of a terminated task, and setting the priority of a task
that is about to terminate very soon; neither case should be an error.
Furthermore, the run-time check is not necessarily feasible to implement
on all systems, since priority changes might be deferred due to inter-processor
communication overhead, so the error might not be detected until after
Set_Priority has returned.
However, we wish to allow implementations to
avoid storing “extra” information about terminated tasks.
Therefore, we make Get_Priority of a terminated task raise an exception;
the implementation need not continue to store the priority of a task
that has terminated.
Program_Error is raised by Set_Priority
and Get_Priority if T is equal to Null_Task_Id.
On a system with a single processor, the setting of the base priority
of a task T
to the new value occurs immediately at the first point
is outside the execution of a protected action.
The priority change is immediate if the target task is on a delay queue
or a ready queue outside of a protected action. However, consider when
Set_Priority is called by a task T1 to set the priority of T2, if T2
is blocked, waiting on an entry call queued on a protected object, the
entry queue needs to be reordered. Since T1 might have a priority that
is higher than the ceiling of the protected object, T1 cannot, in general,
do the reordering. One way to implement this is to wake T2 up and have
T2 do the work. This is similar to the disentangling of queues that needs
to happen when a high-priority task aborts a lower-priority task, which
might have a call queued on a protected object with a low ceiling. We
have an Implementation Permission in D.4
allow this implementation. We could have required an immediate priority
change if on a ready queue during a protected action, but that would
have required extra checks for ceiling violations to meet Bounded (Run-Time)
Error requirements of D.3
and potentially could
cause a protected action to be abandoned in the middle (by raising Program_Error).
That seems bad.
previous version of Ada 9X made it a run-time error for a high-priority
task to set the priority of a lower-priority task that has a queued call
on a protected object with a low ceiling. This was changed because:
The check was not feasible to implement on
all systems, since priority changes might be deferred due to inter-processor
communication overhead. The calling task would continue to execute without
finding out whether the operation succeeded or not.
The run-time check would tend to cause intermittent
system failures — how is the caller supposed to know whether the
other task happens to have a queued call at any given time? Consider
for example an interrupt that needs to trigger a priority change in some
task. The interrupt handler could not safely call Set_Priority without
knowing exactly what the other task is doing, or without severely restricting
the ceilings used in the system. If the interrupt handler wants to hand
the job off to a third task whose job is to call Set_Priority, this won't
help, because one would normally want the third task to have high priority.
Bounded (Run-Time) Errors
Paragraph 11 was
If any subprogram in this package
is called with a parameter T that specifies a task object that no longer
exists, the execution of the program is erroneous.
Ramification: Note that this rule overrides
the above rule saying that Program_Error is raised on Get_Priority of
a terminated task. If the task object still exists, and the task is terminated,
Get_Priority raises Program_Error. However, if the task object no longer
exists, calling Get_Priority causes erroneous execution.
On a multiprocessor, the implementation shall document any conditions
that cause the completion of the setting of the priority of a task to
be delayed later than what is specified for a single processor.
Documentation Requirement: Any conditions
that cause the completion of the setting of the priority of a task to
be delayed for a multiprocessor.
shall document the following metric:
The execution time of a call to Set_Priority, for
the nonpreempting case, in processor clock cycles. This is measured for
a call that modifies the priority of a ready task that is not running
(which cannot be the calling one), where the new base priority of the
affected task is lower than the active priority of the calling task,
and the affected task is not on any entry queue and is not executing
a protected operation.
Documentation Requirement: The metrics
Setting a task's base priority affects task dispatching. First, it can
change the task's active priority. Second, under the FIFO_Within_Priorities
policy it always causes the task to move to the tail of the ready queue
corresponding to its active priority, even if the new base priority is
27 Under the priority queuing policy, setting
a task's base priority has an effect on a queued entry call if the task
is blocked waiting for the call. That is, setting the base priority of
a task causes the priority of a queued entry call from that task to be
updated and the call to be removed and then reinserted in the entry queue
at the new priority (see D.4
), unless the call
originated from the triggering_statement
of an asynchronous_select
28 The effect of two or more Set_Priority
calls executed in parallel on the same task is defined as executing these
calls in some serial order.
The rule for when Tasking_Error is raised for Set_Priority or Get_Priority
is different from the rule for when Tasking_Error is raised on an entry
call (see 9.5.3
). In particular, querying
the priority of a completed or an abnormal task is allowed, so long as
the task is not yet terminated, and setting the priority of a task is
allowed for any task state (including for terminated tasks).
30 Changing the priorities of a set of
tasks can be performed by a series of calls to Set_Priority for each
task separately. For this to work reliably, it should be done within
a protected operation that has high enough ceiling priority to guarantee
that the operation completes without being preempted by any of the affected
Extensions to Ada 95
Priority changes are
now required to be done immediately so long as the target task is not
on an entry queue.
Dynamic_Priorities is now Preelaborated, so it can be used in preelaborated
Wording Changes from Ada 95
This Ada 95 subclause was moved down a level. The paragraph numbers are
the same as those for D.5
in Ada 95.
There is no “standard” policy anymore, so that phrase was
replaced by the name of a specific policy in the notes.
The bounded error for the priority of a task being higher than the ceiling
of an object it is currently in was moved to D.3
so that it applies no matter how the situation arises.
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