linux-kernel - Re: [PATCH] cpufreq, docs: (userspace governor) add that actual freq is >= scaling

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Message-ID: <b62c0462-8185-4eb8-8ac6-7f2abc387768@gmail.com>
Date: Thu, 22 May 2025 06:15:24 -0500
From: Russell Haley <yumpusamongus@...il.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Shashank Balaji <shashank.mahadasyam@...y.com>,
 Viresh Kumar <viresh.kumar@...aro.org>, Jonathan Corbet <corbet@....net>,
 linux-pm@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Shinya Takumi <shinya.takumi@...y.com>
Subject: Re: [PATCH] cpufreq, docs: (userspace governor) add that actual freq
 is >= scaling_setspeed



On 5/22/25 4:47 AM, Rafael J. Wysocki wrote:
> On Thu, May 22, 2025 at 10:51 AM Russell Haley <yumpusamongus@...il.com> wrote:
>>
>>
>> On 5/22/25 3:05 AM, Shashank Balaji wrote:
>>> The userspace governor does not have the CPUFREQ_GOV_STRICT_TARGET flag, which
>>> means the requested frequency may not strictly be followed. This is true in the
>>> case of the intel_pstate driver with HWP enabled. When programming the
>>> HWP_REQUEST MSR, the min_perf is set to `scaling_setspeed`, and the max_perf
>>> is set to the policy's max. So, the hardware is free to increase the frequency
>>> beyond the requested frequency.
>>>
>>> This behaviour can be slightly surprising, given the current wording "allows
>>> userspace to set the CPU frequency". Hence, document this.
>>>
>>
>> In my opinion, the documentation is correct, and it is the
>> implementation in intel_pstate that is wrong. If the user wanted two
>> separate knobs that control the minimum and maximum frequencies, they
>> could leave intel_pstate in "active" mode and change scaling_min_freq
>> and scaling_max_freq.
>>
>> If the user asks for the frequency to be set from userspace, the
>> frequency had damn well better be set from userspace.
> 
> The userspace governor requests a frequency between policy->min and
> policy->max on behalf of user space.  In intel_pstate this translates
> to setting DESIRED_PERF to the requested value which is also the case
> for the other governors.

Huh.  On this Skylake box with kernel 6.14.6, it seems to be setting
Minimum_Performance, and leaving desired at 0.

> echo userspace | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
userspace
> echo 1400000 | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_setspeed
1400000
> sudo x86_energy_perf_policy &| grep REQ
cpu0: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu1: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu2: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu3: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu4: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu5: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu6: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu7: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0

Cheers,
Russell