linux-kernel - Re: [PATCH v2] cpufreq: schedutil: Move max CPU capacity to sugov

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Message-ID: <b2d905b2-5664-442b-bc12-fd759ac9fa9f@gmail.com>
Date:   Mon, 3 Oct 2022 03:41:34 -0500
From:   Russell Haley <yumpusamongus@...il.com>
To:     lukasz.luba@....com
Cc:     dietmar.eggemann@....com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, rafael@...nel.org,
        vincent.guittot@...aro.org, viresh.kumar@...aro.org
Subject: Re: [PATCH v2] cpufreq: schedutil: Move max CPU capacity to
 sugov_policy

>We can do that since all CPUs in the same frequency domain have the
>same max capacity

Do they? In the Intel Alder Lake datasheet [1], it says that a single
power rail supplies all IA ("Intel Architecture") cores, which includes
both P cores and E cores.

I don't have anything that new, but on Haswell and Skylake, despite the
fact that each CPU has a separate policy that lists only itself in
affected_cpus, with the userspace governor I find that every core runs
at the the highest frequency set among all cores. For clarity:

$ grep . cpufreq/policy*/scaling_{setspeed,cur_freq}
cpufreq/policy0/scaling_setspeed:3000000
cpufreq/policy1/scaling_setspeed:2000000
cpufreq/policy0/scaling_cur_freq:2999997
cpufreq/policy1/scaling_cur_freq:3000001

It seems that these cores are in the same frequency domain, even if
cpufreq doesn't know about it. I don't know if this affects the behavior
of the governors in any way, but it might be a bug in intel_pstate that
could one day be fixed. If it is, then any heterogeneous-uarch chips
with both CPU types sharing a voltage rail would have CPUs with
different max capacity in the same frequency domain.

This might present a problem for any future attempt to harmonize
treatment of big.LITTLE between ARM and x86.

[1]:
https://edc.intel.com/content/www/us/en/design/ipla/software-development-platforms/client/platforms/alder-lake-desktop/12th-generation-intel-core-processors-datasheet-volume-1-of-2/processor-power-rails_1/