This config will describe a procedure on how to persistently load kernel modules during a boot time on CentOS or Redhat Linux system. In the below commands will use nf_conntrack_pptp as a sample module. Replace the name of this module with a module name you wish to load.

First, create a script within /etc/sysconfig/modules/ directory with a file extension modules. Where the name of the script can be any name of your choice. In our case the script will be called nf_conntrack_pptp.modules. Enter a following code into your new nf_conntrack_pptp.modules script while replacing the name of the module with the one you wish to load:
#!/bin/sh
exec /sbin/modprobe nf_conntrack_pptp  >/dev/null 2>&1
Lastly, make the script executable:
chmod +x /etc/sysconfig/modules/nf_conntrack_pptp.modules
All done. After you reboot your Linux system you can confirm that the scrip was loaded by using lsmod and grep commands:
 # lsmod | grep nf_conntrack_pptp
nf_conntrack_pptp      19258  0 
nf_conntrack_proto_gre    14287  1 nf_conntrack_pptp
nf_conntrack          105702  8 nf_conntrack_proto_gre,nf_nat,nf_nat_ipv4,
nf_nat_ipv6,xt_conntrack,nf_conntrack_ipv4,nf_conntrack_ipv6,nf_conntrack_pptp