Automatically change cpu frequency governor

I understand from this question that Ubuntu is now using "Intel P-State", and that as a result, there are only two governor options: performance, and powersave.

But is it possible to automatically switch to powersave when on battery, and performance when a charger is connected? I'm surprised that this functionality isn't already implemented by default.

You can use acpid to manage acpi events such as connecting to AC adapter. /etc/acpi/hadler.sh can be configured to change cpu governor on acpi events.

#/etc/acpi/handler.sh

ac_adapter)
    case "$2" in
        AC*)
            case "$4" in
                00000000)
                    echo "powersave" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor    
                ;;
                00000001)
                    echo "performance" >/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
                ;;
            esac
        ;;
        *) logger "ACPI action undefined: $2" ;;
    esac
;;

If you use tlp, you can set

CPU_SCALING_GOVERNOR_ON_AC=performance

on /etc/default/tlp /etc/tlp.conf to do the same.