you mean at that moment all of upstream server has to tried,and it reponse 504? but at that moment only one app server's log throw error info.
A part of access_log like:
10.001 10.001 /cas/login 10.10.2.100:8080 504
10.001 10.001 /cas/login 10.10.2.100:8080 504

If pass to all servers, the log is like below:?
10.001 10.001 /cas/login 10.10.2.100:8080, 10.10.2.101:8080, 10.10.2.102:8080, 10.10.2.103:8080 504


the problem is my app server 's log show ,at that moment only one server of upstream have 504 error.