Collaborative Processor Performance Control (CPPC) — The Linux Kernel 6.14.4-gnu-4-vanilla documentation

# Collaborative Processor Performance Control (CPPC)

## CPPC

CPPC defined in the ACPI spec describes a mechanism for the OS to manage the performance of a logical processor on a contiguous and abstract performance scale. CPPC exposes a set of registers to describe abstract performance scale, to request performance levels and to measure per-cpu delivered performance.

For more details on CPPC please refer to the ACPI specification at:

http://uefi.org/specifications

Some of the CPPC registers are exposed via sysfs under:

    /sys/devices/system/cpu/cpuX/acpi_cppc/

for each cpu X:

    $ ls -lR  /sys/devices/system/cpu/cpu0/acpi_cppc/
    /sys/devices/system/cpu/cpu0/acpi_cppc/:
    total 0
    -r--r--r-- 1 root root 65536 Mar  5 19:38 feedback_ctrs
    -r--r--r-- 1 root root 65536 Mar  5 19:38 highest_perf
    -r--r--r-- 1 root root 65536 Mar  5 19:38 lowest_freq
    -r--r--r-- 1 root root 65536 Mar  5 19:38 lowest_nonlinear_perf
    -r--r--r-- 1 root root 65536 Mar  5 19:38 lowest_perf
    -r--r--r-- 1 root root 65536 Mar  5 19:38 nominal_freq
    -r--r--r-- 1 root root 65536 Mar  5 19:38 nominal_perf
    -r--r--r-- 1 root root 65536 Mar  5 19:38 reference_perf
    -r--r--r-- 1 root root 65536 Mar  5 19:38 wraparound_time

- highest_perf : Highest performance of this processor (abstract scale).

- nominal_perf : Highest sustained performance of this processor (abstract scale).

- lowest_nonlinear_perf : Lowest performance of this processor with nonlinear power savings (abstract scale).

- lowest_perf : Lowest performance of this processor (abstract scale).

- lowest_freq : CPU frequency corresponding to lowest_perf (in MHz).

- nominal_freq : CPU frequency corresponding to nominal_perf (in MHz). The above frequencies should only be used to report processor performance in frequency instead of abstract scale. These values should not be used for any functional decisions.

- feedback_ctrs : Includes both Reference and delivered performance counter. Reference counter ticks up proportional to processor’s reference performance. Delivered counter ticks up proportional to processor’s delivered performance.

- wraparound_time: Minimum time for the feedback counters to wraparound (seconds).

- reference_perf : Performance level at which reference performance counter accumulates (abstract scale).

## Computing Average Delivered Performance

Below describes the steps to compute the average performance delivered by taking two different snapshots of feedback counters at time T1 and T2.

> T1: Read feedback_ctrs as fbc_t1

>   Wait or run some workload

> T2: Read feedback_ctrs as fbc_t2

    delivered_counter_delta = fbc_t2[del] - fbc_t1[del]
    reference_counter_delta = fbc_t2[ref] - fbc_t1[ref]

    delivered_perf = (reference_perf x delivered_counter_delta) / reference_counter_delta