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SubjectRe: [PATCH v1 2/2] cpufreq: ACPI: Update arch scale-invariance max perf ratio if CPPC is not there
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On Thu, 2021-02-04 at 18:34 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> If the maximum performance level taken for computing the
> arch_max_freq_ratio value used in the x86 scale-invariance code is
> higher than the one corresponding to the cpuinfo.max_freq value
> coming from the acpi_cpufreq driver, the scale-invariant utilization
> falls below 100% even if the CPU runs at cpuinfo.max_freq or slightly
> faster, which causes the schedutil governor to select a frequency
> below cpuinfo.max_freq. That frequency corresponds to a frequency
> table entry below the maximum performance level necessary to get to
> the "boost" range of CPU frequencies which prevents "boost"
> frequencies from being used in some workloads.
>
> While this issue is related to scale-invariance, it may be amplified
> by commit db865272d9c4 ("cpufreq: Avoid configuring old governors as
> default with intel_pstate") from the 5.10 development cycle which
> made it extremely easy to default to schedutil even if the preferred
> driver is acpi_cpufreq as long as intel_pstate is built too, because
> the mere presence of the latter effectively removes the ondemand
> governor from the defaults. Distro kernels are likely to include
> both intel_pstate and acpi_cpufreq on x86, so their users who cannot
> use intel_pstate or choose to use acpi_cpufreq may easily be
> affectecd by this issue.
>
> If CPPC is available, it can be used to address this issue by
> extending the frequency tables created by acpi_cpufreq to cover the
> entire available frequency range (including "boost" frequencies) for
> each CPU, but if CPPC is not there, acpi_cpufreq has no idea what
> the maximum "boost" frequency is and the frequency tables created by
> it cannot be extended in a meaningful way, so in that case make it
> ask the arch scale-invariance code to to use the "nominal" performance
> level for CPU utilization scaling in order to avoid the issue at hand.
>
> Fixes: db865272d9c4 ("cpufreq: Avoid configuring old governors as default with intel_pstate")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> arch/x86/kernel/smpboot.c | 1 +
> drivers/cpufreq/acpi-cpufreq.c | 8 ++++++++
> 2 files changed, 9 insertions(+)
>
> Index: linux-pm/drivers/cpufreq/acpi-cpufreq.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/acpi-cpufreq.c
> +++ linux-pm/drivers/cpufreq/acpi-cpufreq.c
> @@ -806,6 +806,14 @@ static int acpi_cpufreq_cpu_init(struct
> state_count++;
> valid_states++;
> data->first_perf_state = valid_states;
> + } else {
> + /*
> + * If the maximum "boost" frequency is unknown, ask the arch
> + * scale-invariance code to use the "nominal" performance for
> + * CPU utilization scaling so as to prevent the schedutil
> + * governor from selecting inadequate CPU frequencies.
> + */
> + arch_set_max_freq_ratio(true);
> }
>
> freq_table = kcalloc(state_count, sizeof(*freq_table), GFP_KERNEL);
> Index: linux-pm/arch/x86/kernel/smpboot.c
> ===================================================================
> --- linux-pm.orig/arch/x86/kernel/smpboot.c
> +++ linux-pm/arch/x86/kernel/smpboot.c
> @@ -1833,6 +1833,7 @@ void arch_set_max_freq_ratio(bool turbo_
> arch_max_freq_ratio = turbo_disabled ? SCHED_CAPACITY_SCALE :
> arch_turbo_freq_ratio;
> }
> +EXPORT_SYMBOL_GPL(arch_set_max_freq_ratio);
>
> static bool turbo_disabled(void)
> {

Reviewed-by: Giovanni Gherdovich <ggherdovich@suse.cz>

Thanks,
Giovanni

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