Comment 38 for bug 1672439

Kernel 4.4.75 appears to have fixed the problem for C states.

          Package | Core | CPU 0 CPU 4
POLL 0.1% | POLL 0.0% | POLL 0.0% 0.0 ms 0.0% 0.0 ms
C1E-SKL 1.4% | C1E-SKL 2.9% | C1E-SKL 3.5% 0.3 ms 2.3% 0.4 ms
C3-SKL 0.9% | C3-SKL 1.1% | C3-SKL 1.5% 0.2 ms 0.8% 0.2 ms
C6-SKL 8.4% | C6-SKL 12.6% | C6-SKL 19.0% 0.6 ms 6.1% 0.4 ms
C7s-SKL 0.2% | C7s-SKL 0.5% | C7s-SKL 0.6% 1.5 ms 0.3% 0.6 ms
C8-SKL 57.6% | C8-SKL 54.1% | C8-SKL 67.1% 1.7 ms 41.0% 3.2 ms
C9-SKL 0.0% | C9-SKL 0.0% | C9-SKL 0.0% 0.0 ms 0.0% 0.0 ms
C10-SKL 27.9% | C10-SKL 24.9% | C10-SKL 3.2% 4.4 ms 46.6% 8.0 ms

                    | Core | CPU 1 CPU 5
                    | POLL 0.4% | POLL 0.0% 0.0 ms 0.8% 3.7 ms
                    | C1E-SKL 1.4% | C1E-SKL 1.6% 0.2 ms 1.2% 0.4 ms
                    | C3-SKL 1.4% | C3-SKL 1.5% 0.3 ms 1.4% 0.8 ms
                    | C6-SKL 6.4% | C6-SKL 9.6% 0.6 ms 3.2% 0.5 ms
                    | C7s-SKL 0.1% | C7s-SKL 0.2% 0.4 ms 0.0% 0.0 ms
                    | C8-SKL 53.5% | C8-SKL 76.6% 2.4 ms 30.2% 2.4 ms
                    | C9-SKL 0.0% | C9-SKL 0.0% 0.0 ms 0.0% 0.0 ms
                    | C10-SKL 33.5% | C10-SKL 7.2% 4.6 ms 59.6% 9.7 ms

                    | Core | CPU 2 CPU 6
                    | POLL 0.0% | POLL 0.0% 0.0 ms 0.0% 0.0 ms
                    | C1E-SKL 0.6% | C1E-SKL 0.7% 0.1 ms 0.6% 0.2 ms
                    | C3-SKL 0.6% | C3-SKL 1.0% 0.2 ms 0.2% 0.1 ms
                    | C6-SKL 7.6% | C6-SKL 12.7% 0.5 ms 2.6% 0.7 ms
                    | C7s-SKL 0.0% | C7s-SKL 0.0% 0.0 ms 0.0% 0.0 ms
                    | C8-SKL 63.6% | C8-SKL 81.4% 2.2 ms 45.6% 3.5 ms
                    | C9-SKL 0.0% | C9-SKL 0.0% 0.0 ms 0.0% 0.0 ms
                    | C10-SKL 19.2% | C10-SKL 0.0% 0.0 ms 38.3% 4.8 ms

                    | Core | CPU 3 CPU 7
                    | POLL 0.0% | POLL 0.0% 0.0 ms 0.0% 0.0 ms
                    | C1E-SKL 0.6% | C1E-SKL 1.0% 0.2 ms 0.2% 0.7 ms
                    | C3-SKL 0.5% | C3-SKL 0.8% 0.2 ms 0.2% 0.4 ms
                    | C6-SKL 6.4% | C6-SKL 12.4% 0.6 ms 0.5% 0.5 ms
                    | C7s-SKL 0.2% | C7s-SKL 0.0% 0.0 ms 0.4% 0.7 ms
                    | C8-SKL 56.6% | C8-SKL 72.6% 2.1 ms 40.5% 5.2 ms
                    | C9-SKL 0.0% | C9-SKL 0.0% 0.0 ms 0.0% 0.0 ms
                    | C10-SKL 32.8% | C10-SKL 10.0% 6.0 ms 55.5% 7.5 ms

carl@carl-5520:~$ sudo tlp stat -p
--- TLP 0.8 --------------------------------------------

+++ Processor
CPU Model = Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz

/sys/devices/system/cpu/cpu0/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq = 3900000 [kHz]
/sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq = 3900000 [kHz]

/sys/devices/system/cpu/cpu1/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu1/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu1/cpufreq/scaling_min_freq = 3900000 [kHz]
/sys/devices/system/cpu/cpu1/cpufreq/scaling_max_freq = 3900000 [kHz]

/sys/devices/system/cpu/cpu2/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu2/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu2/cpufreq/scaling_min_freq = 800000 [kHz]
/sys/devices/system/cpu/cpu2/cpufreq/scaling_max_freq = 800000 [kHz]

/sys/devices/system/cpu/cpu3/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu3/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu3/cpufreq/scaling_min_freq = 800000 [kHz]
/sys/devices/system/cpu/cpu3/cpufreq/scaling_max_freq = 800000 [kHz]

/sys/devices/system/cpu/cpu4/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu4/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu4/cpufreq/scaling_min_freq = 800000 [kHz]
/sys/devices/system/cpu/cpu4/cpufreq/scaling_max_freq = 800000 [kHz]

/sys/devices/system/cpu/cpu5/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu5/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu5/cpufreq/scaling_min_freq = 800000 [kHz]
/sys/devices/system/cpu/cpu5/cpufreq/scaling_max_freq = 800000 [kHz]

/sys/devices/system/cpu/cpu6/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu6/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu6/cpufreq/scaling_min_freq = 800000 [kHz]
/sys/devices/system/cpu/cpu6/cpufreq/scaling_max_freq = 3900000 [kHz]

/sys/devices/system/cpu/cpu7/cpufreq/scaling_driver = intel_pstate
/sys/devices/system/cpu/cpu7/cpufreq/scaling_governor = powersave
/sys/devices/system/cpu/cpu7/cpufreq/scaling_min_freq = 800000 [kHz]
/sys/devices/system/cpu/cpu7/cpufreq/scaling_max_freq = 800000 [kHz]

/sys/devices/system/cpu/intel_pstate/min_perf_pct = 100
/sys/devices/system/cpu/intel_pstate/max_perf_pct = 100
/sys/devices/system/cpu/intel_pstate/no_turbo = 0

x86_energy_perf_policy.cpu0 = powersave
x86_energy_perf_policy.cpu1 = powersave
x86_energy_perf_policy.cpu2 = powersave
x86_energy_perf_policy.cpu3 = powersave
x86_energy_perf_policy.cpu4 = powersave
x86_energy_perf_policy.cpu5 = powersave
x86_energy_perf_policy.cpu6 = powersave
x86_energy_perf_policy.cpu7 = powersave

/proc/sys/kernel/nmi_watchdog = 0

+++ Undervolting
PHC kernel not available.