Are you using latest devstack?. As of last week devstack is running keystone under Apache.   You can stop keystone by doing 
service apache2 stop/start/restart.

Check ps -fe | grep -i apache.  If keystone is under Apache, then it will be listed there.

By S.A.Ali: Try this command /etc/init.d/openstack-keystone status this will tell you the status and process id. Now you can stop it by running the command /etc/init.d/openstack-keystone stop after you are finished you can again restart it by replacing stop with restart execute this command to verify if keystone is running ps -ef | grep -i keystone-all. Hope that helps!!

Maybe some other processes use this port? To see listened ports and PIDs of process which use it run this command under root

netstat -lnp

Error maybe due to the MySQL DB is running in backgrond.

Login from root.

1st; stop the keystone service. #/etc/init.d/openstack-keystone stop

2nd; start the keystone service again; #/etc/init.d/openstack-keystone start

If you got the previous error again. then the mysql db service should be running. so stop it. in centos. use; #service mysqld stop if u get any abnormal errors. then try to find if any available services; #ps -ef|grep -i mysql if any try to login to the db with admin rights to shutdown the db. if not kill the process(not recommend for production). try to start the mysql db service only for testing; #service mysqld start if everything goes well then stop the mysqldb service; #service mysqld stop Now you ready to start the keystone service; #/etc/init.d/openstack-keystone start

Everything should be fine now.