[PATCH V3 2/2] cpufreq: cppc: Add support for frequency invariance

[Ionela Voinescu]

Hey,

Some test results:

On Thursday 18 Feb 2021 at 16:35:38 (+0000), Ionela Voinescu wrote:
[..]
> > +static void __init cppc_freq_invariance_init(void)
> > +{
[..]
> > +
> > +		ret = cppc_get_perf_ctrs(i, &fb_ctrs);
> > +		if (!ret)
> > +			per_cpu(cppc_fi->prev_perf_fb_ctrs, i) = fb_ctrs;
> 

After fixing this one:
			cppc_fi->prev_perf_fb_ctrs = fb_ctrs;

I got the following:

Platform:

 - Juno R2 (CPUs [0-3] are littles, CPUs [4-5] are bigs)
    + PMU counters, used by CPPC through FFH
    + userspace/schedutil


  - Verifying that with userspace governor we see a correct change in
    scale factor:

	root at buildroot:~# dmesg | grep FIE
	[    6.436770] AMU: CPUs[0-3]: AMU counters WON'T be used for FIE.
	[    6.436962] AMU: CPUs[4-5]: AMU counters WON'T be used for FIE.
	[    6.451510] CPPC:CPUs[0-5]: CPPC counters will be used for FIE.

	root at buildroot:~# echo 600000 > policy4/scaling_setspeed
	[  353.939495] CPU4: Invariance(cppc) scale: 512.
	[  353.939497] CPU5: Invariance(cppc) scale: 512.

	root at buildroot:~# echo 1200000 > policy4/scaling_setspeed
	[  372.683511] CPU5: Invariance(cppc) scale: 1024.
	[  372.683518] CPU4: Invariance(cppc) scale: 1024.

	root at buildroot:~# echo 450000 > policy0/scaling_setspeed
	[  641.495513] CPU2: Invariance(cppc) scale: 485.
	[  641.495514] CPU1: Invariance(cppc) scale: 485.
	[  641.495517] CPU0: Invariance(cppc) scale: 485.
	[  641.495542] CPU3: Invariance(cppc) scale: 485.

	root at buildroot:~# echo 950000 > policy0/scaling_setspeed
	[  852.015514] CPU2: Invariance(cppc) scale: 1024.
	[  852.015514] CPU1: Invariance(cppc) scale: 1024.
	[  852.015517] CPU0: Invariance(cppc) scale: 1024.
	[  852.015541] CPU3: Invariance(cppc) scale: 1024.

 - I ran some benchmarks as well (perf, hackbench, dhrystone) on the same
   platform, using the userspace governor at fixed frequency, to evaluate
   the impact of the work we do or don't do on the tick.

   ./perf bench sched pipe
   (10 iterations, higher is better, ops/s, comparisons with
   cpufreq-based FIE)

   cpufreq-based FIE    AMU-based FIE    CPPC-based FIE
   ----------------------------------------------------
   39498.8		40984.7		 38893.4
   std: 3.766%		std: 4.461%	 std: 0.575%
   			diff: 3.625%	 diff: -1.556%

   ./hackbench -l 1000
   (10 iterations, lower is better, seconds, comparison with
   cpufreq-based FIE)

   cpufreq-based FIE    AMU-based FIE    CPPC-based FIE
   ----------------------------------------------------
   6.4207		6.3386		 6.7841
   std: 7.298%		std: 2.252%	 std: 2.460%
   			diff: -1.295%	 diff: 5.356%

   This shows a small regression for the CPPC-based FIE, but within the
   standard deviation.

   I ran some dhrystone benchmarks (./dhrystone -t 2/34/5/6/ -l 5000) as
   well with schedutil governor to understand if an increase in accuracy
   with the AMU/CPPC counters makes a difference. Given the
   characteristics of the platform it's no surprise that the results
   were very similar between the three cases, so I won't bore you with
   the numbers.

Hope it helps,
Ionela.