In my VM(CentOS7.2) I installed the devstack:

But when I check the service, I find the devstack@q-agt.service is not running.

[root@localhost ~]# systemctl status devstack@q-agt.service -l 
● devstack@q-agt.service - Devstack devstack@q-agt.service
   Loaded: loaded (/etc/systemd/system/devstack@q-agt.service; enabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Mon 2017-08-21 22:39:24 CST; 25s ago
  Process: 13595 ExecStart=/usr/bin/neutron-openvswitch-agent --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/ml2/ml2_conf.ini (code=exited, status=1/FAILURE)
 Main PID: 13595 (code=exited, status=1/FAILURE)

Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: ERROR neutron   File "/usr/lib/python2.7/site-packages/eventlet/greenthread.py", line 214, in main
Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: ERROR neutron     result = function(*args, **kwargs)
Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: ERROR neutron   File "/usr/lib/python2.7/site-packages/ryu/lib/hub.py", line 65, in _launch
Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: ERROR neutron     raise e
Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: ERROR neutron Exception: Could not retrieve schema from tcp:127.0.0.1:6640: Connection refused
Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: ERROR neutron 
Aug 21 22:39:24 localhost.localdomain neutron-openvswitch-agent[13595]: INFO oslo_rootwrap.client [-] Stopping rootwrap daemon process with pid=13632
Aug 21 22:39:24 localhost.localdomain systemd[1]: devstack@q-agt.service: main process exited, code=exited, status=1/FAILURE
Aug 21 22:39:24 localhost.localdomain systemd[1]: Unit devstack@q-agt.service entered failed state.
Aug 21 22:39:24 localhost.localdomain systemd[1]: devstack@q-agt.service failed.

I tried to restart this service but failed, how to avoid it?

EDIT

And in the /var/log/message I found the below error message :

...
Aug 21 22:50:16 localhost ovsdb-server: ovs|09137|socket_util|ERR|6640:127.0.0.1: bind: Permission denied
Aug 21 22:50:16 localhost ovsdb-server: ovs|09138|socket_util|ERR|6640:127.0.0.1: bind: Permission denied
Aug 21 22:50:16 localhost ovsdb-server: ovs|09139|socket_util|ERR|6640:127.0.0.1: bind: Permission denied
Aug 21 22:50:16 localhost ovsdb-server: ovs|09140|socket_util|ERR|6640:127.0.0.1: bind: Permission denied
Aug 21 22:50:16 localhost ovs-vsctl: ovs|00002|db_ctl_base|ERR|transaction error: {"details":"Transaction causes multiple rows in \"Manager\" table to have identical values (\"ptcp:6640:127.0.0.1\") for index on column \"target\".  First row, with UUID 5ec22779-d307-403c-a2d7-e61a739be275, was inserted by this transaction.  Second row, with UUID cb472941-4441-4b92-8777-694aa1278310, existed in the database before this transaction and was not modified by the transaction.","error":"constraint violation"}
Aug 21 22:50:16 localhost ovsdb-server: ovs|09141|socket_util|ERR|6640:127.0.0.1: bind: Permission denied
Aug 21 22:50:16 localhost ovsdb-server: ovs|09142|socket_util|ERR|6640:127.0.0.1: bind: Permission denied

EDIT-2

I use the journalctl -xe get the bellow error:

[root@localhost log]# journalctl -xe 
Aug 21 ...