YARN-11660. Fix performance regression for SingleConstraintAppPlacementAllocator (#6623) Contributed by Junfan Zhang.

Reviewed-by: Shilun Fan <slfan1989@apache.org>
Reviewed-by: Dinesh Chitlangia <dineshc@apache.org>
Signed-off-by: Shilun Fan <slfan1989@apache.org>

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/placement/SingleConstraintAppPlacementAllocator.java

@@ -309,6 +309,8 @@ public class SingleConstraintAppPlacementAllocator<N extends SchedulerNode>
     // Deduct pending #allocations by 1
     ResourceSizing sizing = schedulingRequest.getResourceSizing();
     sizing.setNumAllocations(sizing.getNumAllocations() - 1);
+
+    appSchedulingInfo.decPendingResource(targetNodePartition, sizing.getResources());
   }
 
   @Override