ACPI Low Power Idle (LPI)

Important

This feature might not be applicable to all Platforms. Please check individual Platform pages, section Supported Features to confirm if this feature is listed as supported.

Overview of LPI test

ACPI Low Power Idle (LPI) mechanism allows an operating system to manage the power states of the processor power domain hierarchy. Neoverse Reference Design platforms support the c-states c0 (run state), c1 (WFI) and c3 (WFI with core powered down).

This document describes the procedure to validate LPI functionality, determining the number of times a particular CPU core switched to idle state and the total time the core has been in a idle state.

Download and build the required platform software

For downloading and building the platform firmware, refer Buildroot boot. To enable LPI from ACPI, update the LPI_EN variable from SgiPlatform.dsc.inc before build. Also remember to enable stress-ng binary from the buildroot config.

Procedure for validating LPI states

  1. Boot the platform to buildroot command line prompt.

  2. Run the command ‘nproc’ to get the cpu count in the system.

  3. Read the idle state descriptor entry to know about the c-state information.

    cat /sys/devices/system/cpu/cpu<x>/cpuidle/state<j>/desc

Here, x = 0, 1, 2, ... (nproc -1)
      y = 0, 1, 2, ...

    generally for RD platform:

    state0: c1 (LPI1) state for CPUx

    state1: c3 (LPI3) state for CPUx

    state2: available only for plaforms having power control for CPU container and is the combined c3 (LPI3 for core and LPI2 for cluster) state for CPU and cluster.

  4. To get the LPI statistics, read the ‘usage’ and ‘time’ entries:

    cat /sys/devices/system/cpu/cpu<x>/cpuidle/state<y>/usage
cat /sys/devices/system/cpu/cpu<x>/cpuidle/state<y>/time

  5. Wake up all CPUs from sleep. The example shown below uses the ‘stress-ng’ utility. Run stress-ng utility for one second for all CPUs using the command

    stress-ng -c <num_cpu> -t 1

Here num_cpu is the value obtained on step 2

  6. Repeat step 4 and compare the usage and time values.

In a system with idle states enabled, the expectation is the ‘usage’ count should increment on each suspend-resume cycle. The value for ‘time’ specifies the total time period the core was in that particular state.

Note

In a system that supports state2, the usage count will increment for either state1 or for state2. This is applicable when a core is the last one to undergo sleep inside a container, then the core will request for a combined sleep state instead of core only power down.