Re: [PATCH v1] cpufreq: CPPC: Fix performance/frequency conversion

["Rafael J. Wysocki" <rafael@kernel.org>]

On Mon, Nov 15, 2021 at 11:19 AM Pierre Gondois <pierre.gondois@arm.com> wrote:
>
> Hello Rafael,
>
> On 11/5/21 15:40, Rafael J. Wysocki wrote:
> > On Fri, Oct 22, 2021 at 9:51 AM Pierre Gondois <Pierre.Gondois@arm.com> wrote:
> >> CPUfreq governors request CPU frequencies using information
> >> on current CPU usage. The CPPC driver converts them to
> >> performance requests. Frequency targets are computed as:
> >>   target_freq = (util / cpu_capacity) * max_freq
> >> target_freq is then clamped between [policy->min, policy->max].
> >>
> >> The CPPC driver converts performance values to frequencies
> >> (and vice-versa) using cppc_cpufreq_perf_to_khz() and
> >> cppc_cpufreq_khz_to_perf(). These functions both use two different
> >> factors depending on the range of the input value. For
> >> cppc_cpufreq_khz_to_perf():
> >> - (NOMINAL_PERF / NOMINAL_FREQ) or
> >> - (LOWEST_PERF / LOWEST_FREQ)
> >> and for cppc_cpufreq_perf_to_khz():
> >> - (NOMINAL_FREQ / NOMINAL_PERF) or
> >> - ((NOMINAL_PERF - LOWEST_FREQ) / (NOMINAL_PERF - LOWEST_PERF))
> >>
> >> This means the functions are not inverse for some values:
> >> (perf_to_khz(khz_to_perf(x)) != x)
> >>
> >> This patch makes use of one single conversion factor, being
> >> (MAX_PERF / MAX_FREQ).
> >>
> >> As LOWEST_FREQ is not used during conversion, the LOWEST_FREQ
> >> advertised through policy->cpuinfo.min_freq might be different
> >> from the LOWEST_FREQ value available in the CPPC object,
> >> but the conversion will be correct.
> > Well, this assumes that there is a linear perf <-> freq mapping which
> > is a change in behavior.
> The perf <-> freq relation is currently linear on 2 distinct segments.
>
> The patch is also intending handle the case of CPUs whose frequency and
> performance values are not proportional.
>
> Example for the current code:
>   MAX_FREQ = 1.000.000
>   MIN_FREQ = 300.000
>   MAX_PERF = 100 ("nominal_perf" in the code)
>   MIN_PERF = 40  ("lowest_perf" in the code)
> With these values, frequencies and performances are not proportional as
> (MAX_FREQ / MAX_PERF) != (MIN_FREQ / MIN_PERF).
>
> A util of 40% gives:
>   target_freq = 40% * MAX_FREQ.
> cppc_cpufreq_khz_to_perf() then requests:
>   target_perf = target_freq * (MIN_PERF / MIN_FREQ)
>   target_perf = 40% * 1.000.000 * (40 / 300.000)
>   target_perf = 53.3
> A performance request of 40 would have been sufficient.
>
> The error comes from the utilization of 2 different factors.
> policy->max (frequency) is computed with this factor:
> - (MAX_FREQ / MAX_PERF)
> and frequency requests are mostly converted to performance
> values with this factor:
> - (MIN_PERF / MIN_FREQ)
>
> Using one single factor per conversion function, being
> (MAX_PERF / MAX_FREQ) for cppc_cpufreq_khz_to_perf():
>   target_perf = target_freq * (MAX_PERF / MAX_FREQ)
>   target_perf = 40% * 1.000.000 * (100 / 1.000.000)
>   target_perf = 40
> > While I agree that consistency is a good thing in general, won't this
> > cause the values visible via sysfs to change?  And if it does, won't
> > it confuse anyone or break anything in user space?
> Yes it changes the minimum frequency advertised on sysfs. It might
> effectively be confusing. It should be possible to still advertise the
> minimum frequency in the ACPI _CPC object while using one conversion
> factor, but this will require more changes.

So why don't we make them?