Re: [PATCH v4] Force cppc_cpufreq to report values in KHz to fix user space reporting

Alexey Klimov
2016-07-14T10:03:20Z

Hi Al,

On Tue, Jul 12, 2016 at 11:16:11AM -0600, Al Stone wrote:
> When CPPC is being used by ACPI on arm64, user space tools such as
> cpupower report CPU frequency values from sysfs that are incorrect.
>
> What the driver was doing was reporting the values given by ACPI tables
> in whatever scale was used to provide them. However, the ACPI spec
> defines the CPPC values as unitless abstract numbers. Internal kernel
> structures such as struct perf_cap, in contrast, expect these values
> to be in KHz. When these struct values get reported via sysfs, the
> user space tools also assume they are in KHz, causing them to report
> incorrect values (for example, reporting a CPU frequency of 1MHz when
> it should be 1.8GHz).
>
> While the investigation for a long term fix proceeds (several options
> are being explored, some of which may require spec changes or other
> much more invasive fixes), this patch forces the values read by CPPC
> to be read in KHz, regardless of what they actually represent.
>
> The downside is that this approach has some assumptions:
>
> (1) It relies on SMBIOS3 being used, *and* that the Max Frequency
> value for a processor is set to a non-zero value.
>
> (2) It assumes that all processors run at the same speed, or that
> the CPPC values have all been scaled to reflect relative speed.
> This patch retrieves the largest CPU Max Frequency from a type 4 DMI
> record that it can find. This may not be an issue, however, as a
> sampling of DMI data on x86 and arm64 indicates there is often only
> one such record regardless. Since CPPC is relatively new, it is
> unclear if the ACPI ASL will always be written to reflect any sort
> of relative performance of processors of differing speeds.
>
> (3) It assumes that performance and frequency both scale linearly.
>
> For arm64 servers, this may be sufficient, but it does rely on
> firmware values being set correctly. Hence, other approaches are
> also being considered.
>
> This has been tested on three arm64 servers, with and without DMI, with
> and without CPPC support.
>
> Changes for v4:
> -- Replaced magic constants with #defines (Rafael Wysocki)
> -- Renamed cppc_unitless_to_khz() to cppc_to_khz() (Rafael Wysocki)
> -- Replaced hidden initialization with a clearer form (Rafael Wysocki)
> -- Instead of picking up the first Max Speed value from DMI, we will
> now get the largest Max Speed; still an approximation, but slightly
> less subject to error (Rafael Wysocki)
> -- Kconfig for cppc_cpufreq now depends on DMI, instead of selecting
> it, in order to make sure DMI is set up properly (Rafael Wysocki)
>
> Changes for v3:
> -- Added clarifying commentary re short-term vs long-term fix (Alexey
> Klimov)
> -- Added range checking code to ensure proper arithmetic occurs,
> especially no division by zero (Alexey Klimov)
>
> Changes for v2:
> -- Corrected thinko: needed to have DEPENDS on DMI in Kconfig.arm,
> not SELECT DMI (found by build daemon)
>
> Signed-off-by: Al Stone <ahs3@redhat.com>
> ---
> drivers/acpi/cppc_acpi.c | 106 +++++++++++++++++++++++++++++++++++++++++---
> drivers/cpufreq/Kconfig.arm | 2 +-
> 2 files changed, 102 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 8adac69..6e6df9c 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -40,8 +40,18 @@
> #include <linux/cpufreq.h>
> #include <linux/delay.h>
> #include <linux/ktime.h>
> +#include <linux/dmi.h>
> +
> +#include <asm/unaligned.h>
>
> #include <acpi/cppc_acpi.h>
> +
> +/* Minimum struct length needed for the DMI processor entry we want */
> +#define DMI_ENTRY_PROCESSOR_MIN_LENGTH 48
> +
> +/* Offest in the DMI processor structure for the max frequency */
> +#define DMI_PROCESSOR_MAX_SPEED 0x14
> +
> /*
> * Lock to provide mutually exclusive access to the PCC
> * channel. e.g. When the remote updates the shared region
> @@ -709,6 +719,56 @@ static int cpc_write(struct cpc_reg *reg, u64 val)
> return ret_val;
> }
>
> +static u64 cppc_dmi_khz;
> +
> +static void cppc_find_dmi_mhz(const struct dmi_header *dm, void *private)
> +{
> + const u8 *dmi_data = (const u8 *)dm;
> + u16 *mhz = (u16 *)private;
> +
> + if (dm->type == DMI_ENTRY_PROCESSOR &&
> + dm->length >= DMI_ENTRY_PROCESSOR_MIN_LENGTH) {
> + u16 val = (u16)get_unaligned((const u16 *)
> + (dmi_data + DMI_PROCESSOR_MAX_SPEED));
> + *mhz = val > *mhz ? val : *mhz;
> + }
> +}
> +
> +
> +static u64 cppc_get_dmi_khz(void)
> +{
> + u16 mhz = 0;
> +
> + dmi_walk(cppc_find_dmi_mhz, &mhz);
> +
> + /*
> + * Real stupid fallback value, just in case there is no
> + * actual value set.
> + */
> + mhz = mhz ? mhz : 1;
> +
> + return (1000 * mhz);
> +}
> +
> +static u64 cppc_to_khz(u64 min_in, u64 max_in, u64 val)
> +{
> + /*
> + * The incoming val should be min <= val <= max. Our
> + * job is to convert that to KHz so it can be properly
> + * reported to user space via cpufreq_policy.
> + */
> + u64 curval = val;
> + u64 maxf = max_in;
> + u64 minf = min_in;
> +
> + /* range check the input values */
> + curval = curval < minf ? minf : curval;
> + curval = curval > maxf ? maxf : curval;
> + minf = minf >= maxf ? maxf - 1 : minf;

In the pedantic world kernel should warn in dmesg about nominal value that is
out of range. Or min being larger than max.
Not really an issue but for debugging purposes..

> + return ((curval - minf) * cppc_dmi_khz) / (maxf - minf);
> +}
> +
> /**
> * cppc_get_perf_caps - Get a CPUs performance capabilities.
> * @cpunum: CPU from which to get capabilities info.
> @@ -748,17 +808,53 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
> }
> }
>
> - cpc_read(&highest_reg->cpc_entry.reg, &high);
> - perf_caps->highest_perf = high;
> + /*
> + * Since these values in perf_caps will be used in setting
> + * up the cpufreq policy, they must always be stored in units
> + * of KHz. If they are not, user space tools will become very
> + * confused since they assume these are in KHz when reading
> + * sysfs.
> + *
> + * NB: there may be better approaches to this problem that, as
> + * of this writing, are still being explored. Ideally, this is
> + * a short term solution since correlating CPPC abstract values
> + * with CPU frequency may or may not reflect actual performance.
> + *
> + * The reason longer term solutions are being explored is because
> + * this solution requires we make the following assumptions:
> + *
> + * (1) It relies on SMBIOS3 being used, *and* that the Max
> + * Frequency value for a processor is set to a non-zero value.
> + *
> + * (2) It assumes that all processors run at the same speed, or
> + * that the CPPC values have all been scaled to reflect any
> + * relative differences. This code retrieves the largest CPU
> + * Max Frequency from a type 4 DMI record that it can find.
> + * This may not be an issue, however, as a sampling of DMI
> + * data on x86 and arm64 indicates there is often only one
> + * such record regardless.
> + *
> + * (3) It assumes that performance and frequency both scale
> + * linearly.
> + *
> + * None of these are particularly horrible assumptions. But, they
> + * are assumptions and ultimately we'd like to be able to report
> + * performance without quite so many of them.
> + *
> + */
> + cppc_dmi_khz = cppc_get_dmi_khz();
>
> + cpc_read(&highest_reg->cpc_entry.reg, &high);
> cpc_read(&lowest_reg->cpc_entry.reg, &low);
> - perf_caps->lowest_perf = low;
> +
> + perf_caps->highest_perf = cppc_to_khz(low, high, high);
> + perf_caps->lowest_perf = cppc_to_khz(low, high, low);

Just to check. Do I understand correctly that cpufreq subsystem is populated
with this converted values (policy->min and max), then cpufreq sends request to
set new target_freq in converted units to CPPC that in its turn is not aware
about convertation or do i miss something?
There should be convertation back to abstract scale for cppc to correctly
understand and handle request to set new desired performance, shouldn't it?

>
> cpc_read(&ref_perf->cpc_entry.reg, &ref);
> - perf_caps->reference_perf = ref;
> + perf_caps->reference_perf = cppc_to_khz(low, high, ref);
>
> cpc_read(&nom_perf->cpc_entry.reg, &nom);
> - perf_caps->nominal_perf = nom;
> + perf_caps->nominal_perf = cppc_to_khz(low, high, nom);
>
> if (!ref)
> perf_caps->reference_perf = perf_caps->nominal_perf;
> diff --git a/drivers/cpufreq/Kconfig.arm b/drivers/cpufreq/Kconfig.arm
> index 14b1f93..b4aae52 100644
> --- a/drivers/cpufreq/Kconfig.arm
> +++ b/drivers/cpufreq/Kconfig.arm
> @@ -253,7 +253,7 @@ config ARM_PXA2xx_CPUFREQ
>
> config ACPI_CPPC_CPUFREQ
> tristate "CPUFreq driver based on the ACPI CPPC spec"
> - depends on ACPI
> + depends on ACPI && DMI
> select ACPI_CPPC_LIB
> default n
> help
> --

Best regards,
Alexey