score: Implement scheduler helping protocol

The following scheduler operations return a thread in need for help

    - unblock,
    - change priority, and
    - yield.

A thread in need for help is a thread that encounters a scheduler state
change from scheduled to ready or a thread that cannot be scheduled in
an unblock operation.  Such a thread can ask threads which depend on
resources owned by this thread for help.

Add a new ask for help scheduler operation.  This operation is used by
_Scheduler_Ask_for_help() to help threads in need for help returned by
the operations mentioned above.  This operation is also used by
_Scheduler_Thread_change_resource_root() in case the root of a resource
sub-tree changes.  A use case is the ownership change of a resource.

In case it is not possible to schedule a thread in need for help, then
the corresponding scheduler node will be placed into the set of ready
scheduler nodes of the scheduler instance.  Once a state change from
ready to scheduled happens for this scheduler node it may be used to
schedule the thread in need for help.

1 files changed, 7 insertions, 0 deletions

diff --git a/cpukit/score/include/rtems/score/schedulersimplesmp.h b/cpukit/score/include/rtems/score/schedulersimplesmp.h
index de338ab168..11310c6898 100644
--- a/cpukit/score/include/rtems/score/schedulersimplesmp.h
+++ b/cpukit/score/include/rtems/score/schedulersimplesmp.h
@@ -65,6 +65,7 @@ typedef struct {
     _Scheduler_simple_SMP_Block, \
     _Scheduler_simple_SMP_Unblock, \
     _Scheduler_simple_SMP_Change_priority, \
+    _Scheduler_simple_SMP_Ask_for_help, \
     _Scheduler_simple_SMP_Node_initialize, \
     _Scheduler_default_Node_destroy, \
     _Scheduler_simple_SMP_Update_priority, \
@@ -99,6 +100,12 @@ Thread_Control *_Scheduler_simple_SMP_Change_priority(
   bool                     prepend_it
 );
 
+Thread_Control *_Scheduler_simple_SMP_Ask_for_help(
+  const Scheduler_Control *scheduler,
+  Thread_Control          *offers_help,
+  Thread_Control          *needs_help
+);
+
 void _Scheduler_simple_SMP_Update_priority(
   const Scheduler_Control *scheduler,
   Thread_Control          *thread,