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

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Message-ID: <CAJZ5v0jrwDJ-TQuay_OkwfsWr2z9COg=oDY7q0FqRBC-3_br2Q@mail.gmail.com>
Date: Thu, 22 May 2025 13:54:36 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Russell Haley <yumpusamongus@...il.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, 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 Thu, May 22, 2025 at 1:15 PM Russell Haley <yumpusamongus@...il.com> wrote:
>
>
>
> 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

OK, let me double check the code.