I've set "Maximum Concurrent Builds Per Node" to 1 and checked "Throttle Matrix master builds" and "Throttle Matrix configuration builds" in my multi configuration job which can be called by several jobs. I specified a user-defined axis with multiple values as well as a "Label expression" axis which has only one "Git_Linux" value. There are 3 slaves have this label in my build cluster.
Under such configuration, the master job are not throttled well. When the master job is called simultaneously by different jobs, there was great possibility that this multi configuration job was to run on one node.
It is likely something related to the label. Because when I specify the "Label expression" with name of one specified slave node, everything seems to be OK.