Comment # 13 on bug 1131437 from Jan Kara

4.18 vanilla kernel numbers with intel_idle.max_cstate=1:

Amean      1       26.78 (   0.00%)       26.63 (   0.55%)       29.06 ( 
-8.51%)       26.06 (   2.70%)       26.81 (  -0.09%)
Amean      2       31.79 (   0.00%)       31.85 (  -0.19%)       32.54 ( 
-2.36%)       34.30 (  -7.90%)       33.63 (  -5.78%)
Amean      4       39.09 (   0.00%)       41.04 (  -4.99%)       37.31 (  
4.55%)       38.27 (   2.09%)       35.77 (   8.49%)
Stddev     1        3.31 (   0.00%)        3.60 (  -8.60%)        5.65 (
-70.56%)        2.67 (  19.45%)        3.98 ( -20.17%)
Stddev     2        6.04 (   0.00%)        5.86 (   2.84%)        5.76 (  
4.61%)        6.33 (  -4.83%)        5.84 (   3.21%)
Stddev     4        5.90 (   0.00%)        5.69 (   3.56%)        6.69 (
-13.35%)        6.26 (  -6.13%)        6.62 ( -12.13%)

So not significantly different to numbers without intel_idle.max_cstate=1. But
variation is about comparable to the one with 5.0 and intel_idle.max_cstate=1
(comment 5).

I also did 4.18 run with intel_idle.max_cstate=1 and 'performance' frequency
governor. Results are:

Amean      1       25.07 (   0.00%)       25.04 (   0.13%)       25.06 (  
0.07%)       25.03 (   0.19%)       25.03 (   0.18%)
Amean      2       28.16 (   0.00%)       27.20 (   3.42%)       27.21 (  
3.38%)       27.47 (   2.46%)       27.75 (   1.45%)
Amean      4       29.58 (   0.00%)       29.50 (   0.27%)       30.03 ( 
-1.50%)       30.57 (  -3.33%)       29.49 (   0.31%)
Stddev     1        0.59 (   0.00%)        0.70 ( -19.15%)        0.55 (  
6.57%)        0.56 (   5.15%)        0.54 (   9.03%)
Stddev     2        1.41 (   0.00%)        1.49 (  -5.43%)        1.46 ( 
-3.51%)        1.10 (  22.32%)        1.24 (  12.20%)
Stddev     4        2.62 (   0.00%)        2.71 (  -3.30%)        2.75 ( 
-4.91%)        2.78 (  -6.16%)        3.14 ( -19.70%)

So in this case the variation is significantly reduced. I'll see if I can
identify which patches make Leap 15.0 kernel similarly stable.