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

[Ionela Voinescu]

On Monday 28 Jun 2021 at 14:14:14 (+0200), Vincent Guittot wrote:
> 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
> 

Thanks! Yes, given the high wraparound time I thought they were 64 bit.
All variables in software are 64 bit, but looking at bit width in the
_CPC entries, the platform counters are 32 bit counters.

> There are 32bits and the overflow need to be handled by cppc_cpufreq
> driver

I'm wondering if this would be best handled in the function that reads
the counters or in the cppc_cpufreq driver that uses them. Probably the
latter, as you say, as the read function should only return the raw
values, but it does complicate things.

Thanks,
Ionela.



> > 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.