linux-kernel - Re: [PATCH 2/2][RFC v2] ACPI: Update cpuinfo.max after bootup if necessary

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Date:   Thu, 28 Feb 2019 23:56:48 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <len.brown@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yu Chen <yu.chen.surf@...il.com>
Subject: Re: [PATCH 2/2][RFC v2] ACPI: Update cpuinfo.max after bootup if necessary

On Thu, Feb 28, 2019 at 6:59 PM Chen Yu <yu.c.chen@...el.com> wrote:
>
> On Dell Inc. XPS13 9333, the BIOS changes the value of
> MSR_IA32_MISC_ENABLE_TURBO_DISABLE at runtime (e.g., when
> the power source changes), the maximum frequency of the
> CPU is not updated accordingly. This is due to the policy's
> cpuinfo.max is not updated when _PPC notifier fires.
>
> Fix this problem by updating the policy's cpuinfo.max when
> necessary.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=200759
> Reported-and-tested-by: Gabriele Mazzotta <gabriele.mzt@...il.com>
> Originally-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>  drivers/cpufreq/cpufreq.c      |  2 ++
>  drivers/cpufreq/intel_pstate.c | 15 +++++++++++++--
>  2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index e35a886e00bc..95e08816b512 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2237,6 +2237,8 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>
>         policy->min = new_policy->min;
>         policy->max = new_policy->max;
> +       policy->cpuinfo.max_freq = new_policy->cpuinfo.max_freq;
> +       policy->cpuinfo.min_freq = new_policy->cpuinfo.min_freq;
>         trace_cpu_frequency_limits(policy);
>
>         policy->cached_target_freq = UINT_MAX;
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index dd66decf2087..841c74f69f66 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -2081,11 +2081,17 @@ static void intel_pstate_adjust_policy_max(struct cpufreq_policy *policy,
>
>  static int intel_pstate_verify_policy(struct cpufreq_policy *policy)
>  {
> +       int max_freq;
>         struct cpudata *cpu = all_cpu_data[policy->cpu];
>
>         update_turbo_state();
> +       max_freq = intel_pstate_get_max_freq(cpu);
> +
> +       if (acpi_ppc && max_freq != policy->cpuinfo.max_freq)
> +               policy->cpuinfo.max_freq = policy->max = max_freq;

Updating cpuinfo.max_freq here only causes the current limit to be
reported via sysfs, because intel_pstate doesn't actually use
cpuinfo.max_freq for anything and the core doesn't enforce it as a
limit.

All of the computations in the active mode in the driver actually use
the current limit anyway AFAICS.

> +
>         cpufreq_verify_within_limits(policy, policy->cpuinfo.min_freq,
> -                                    intel_pstate_get_max_freq(cpu));
> +                                    max_freq);
>
>         if (policy->policy != CPUFREQ_POLICY_POWERSAVE &&
>             policy->policy != CPUFREQ_POLICY_PERFORMANCE)
> @@ -2192,11 +2198,16 @@ static struct cpufreq_driver intel_pstate = {
>
>  static int intel_cpufreq_verify_policy(struct cpufreq_policy *policy)
>  {
> +       int max_freq;
>         struct cpudata *cpu = all_cpu_data[policy->cpu];
>
>         update_turbo_state();
> +       max_freq = intel_pstate_get_max_freq(cpu);
> +
> +       if (acpi_ppc && max_freq != policy->cpuinfo.max_freq)
> +               policy->cpuinfo.max_freq = policy->max = max_freq;

In this case (passive mode) updating cpuinfo.max_freq will actually
cause governors to use the new value in computations, so the P-state
selection will work somewhat differently, but that isn't really
consistent with what acpi-cpufreq does and with setting no_turbo in
the intel_pstate sysfs to 1 without this patch.

With acpi-cpufreq cpuinfo.max_freq is always the max frequency in the
_PSS table regardless of what the _PSS limit is.  Also setting
no_turbo to 1 in intel_pstate without this patch doesn't cause
cpuinfo.max_freq to change and I don't really think that it should.

I would be tempted to always initialize cpuinfo.max_freq to the max
turbo frequency, but there is a concern about systems in which
MSR_IA32_MISC_ENABLE_TURBO_DISABLE is never set on the fly (just in
the BIOS setup as it should be) and where it doesn't make sense to
consider turbo frequencies at all.

>         cpufreq_verify_within_limits(policy, policy->cpuinfo.min_freq,
> -                                    intel_pstate_get_max_freq(cpu));
> +                                    max_freq);
>
>         intel_pstate_adjust_policy_max(policy, cpu);
>
> --

It looks like I need to think about this a bit more.