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

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Date:   Tue, 29 Jun 2021 10:50:28 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Will Deacon <will@...nel.org>, linux-pm@...r.kernel.org,
        Qian Cai <quic_qiancai@...cinc.com>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH V3 0/4] cpufreq: cppc: Add support for frequency
 invariance

On 28-06-21, 12:54, Ionela Voinescu wrote:
> 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:

Interesting.

> root@...get:/sys/devices/system/cpu/cpufreq/policy0# echo 1056000 > scaling_setspeed
> root@...get:/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@...get:/sys/devices/system/cpu/cpufreq/policy0# echo 1936000 > scaling_setspeed
> root@...get:/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@...get:/sys/devices/system/cpu/cpufreq/policy0#
> root@...get:/sys/devices/system/cpu/cpufreq/policy0# echo 2200000 > scaling_setspeed
> root@...get:/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.

I think we also saw numbers like this, which didn't explain a lot on
ThunderX2. We thought they may be due to rounding issues, but the
offlining stuff adds an interesting factor to that.

-- 
viresh