PowerEdge R730xd: How to configure fixed CPU frequency?

​We are running some performance tests on our R730xds, but see CPU frequency jump up and down even though we're running in "Performance" mode.​

​The OS is Ubuntu 14.04 with a 4.4 kernel. It seems to switch from "performance" to "powersave" mode :-(​

​Currently, it is running the "intel_pstate" (/sys/devices/system/cpu/intel_pstate) scaling driver​
​ (/sys/devices/system/cpu/cpu0/cpufreq/scaling_driver). But because​
​ the OS thinks HWP is not supported, it's running in a less efficient​
​ "legacy" mode designed for Sandy Bridge and earlier​
​ (/sys/kernel/debug/pstate_snb).​

​How can we make the system run at a fixed CPU frequency? CPUs are Intel Xeon E5-2640 v3.​

​Any help would be much appreciated.​