Re: [PATCH v9 2/5] cpufreq: Introduce an optional cpuinfo_avg_freq sysfs entry

Sumit Gupta
2025-01-29T11:30:00Z

On 21/01/25 14:14, Beata Michalska wrote:
>
>
> Currently the CPUFreq core exposes two sysfs attributes that can be used
> to query current frequency of a given CPU(s): namely cpuinfo_cur_freq
> and scaling_cur_freq. Both provide slightly different view on the
> subject and they do come with their own drawbacks.
>
> cpuinfo_cur_freq provides higher precision though at a cost of being
> rather expensive. Moreover, the information retrieved via this attribute
> is somewhat short lived as frequency can change at any point of time
> making it difficult to reason from.
>
> scaling_cur_freq, on the other hand, tends to be less accurate but then
> the actual level of precision (and source of information) varies between
> architectures making it a bit ambiguous.
>
> The new attribute, cpuinfo_avg_freq, is intended to provide more stable,
> distinct interface, exposing an average frequency of a given CPU(s), as
> reported by the hardware, over a time frame spanning no more than a few
> milliseconds. As it requires appropriate hardware support, this
> interface is optional.
>
> Note that under the hood, the new attribute relies on the information
> provided by arch_freq_get_on_cpu, which, up to this point, has been
> feeding data for scaling_cur_freq attribute, being the source of
> ambiguity when it comes to interpretation. This has been amended by
> restoring the intended behavior for scaling_cur_freq, with a new
> dedicated config option to maintain status quo for those, who may need
> it.
>
> CC: Jonathan Corbet <corbet@lwn.net>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Borislav Petkov <bp@alien8.de>
> CC: Dave Hansen <dave.hansen@linux.intel.com>
> CC: H. Peter Anvin <hpa@zytor.com>
> CC: Phil Auld <pauld@redhat.com>
> CC: x86@kernel.org
> CC: linux-doc@vger.kernel.org
>
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> ---
> Documentation/admin-guide/pm/cpufreq.rst | 16 ++++++++++++-
> drivers/cpufreq/Kconfig.x86 | 12 ++++++++++
> drivers/cpufreq/cpufreq.c | 30 +++++++++++++++++++++++-
> 3 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst
> index a21369eba034..e9969174026c 100644
> --- a/Documentation/admin-guide/pm/cpufreq.rst
> +++ b/Documentation/admin-guide/pm/cpufreq.rst
> @@ -248,6 +248,19 @@ are the following:
> If that frequency cannot be determined, this attribute should not
> be present.
>
> +``cpuinfo_avg_freq``
> + An average frequency (in KHz) of all CPUs belonging to a given policy,
> + derived from a hardware provided feedback and reported on a time frame
> + spanning at most few milliseconds.
> +
> + This is expected to be based on the frequency the hardware actually runs
> + at and, as such, might require specialised hardware support (such as AMU
> + extension on ARM). If one cannot be determined, this attribute should
> + not be present.
> +
> + Note, that failed attempt to retrieve current frequency for a given
> + CPU(s) will result in an appropriate error.
> +

Minor nit:
Should we also add: Idle CPU's on ARM will return EAGAIN (Resource
temporarily unavailable) error?

> ``cpuinfo_max_freq``
> Maximum possible operating frequency the CPUs belonging to this policy
> can run at (in kHz).
> @@ -293,7 +306,8 @@ are the following:
> Some architectures (e.g. ``x86``) may attempt to provide information
> more precisely reflecting the current CPU frequency through this
> attribute, but that still may not be the exact current CPU frequency as
> - seen by the hardware at the moment.
> + seen by the hardware at the moment. This behavior though, is only
> + available via c:macro:``CPUFREQ_ARCH_CUR_FREQ`` option.
>
> ``scaling_driver``
> The scaling driver currently in use.
> diff --git a/drivers/cpufreq/Kconfig.x86 b/drivers/cpufreq/Kconfig.x86
> index 97c2d4f15d76..212e1b9afe21 100644
> --- a/drivers/cpufreq/Kconfig.x86
> +++ b/drivers/cpufreq/Kconfig.x86
> @@ -340,3 +340,15 @@ config X86_SPEEDSTEP_RELAXED_CAP_CHECK
> option lets the probing code bypass some of those checks if the
> parameter "relaxed_check=1" is passed to the module.
>
> +config CPUFREQ_ARCH_CUR_FREQ
> + default y
> + bool "Current frequency derived from HW provided feedback"
> + help
> + This determines whether the scaling_cur_freq sysfs attribute returns
> + the last requested frequency or a more precise value based on hardware
> + provided feedback (as architected counters).
> + Given that a more precise frequency can now be provided via the
> + cpuinfo_avg_cur_freq attribute, by enabling this option,

s/cpuinfo_avg_cur_freq/cpuinfo_cur_freq/?

Overall looks good to me.

Reviewed-by: Sumit Gupta <sumitg@nvidia.com>