Comment 27 for bug 1188647

@Phillip: I would like to get to the root of your issue, but I don't know that this is the place to do it.

What is your processor? Mine is: Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz
What kernel version do you run? I am running 4.3RC1 with a modified intel_pstate driver.

If I start a CPU burning process on CPU 7 I get:

doug@s15:~/temp$ grep MHz /proc/cpuinfo
cpu MHz : 3699.757
cpu MHz : 3627.640
cpu MHz : 3684.351
cpu MHz : 3800.429
cpu MHz : 3524.710
cpu MHz : 3522.187
cpu MHz : 3540.250
cpu MHz : 3799.898

Then if I set an upper limit:

echo "75" | sudo tee /sys/devices/system/cpu/intel_pstate/max_perf_pct

I get:

doug@s15:~/temp$ grep MHz /proc/cpuinfo
cpu MHz : 2799.953
cpu MHz : 2799.953
cpu MHz : 2799.953
cpu MHz : 2799.953
cpu MHz : 2853.078
cpu MHz : 2799.953
cpu MHz : 2799.953
cpu MHz : 2799.953

as expected.
As a sanity check, my CPU prints a time message occasionally, so we would expect it to have a bigger gap between prints now:

doug@s15:~/c$ taskset -c 7 ./test1
Elapsed: 174.06 s. Delta: 174.06 s. Ave (10): 17.41 s. user cpu: 173.88 s. sys cpu: 0.06 s.
Elapsed: 348.04 s. Delta: 173.98 s. Ave (10): 34.80 s. user cpu: 347.74 s. sys cpu: 0.06 s.
Elapsed: 557.31 s. Delta: 209.27 s. Ave (10): 55.73 s. user cpu: 556.88 s. sys cpu: 0.06 s.
Elapsed: 792.06 s. Delta: 234.75 s. Ave (10): 79.21 s. user cpu: 791.47 s. sys cpu: 0.07 s.

174 / 235 = 74%