Option 1) Adding this to /etc/security/limits.conf
mysql soft nofile 65535 mysql hard nofile 65535
Option 2)Modifying systemd configuration file
# vim /etc/systemd/system/mysql.service
And you will need to add following lines
LimitNOFILE=infinity LimitMEMLOCK=infinity
For your information, you can put correct numbers instead of infinity