Re: [RFC/RFT][PATCH] cpufreq: intel_pstate: Work in passive mode with HWP enabled

[Francisco Jerez <francisco.jerez.plata@intel.com>]

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"Rafael J. Wysocki" <rafael@kernel.org> writes:

>  to On Mon, May 25, 2020 at 10:57 PM Francisco Jerez
> <francisco.jerez.plata@intel.com> wrote:
>>
>> "Rafael J. Wysocki" <rafael@kernel.org> writes:
>>
>> > On Mon, May 25, 2020 at 3:39 AM Francisco Jerez
>> > <francisco.jerez.plata@intel.com> wrote:
>> >>
>> >> "Rafael J. Wysocki" <rjw@rjwysocki.net> writes:
>> >>
>> >> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> >> >
>> >> > Allow intel_pstate to work in the passive mode with HWP enabled and
>> >> > make it translate the target frequency supplied by the cpufreq
>> >> > governor in use into an EPP value to be written to the HWP request
>> >> > MSR (high frequencies are mapped to low EPP values that mean more
>> >> > performance-oriented HWP operation) as a hint for the HWP algorithm
>> >> > in the processor, so as to prevent it and the CPU scheduler from
>> >> > working against each other at least when the schedutil governor is
>> >> > in use.
>> >> >
>> >> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> >> > ---
>> >> >
>> >> > This is a prototype not intended for production use (based on linux-next).
>> >> >
>> >> > Please test it if you can (on HWP systems, of course) and let me know the
>> >> > results.
>> >> >
>> >> > The INTEL_CPUFREQ_TRANSITION_DELAY_HWP value has been guessed and it very well
>> >> > may turn out to be either too high or too low for the general use, which is one
>> >> > reason why getting as much testing coverage as possible is key here.
>> >> >
>> >> > If you can play with different INTEL_CPUFREQ_TRANSITION_DELAY_HWP values,
>> >> > please do so and let me know the conclusions.
>> >> >
>> >> > Cheers,
>> >> > Rafael
>> >> >
>> >> > ---
>> >> >  drivers/cpufreq/intel_pstate.c |  169 +++++++++++++++++++++++++++++++----------
>> >> >  1 file changed, 131 insertions(+), 38 deletions(-)
>> >> >
>> >> > Index: linux-pm/drivers/cpufreq/intel_pstate.c
>> >> > ===================================================================
>> >> > --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
>> >> > +++ linux-pm/drivers/cpufreq/intel_pstate.c
>> >> > @@ -36,6 +36,7 @@
>> >> >  #define INTEL_PSTATE_SAMPLING_INTERVAL       (10 * NSEC_PER_MSEC)
>> >> >
>> >> >  #define INTEL_CPUFREQ_TRANSITION_LATENCY     20000
>> >> > +#define INTEL_CPUFREQ_TRANSITION_DELAY_HWP   5000
>> >> >  #define INTEL_CPUFREQ_TRANSITION_DELAY               500
>> >> >
>> >> >  #ifdef CONFIG_ACPI
>> >> > @@ -95,6 +96,8 @@ static inline int32_t percent_ext_fp(int
>> >> >       return div_ext_fp(percent, 100);
>> >> >  }
>> >> >
>> >> > +#define HWP_EPP_TO_BYTE(x)   (((u64)x >> 24) & 0xFF)
>> >> > +
>> >> >  /**
>> >> >   * struct sample -   Store performance sample
>> >> >   * @core_avg_perf:   Ratio of APERF/MPERF which is the actual average
>> >> > @@ -2175,7 +2178,10 @@ static int intel_pstate_verify_policy(st
>> >> >
>> >> >  static void intel_cpufreq_stop_cpu(struct cpufreq_policy *policy)
>> >> >  {
>> >> > -     intel_pstate_set_min_pstate(all_cpu_data[policy->cpu]);
>> >> > +     if (hwp_active)
>> >> > +             intel_pstate_hwp_force_min_perf(policy->cpu);
>> >> > +     else
>> >> > +             intel_pstate_set_min_pstate(all_cpu_data[policy->cpu]);
>> >> >  }
>> >> >
>> >> >  static void intel_pstate_stop_cpu(struct cpufreq_policy *policy)
>> >> > @@ -2183,12 +2189,10 @@ static void intel_pstate_stop_cpu(struct
>> >> >       pr_debug("CPU %d exiting\n", policy->cpu);
>> >> >
>> >> >       intel_pstate_clear_update_util_hook(policy->cpu);
>> >> > -     if (hwp_active) {
>> >> > +     if (hwp_active)
>> >> >               intel_pstate_hwp_save_state(policy);
>> >> > -             intel_pstate_hwp_force_min_perf(policy->cpu);
>> >> > -     } else {
>> >> > -             intel_cpufreq_stop_cpu(policy);
>> >> > -     }
>> >> > +
>> >> > +     intel_cpufreq_stop_cpu(policy);
>> >> >  }
>> >> >
>> >> >  static int intel_pstate_cpu_exit(struct cpufreq_policy *policy)
>> >> > @@ -2296,7 +2300,8 @@ static int intel_cpufreq_verify_policy(s
>> >> >  #define      INTEL_PSTATE_TRACE_TARGET 10
>> >> >  #define      INTEL_PSTATE_TRACE_FAST_SWITCH 90
>> >> >
>> >> > -static void intel_cpufreq_trace(struct cpudata *cpu, unsigned int trace_type, int old_pstate)
>> >> > +static void intel_cpufreq_trace(struct cpudata *cpu, unsigned int trace_type,
>> >> > +                             int from, int to)
>> >> >  {
>> >> >       struct sample *sample;
>> >> >
>> >> > @@ -2309,8 +2314,8 @@ static void intel_cpufreq_trace(struct c
>> >> >       sample = &cpu->sample;
>> >> >       trace_pstate_sample(trace_type,
>> >> >               0,
>> >> > -             old_pstate,
>> >> > -             cpu->pstate.current_pstate,
>> >> > +             from,
>> >> > +             to,
>> >> >               sample->mperf,
>> >> >               sample->aperf,
>> >> >               sample->tsc,
>> >> > @@ -2318,40 +2323,110 @@ static void intel_cpufreq_trace(struct c
>> >> >               fp_toint(cpu->iowait_boost * 100));
>> >> >  }
>> >> >
>> >> > -static int intel_cpufreq_target(struct cpufreq_policy *policy,
>> >> > -                             unsigned int target_freq,
>> >> > -                             unsigned int relation)
>> >> > +static void intel_cpufreq_update_hwp_request(struct cpudata *cpu, u8 new_epp)
>> >> >  {
>> >> > -     struct cpudata *cpu = all_cpu_data[policy->cpu];
>> >> > -     struct cpufreq_freqs freqs;
>> >> > -     int target_pstate, old_pstate;
>> >> > +     u64 value, prev;
>> >> >
>> >> > -     update_turbo_state();
>> >> > +     prev = READ_ONCE(cpu->hwp_req_cached);
>> >> > +     value = prev;
>> >> >
>> >> > -     freqs.old = policy->cur;
>> >> > -     freqs.new = target_freq;
>> >> > +     /*
>> >> > +      * The entire MSR needs to be updated in order to update the EPP field
>> >> > +      * in it, so opportunistically update the min and max too if needed.
>> >> > +      */
>> >> > +     value &= ~HWP_MIN_PERF(~0L);
>> >> > +     value |= HWP_MIN_PERF(cpu->min_perf_ratio);
>> >> > +
>> >> > +     value &= ~HWP_MAX_PERF(~0L);
>> >> > +     value |= HWP_MAX_PERF(cpu->max_perf_ratio);
>> >> > +
>> >> > +     if (boot_cpu_has(X86_FEATURE_HWP_EPP)) {
>> >> > +             intel_cpufreq_trace(cpu, INTEL_PSTATE_TRACE_TARGET,
>> >> > +                                 HWP_EPP_TO_BYTE(prev), new_epp);
>> >> > +
>> >> > +             value &= ~GENMASK_ULL(31, 24);
>> >> > +             value |= HWP_ENERGY_PERF_PREFERENCE(new_epp);
>> >> > +     }
>> >> > +
>> >> > +     if (value != prev) {
>> >> > +             WRITE_ONCE(cpu->hwp_req_cached, value);
>> >> > +             wrmsrl_on_cpu(cpu->cpu, MSR_HWP_REQUEST, value);
>> >> > +     }
>> >> > +}
>> >> > +
>> >> > +/**
>> >> > + * intel_cpufreq_adjust_hwp_request - Adjust the HWP reuqest register.
>> >> > + * @cpu: Target CPU.
>> >> > + * @max_freq: Maximum frequency to consider.
>> >> > + * @target_freq: Target frequency selected by the governor.
>> >> > + *
>> >> > + * Translate the target frequency into a new EPP value to be written into the
>> >> > + * HWP request MSR of @cpu as a hint for the HW-driven P-state selection.
>> >> > + *
>> >> > + * The purpose of this is to avoid situations in which the kernel and the HWP
>> >> > + * algorithm work against each other by giving a hint about the expectations of
>> >> > + * the former to the latter.
>> >> > + *
>> >> > + * The mapping betweeen the target frequencies and the hint values need not be
>> >> > + * exact, but it must be monotonic, so that higher target frequencies always
>> >> > + * indicate more performance-oriented P-state selection.
>> >> > + */
>> >> > +static void intel_cpufreq_adjust_hwp_request(struct cpudata *cpu, s64 max_freq,
>> >> > +                                          unsigned int target_freq)
>> >> > +{
>> >> > +     s64 epp_fp = div_fp(255 * (max_freq - target_freq), max_freq);
>> >> > +
>> >> > +     intel_cpufreq_update_hwp_request(cpu, fp_toint(epp_fp));
>> >> > +}
>> >> > +
>> >>
>> >> Hey Rafael, I'm building a kernel with this in order to give it a try on
>> >> my system, but I'm skeptical that translating the target frequency to an
>> >> EPP value will work reliably.  AFAIA the EPP value only has an indirect
>> >> influence on the processor's performance by adjusting the trade-off
>> >> between its responsiveness (rather than the actual clock frequency which
>> >> it will sustain in the long run) and its energy usage, in a largely
>> >> unspecified and non-linear way (non-linear like the effect of switching
>> >> CPU energy optimization features on and off, or like its effect on the
>> >> energy balancing behavior of the processor which can have a paradoxical
>> >> effect on performance).
>> >>
>> >> I doubt that the scheduling-based CPU utilization is a good predictor
>> >> for the optimal trade-off between those two variables.
>> >
>> > While I agree that this is not perfect, there barely is anything else
>> > that can be used for this purpose.
>> >
>> > Using the desired field or trying to adjust the limits relatively
>> > often would basically cause the P-state selection to be driven
>> > entirely by the kernel which simply doesn't know certain things only
>> > known to the P-unit, so it is reasonable to leave some level of
>> > control to the latter, so as to allow it to use the information known
>> > to it only.
>> >
>> > However, if it is allowed to do whatever it likes without any hints
>> > from the kernel, it may very well go against the scheduler's decisions
>> > which is not going to be optimal.
>> >
>> > I'm simply not sure if there is any other way to give such hints to it
>> > that through the EPP.
>> >
>>
>> Why not HWP_MIN_PERF?  That would leave the HWP quite some room for
>> maneuvering (the whole HWP_MIN_PERF-HWP_MAX_PERF P-state range, it's not
>> like P-state selection would be entirely driven by the kernel), while
>> avoiding every failure scenario I was describing in my previous reply.
>
> Actually, I have been thinking about the HWP min as an alternative
> that may be worth evaluating.
>
> However, I would rather set the HWP min to something like 80% if the
> cpufreq request.
>
> Let me cut an alternative patch.

Heh...  Good.  While you're at it, it would be nice to make sure we have
a way for the governor to control both the minimum and maximum P-state
in some efficient way, rather than it simply providing a single target,
with [1] in mind.

[1] https://lwn.net/Articles/818827/

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