I used netstat to acquire RefCnt column. And there are 2 attached processes via /var/run/mysqld/mysqld.sock. How to find their PIDs?
# netstat -a -p -A unix
Proto RefCnt Flags Type State I-Node PID/Program name Path
...
unix 2 [ ACC ] STREAM LISTENING 1432153 7517/mysqld /var/run/mysqld/mysqld.sock
...