linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Use the latest guaranteed freq during verify

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Date:   Thu, 17 Dec 2020 06:23:44 -0800
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Use the latest guaranteed freq
 during verify

On Thu, 2020-12-17 at 06:19 -0800, Srinivas Pandruvada wrote:
> On Thu, 2020-12-17 at 14:58 +0100, Rafael J. Wysocki wrote:
> > On Thu, Dec 17, 2020 at 11:44 AM Srinivas Pandruvada
> > <srinivas.pandruvada@...ux.intel.com> wrote:
> > > 
> > > This change tries to address an issue, when BIOS disabled turbo
> > > but HWP_CAP guaranteed is changed later and user space wants to
> > > take
> > > advantage of this increased guaranteed performance.
> > > 
> > > The HWP_CAP.GUARANTEED value is not a static value. It can be
> > > changed
> > > by some out of band agent or during Intel Speed Select
> > > performance
> > > level change. The HWP_CAP.MAX still shows max possible
> > > performance
> > > when
> > > BIOS disabled turbo. So guaranteed can still change as long as
> > > this
> > > is
> > > same or below HWP_CAP.MAX.
> > > 
> > > When guaranteed is changed, the sysfs base_frequency attributes
> > > shows
> > > the latest guaranteed frequency. This attribute can be used by
> > > user
> > > space software to update scaling min/max frequency.
> > > 
> > > Currently the setpolicy callback already uses the latest HWP_CAP
> > > values when setting HWP_REQ. But the verify callback will still
> > > restrict
> > > the user settings to the to old guaranteed value. So if the
> > > guaranteed
> > > is increased, user space can't take advantage of it.
> > > 
> > > To solve this similar to setpolicy callback, read the latest
> > > HWP_CAP
> > > values and use it to restrict the maximum setting. This is done
> > > by
> > > calling intel_pstate_get_hwp_max(), which already accounts for
> > > user
> > > and BIOS turbo disable to get the current max performance.
> > > 
> > > This issue is side effect of fixing the issue of scaling
> > > frequency
> > > limits by the
> > >  'commit eacc9c5a927e ("cpufreq: intel_pstate:
> > >  Fix intel_pstate_get_hwp_max() for turbo disabled")'
> > > The fix resulted in correct setting of reduced scaling
> > > frequencies,
> > > but this resulted in capping HWP.REQ to HWP_CAP.GUARANTEED in
> > > this
> > > case.
> > > 
> > > Cc: 5.8+ <stable@...r.kernel.org> # 5.8+
> > > Signed-off-by: Srinivas Pandruvada <
> > > srinivas.pandruvada@...ux.intel.com>
> > > ---
> > >  drivers/cpufreq/intel_pstate.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/drivers/cpufreq/intel_pstate.c
> > > b/drivers/cpufreq/intel_pstate.c
> > > index 2a4db856222f..7081d1edb22b 100644
> > > --- a/drivers/cpufreq/intel_pstate.c
> > > +++ b/drivers/cpufreq/intel_pstate.c
> > > @@ -2199,6 +2199,12 @@ static void
> > > intel_pstate_clear_update_util_hook(unsigned int cpu)
> > > 
> > >  static int intel_pstate_get_max_freq(struct cpudata *cpu)
> > >  {
> > > +       if (hwp_active) {
> > > +               int turbo_max, max_state;
> > > +
> > > +               intel_pstate_get_hwp_max(cpu->cpu, &turbo_max,
> > > &max_state);
> > 
> > This would cause intel_pstate_get_hwp_max() to be called twice in
> > intel_pstate_update_perf_limits() which is not perfect.
> 
> We can optimize by using cached value.
> 
> 
> diff --git a/drivers/cpufreq/intel_pstate.c
> b/drivers/cpufreq/intel_pstate.c
> index 7081d1edb22b..d345c9ef240c 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -2223,7 +2223,11 @@ static void
> intel_pstate_update_perf_limits(struct cpudata *cpu,
>          * rather than pure ratios.
>          */
>         if (hwp_active) {
> -               intel_pstate_get_hwp_max(cpu->cpu, &turbo_max,
> &max_state);
> +               if (global.no_turbo || global.turbo_disabled)
> +                       max_state = HWP_GUARANTEED_PERF(cpu-
> > hwp_cap_cached);
> +               else
> +                       max_state = HWP_HIGHEST_PERF(cpu-
> > hwp_cap_cached);
Can use  ternary operator instead of if..else. to further simplify.

> +               turbo_max = HWP_HIGHEST_PERF(cpu->hwp_cached);
>         } else {
>                 max_state = global.no_turbo || global.turbo_disabled
> ?
>                         cpu->pstate.max_pstate : cpu-
> > pstate.turbo_pstate;
> 
> 
> Thanks,
> Srinivas
> 
> 
> > 
> > > +               return max_state * cpu->pstate.scaling;
> > > +       }
> > >         return global.turbo_disabled || global.no_turbo ?
> > >                         cpu->pstate.max_freq : cpu-
> > > > pstate.turbo_freq;
> > >  }
> > > --
>