CoreFreq for undervolting/overclocking CPU in Linux

Hi there!
Can CoreFreq be used to undervolt/overclock a CPU?
Reason being, I see the option (it tells me my Celeron G4900 can be safely overclocked from 3,1Ghz to like 3,5Ghz); so I tried a safer 3,3Ghz OC, but it did nothing.

You need to use intel-undervolt tool. You can grab it from the following url:

Thanks!
Does this also work for an Intel Xeon Phi pcie card?

I am not sure about that.