Re: [PATCH V3 0/4] cpufreq: cppc: Add support for frequency invariance

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi guys,

On Monday 21 Jun 2021 at 14:49:33 (+0530), Viresh Kumar wrote:
> Hello,
> 
> Changes since V2:
> 
> - We don't need start_cpu() and stop_cpu() callbacks anymore, we can make it
>   work using policy ->init() and exit() alone.
> 
> - Two new cleanup patches 1/4 and 2/4.
> 
> - Improved commit log of 3/4.
> 
> - Dropped WARN_ON(local_freq_scale > 1024), since this can occur on counter's
>   overlap (seen with Vincent's setup).
> 

If you happen to have the data around, I would like to know more about
your observations on ThunderX2.


I tried ThunderX2 as well, with the following observations:

Booting with userspace governor and all CPUs online, the CPPC frequency
scale factor was all over the place (even much larger than 1024).

My initial assumptions:
 - Counters do not behave properly in light of SMT
 - Firmware does not do a good job to keep the reference and core
   counters monotonic: save and restore at core off.

So I offlined all CPUs with the exception of 0, 32, 64, 96 - threads of
a single core (part of policy0). With this all works very well:

root@target:/sys/devices/system/cpu/cpufreq/policy0# echo 1056000 > scaling_setspeed
root@target:/sys/devices/system/cpu/cpufreq/policy0#
[ 1863.095370] CPU96: cppc scale: 697.
[ 1863.175370] CPU0: cppc scale: 492.
[ 1863.215367] CPU64: cppc scale: 492.
[ 1863.235366] CPU96: cppc scale: 492.
[ 1863.485368] CPU32: cppc scale: 492.

root@target:/sys/devices/system/cpu/cpufreq/policy0# echo 1936000 > scaling_setspeed
root@target:/sys/devices/system/cpu/cpufreq/policy0#
[ 1891.395363] CPU96: cppc scale: 558.
[ 1891.415362] CPU0: cppc scale: 595.
[ 1891.435362] CPU32: cppc scale: 615.
[ 1891.465363] CPU96: cppc scale: 635.
[ 1891.495361] CPU0: cppc scale: 673.
[ 1891.515360] CPU32: cppc scale: 703.
[ 1891.545360] CPU96: cppc scale: 738.
[ 1891.575360] CPU0: cppc scale: 779.
[ 1891.605360] CPU96: cppc scale: 829.
[ 1891.635360] CPU0: cppc scale: 879.

root@target:/sys/devices/system/cpu/cpufreq/policy0#
root@target:/sys/devices/system/cpu/cpufreq/policy0# echo 2200000 > scaling_setspeed
root@target:/sys/devices/system/cpu/cpufreq/policy0#
[ 1896.585363] CPU32: cppc scale: 1004.
[ 1896.675359] CPU64: cppc scale: 973.
[ 1896.715359] CPU0: cppc scale: 1024.

I'm doing a rate limited printk only for increase/decrease values over
64 in the scale factor value.

This showed me that SMT is handled properly.

Then, as soon as I start onlining CPUs 1, 33, 65, 97, the scale factor
stops being even close to correct, for example:

[238394.770328] CPU96: cppc scale: 22328.
[238395.628846] CPU96: cppc scale: 245.
[238516.087115] CPU96: cppc scale: 930.
[238523.385009] CPU96: cppc scale: 245.
[238538.767473] CPU96: cppc scale: 936.
[238538.867546] CPU96: cppc scale: 245.
[238599.367932] CPU97: cppc scale: 2728.
[238599.859865] CPU97: cppc scale: 452.
[238647.786284] CPU96: cppc scale: 1438.
[238669.604684] CPU96: cppc scale: 27306.
[238676.805049] CPU96: cppc scale: 245.
[238737.642902] CPU97: cppc scale: 2035.
[238737.664995] CPU97: cppc scale: 452.
[238788.066193] CPU96: cppc scale: 2749.
[238788.110192] CPU96: cppc scale: 245.
[238817.231659] CPU96: cppc scale: 2698.
[238818.083687] CPU96: cppc scale: 245.
[238845.466850] CPU97: cppc scale: 2990.
[238847.477805] CPU97: cppc scale: 452.
[238936.984107] CPU97: cppc scale: 1590.
[238937.029079] CPU97: cppc scale: 452.
[238979.052464] CPU97: cppc scale: 911.
[238980.900668] CPU97: cppc scale: 452.
[239149.587889] CPU96: cppc scale: 803.
[239151.085516] CPU96: cppc scale: 245.
[239303.871373] CPU64: cppc scale: 956.
[239303.906837] CPU64: cppc scale: 245.
[239308.666786] CPU96: cppc scale: 821.
[239319.440634] CPU96: cppc scale: 245.
[239389.978395] CPU97: cppc scale: 4229.
[239391.969562] CPU97: cppc scale: 452.
[239415.894738] CPU96: cppc scale: 630.
[239417.875326] CPU96: cppc scale: 245.

The counter values shown by feedback_ctrs do not seem monotonic even
when only core 0 threads are online.

ref:2812420736 del:166051103
ref:3683620736 del:641578595
ref:1049653440 del:1548202980
ref:2099053440 del:2120997459
ref:3185853440 del:2714205997
ref:712486144  del:3708490753
ref:3658438336 del:3401357212
ref:1570998080 del:2279728438

For now I was just wondering if you have seen the same and whether you
have an opinion on this.

> This is tested on my Hikey platform (without the actual read/write to
> performance counters), with this script for over an hour:
> 
> while true; do
>     for i in `seq 1 7`;
>     do
>         echo 0 > /sys/devices/system/cpu/cpu$i/online;
>     done;
> 
>     for i in `seq 1 7`;
>     do
>         echo 1 > /sys/devices/system/cpu/cpu$i/online;
>     done;
> done
> 
> 
> The same is done by Vincent on ThunderX2 and no issues were seen.

Hotplug worked fine for me as well on both platforms I tested (Juno R2
and ThunderX2).

Thanks,
Ionela.