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

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Message-ID: <aDV2HPfybqnbzJ9N@JPC00244420>
Date: Tue, 27 May 2025 17:21:48 +0900
From: Shashank Balaji <shashank.mahadasyam@...y.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Russell Haley <yumpusamongus@...il.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

Hi Rafael,

On Fri, May 23, 2025 at 09:06:04PM +0200, Rafael J. Wysocki wrote:
> On Fri, May 23, 2025 at 6:25 AM Shashank Balaji
> <shashank.mahadasyam@...y.com> wrote:
> > ...
> > Consider the following on a Raptor Lake machine:
> > ...
> >
> > 3. Same as above, except with strictuserspace governor, which is a
> > custom kernel module which is exactly the same as the userspace
> > governor, except it has the CPUFREQ_GOV_STRICT_TARGET flag set:
> >
> >         # echo strictuserspace > cpufreq/policy0/scaling_governor
> >         # x86_energy_perf_policy -c 0 2>&1 | grep REQ
> >         cpu0: HWP_REQ: min 26 max 26 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
> >         pkg0: HWP_REQ_PKG: min 1 max 255 des 0 epp 128 window 0x0 (0*10^0us)
> >         # echo 3000000 > cpufreq/policy0/scaling_setspeed
> >         # x86_energy_perf_policy -c 0 2>&1 | grep REQ
> >         cpu0: HWP_REQ: min 39 max 39 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
> >         pkg0: HWP_REQ_PKG: min 1 max 255 des 0 epp 128 window 0x0 (0*10^0us)
> >
> >         With the strict flag set, intel_pstate honours this by setting
> >         the min and max freq same.
> >
> > desired_perf is always 0 in the above cases. The strict flag check is done in
> > intel_cpufreq_update_pstate, which sets max_pstate to target_pstate if policy
> > has strict target, and cpu->max_perf_ratio otherwise.
> >
> > As Russell and Rafael have noted, CPU frequency is subject to hardware
> > coordination and optimizations. While I get that, shouldn't software try
> > its best with whatever interface it has available? If a user sets the
> > userspace governor, that's because they want to have manual control over
> > CPU frequency, for whatever reason. The kernel should honor this by
> > setting the min and max freq in HWP_REQUEST equal. The current behaviour
> > explicitly lets the hardware choose higher frequencies.
> 
> Well, the userspace governor ends up calling the same function,
> intel_cpufreq_target(), as other cpufreq governors except for
> schedutil.  This function needs to work for all of them and for some
> of them setting HWP_MIN_PERF to the same value as HWP_MAX_PERF would
> be too strict.  HWP_DESIRED_PERF can be set to the same value as
> HWP_MIN_PERF, though (please see the attached patch).
> 
> > Since Russell pointed out that the "actual freq >= target freq" can be
> > achieved by leaving intel_pstate active and setting scaling_{min,max}_freq
> > instead (for some reason this slipped my mind), I now think the strict target
> > flag should be added to the userspace governor, leaving the documentation as
> > is. Maybe a warning like "you may want to set this exact frequency, but it's
> > subject to hardware coordination, so beware" can be added.
> 
> If you expect the userspace governor to set the frequency exactly
> (module HW coordination), that's the only way to make it do so without
> potentially affecting the other governors.

I don't mean to say that intel_cpufreq_target() should be modified. I'm
suggesting that the CPUFREQ_GOV_STRICT_TARGET flag be added to the
userspace governor. That'll ensure that HWP_MIN_PERF and
HWP_MAX_PERF are set to the target frequency. intel_cpufreq_target()
already correctly deals with the strict target flag. To test this, I
registered a custom governor, same as the userspace governor, except
with the strict target flag set. Please see case 3 above.

If this flag is added to the userspace governor, then whatever the
documentation says right now will actually be true. No need to modify
the documentation then.

Regards,
Shashank