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.