Contiunous rise of mysqld memory consumption
We tracked it down to glibc. With glibc-2.12-1.149.el6_6.4.x86_64, memory is not fully freed when closing tables.

It seems that under high concurrent connections, accessing partitioned tables increases Open_tables which increases memory usage, however memory is not fully freed when the table is closed using glibc.

Newer glibcs faced the same problem as well.
Switching to jemalloc-3.6.0-1.el6.x86_64 has resolved our problem.

Test environment
CentOS release 6.6 (Final)
kernel 2.6.32-504.el6.x86_64
mysql-5.6.16  

physical ram 2G
bufferpool 256M
other default  

memory usage with gblic-2.12-1.166.el6_7.7.x86_64
1st round
mysqlslap concurrent connection: 70, accessed partions: 800 -> 1.3G
"flush table" -> 1.0G  

2nd round
mysqlslap concurrent connection: 70, accessed partions: 800 -> 1.6G
"flush table" -> 1.2G  

3rd round
mysqlslap concurrent connection: 70, accessed partions: 800 -> 1.6G
"flush table" -> 1.4G  

memory usage with jemalloc-3.6.0-1.el6.x86_6
1st round
mysqlslap concurrent connection: 70, accessed partions: 800 -> 1.3G
"flush table" -> 582M  

2nd round
mysqlslap concurrent connection: 70, accessed partions: 800 -> 1.3G
"flush table" -> 583M  

3rd round
mysqlslap concurrent connection: 70, accessed partions: 800 -> 1.2G
"flush table" -> 584M