Why are changes to system CPU settings from 'Minimum Power' to 'Maximum Performance' mode not being actioned?

Issue

• On a server where the CPU frequency or power saving mode (P-state) has been changed via the onboard Baseboard Management Controller (BMC), for example via a web based GUI such as Fujitsu iRMC or Dell DRAC, it's not possible to subsequently change the setting again without a reboot.
• For instance, if it was set to 'Minimum Power', changing it to 'Maximum Performance' has no effect on the CPU frequency or power state, which means that it's not possible to tightly control the CPU frequency thereby restricting power-saving capabilities.
• This can be a further problem in some systems where this mechanism is used for thermal control and thermal disaster prevention. The BMC reads thermal monitors and forces the Operating System to lower P-states using the ACPI _PPC method when critical temperature is approached.