code: private void checkDecommissioning(DatanodeDescriptor nodeReg, String ipAddr) 
      throws IOException {
      // If the registered node is in exclude list, then decommission it
      if (inExcludedHostsList(nodeReg, ipAddr)) {
        startDecommission(nodeReg);
      }
    }
docs: Decommission the node if it is in exclude list.