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

[Vincent Guittot]

On Mon, 28 Jun 2021 at 13:54, Ionela Voinescu <ionela.voinescu@xxxxxxx> wrote:
>
> 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.
>

With the counter being 32bits and the freq scaling being update at
tick, you can easily get a overflow on it in idle system. I can easily
imagine that when you unplug CPUs there is enough activity on the CPU
to update it regularly whereas with all CPUs, the idle time is longer
that the counter overflow

> 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.