Upgrade to postgres 10 - Postmaster not getting shutdown

Hi

I am trying to upgrade to postgres 10 from 9.5.4. On doing the
compatibility check I am getting the below error.

ubuntu(at)vrni-platform-release:/tmp$ sudo service postgresql stop
* Stopping PostgreSQL 10 database server

[ OK ]
* Stopping PostgreSQL 9.5 database server

[ OK ]

ubuntu(at)vrni-platform-release:/tmp$ sudo -H -u postgres
/usr/lib/postgresql/10/bin/pg_upgrade -b /usr/lib/postgresql/9.5/bin -B
/usr/lib/postgresql/10/bin -d /var/lib/postgresql/data/postgresql0 -D
/var/lib/postgresql/10/data/postgresql0 -o ' -c
config_file=/etc/postgresql/9.5/main/postgresql.conf' -O ' -c
config_file=/etc/postgresql/10/main/postgresql.conf' --check -k

There seems to be a postmaster servicing the new cluster.
Please shutdown that postmaster and try again.
Failure, exiting

Can some one let me know what is going wrong here and how can I get around
this?

Thanks,
Debraj