score: Update priority only if necessary

In _Thread_queue_Flush_critical(), update the priority of the thread
queue owner only if necessary.  The scheduler update priority operation
could be expensive.

cpukit/include/rtems/score/threadqimpl.h

@@ -366,8 +366,8 @@ RTEMS_INLINE_ROUTINE void _Thread_queue_Context_clear_priority_updates(
  *
  * @return The priority update count of @a queue_context.
  */
-RTEMS_INLINE_ROUTINE size_t _Thread_queue_Context_save_priority_updates(
-  Thread_queue_Context *queue_context
+RTEMS_INLINE_ROUTINE size_t _Thread_queue_Context_get_priority_updates(
+  const Thread_queue_Context *queue_context
 )
 {
   return queue_context->Priority.update_count;

cpukit/score/src/threadchangepriority.c

@@ -212,7 +212,7 @@ void _Thread_Priority_perform_actions(
    */
 
   the_thread = start_of_path;
-  update_count = _Thread_queue_Context_save_priority_updates( queue_context );
+  update_count = _Thread_queue_Context_get_priority_updates( queue_context );
 
   while ( true ) {
     Thread_queue_Queue *queue;

cpukit/score/src/threadqflush.c

@@ -71,15 +71,15 @@ size_t _Thread_queue_Flush_critical(
   Thread_queue_Context          *queue_context
 )
 {
-  size_t          flushed;
-  Chain_Control   unblock;
-  Thread_Control *owner;
-  Chain_Node     *node;
-  Chain_Node     *tail;
+  size_t        flushed;
+  size_t        priority_updates;
+  Chain_Control unblock;
+  Chain_Node   *node;
+  Chain_Node   *tail;
 
   flushed = 0;
+  priority_updates = 0;
   _Chain_Initialize_empty( &unblock );
-  owner = queue->owner;
 
   while ( true ) {
     Thread_queue_Heads *heads;
@@ -99,8 +99,7 @@ size_t _Thread_queue_Flush_critical(
 
     /*
      * We do not have enough space in the queue context to collect all priority
-     * updates, so clear it each time.  We unconditionally do the priority
-     * update for the owner later if it exists.
+     * updates, so clear it each time and accumulate the priority updates.
      */
     _Thread_queue_Context_clear_priority_updates( queue_context );
 
@@ -120,6 +119,8 @@ size_t _Thread_queue_Flush_critical(
       );
     }
 
+    priority_updates +=
+      _Thread_queue_Context_get_priority_updates( queue_context );
     ++flushed;
   }
 
@@ -145,9 +146,12 @@ size_t _Thread_queue_Flush_critical(
       node = next;
     } while ( node != tail );
 
-    if ( owner != NULL ) {
+    if ( priority_updates != 0 ) {
+      Thread_Control  *owner;
       ISR_lock_Context lock_context;
 
+      owner = queue->owner;
+      _Assert( owner != NULL );
       _Thread_State_acquire( owner, &lock_context );
       _Scheduler_Update_priority( owner );
       _Thread_State_release( owner, &lock_context );