I’ve AMD Athlon II processors based server and it does not change frequency on all logical cores under CentOS version 5.5 or RHEL Linux. How do I fix this problem?

The default CPU frequency scaling method uses powernow-k8 frequency driver module, and the ACPI data provided by BIOS, are not compatible with each other. It may not work at all to save power. To fix this problem Edit /etc/modprobe.conf file, enter:
# vi /etc/modprobe.conf
Append the following line:

options powernow-k8 preregister_acpi_perf=0

Save and close the file. It will work after reboot, however you can fix this for current setup by typing the following command as root user:
# /etc/init.d/ cpuspeed stop
# /sbin/rmmod powernow_k8
# /sbin/modprobe powernow-k8 preregister_acpi_perf=0
# /etc/init.d/cpuspeed start