8.1. PSCI Performance Measurement

TF-A provides two instrumentation tools for performing analysis of the PSCI implementation:

PSCI STAT
Runtime Instrumentation

This page explains how they may be enabled and used to perform all varieties of analysis.

8.1.1. Performance Measurement Framework

The Performance Measurement Framework PMF is a framework that provides mechanisms for collecting and retrieving timestamps at runtime from the Performance Measurement Unit (PMU). The PMU is a generalized abstraction for accessing CPU hardware registers used to measure hardware events. This means, for instance, that the PMU might be used to place instrumentation points at logical locations in code for tracing purposes.

TF-A utilises the PMF as a backend for the two instrumentation services it provides–PSCI Statistics and Runtime Instrumentation. The PMF is used by these services to facilitate collection and retrieval of timestamps. For instance, the PSCI Statistics service registers the PMF service psci_svc to track its residency statistics.

This is reserved a unique ID, name, and space in memory by the PMF. The framework provides a convenient interface for PSCI Statistics to retrieve values from psci_svc at runtime. Alternatively, the service may be configured such that the PMF dumps those values to the console. A platform may choose to expose SMCs that allow retrieval of these timestamps from the service.

This feature is enabled with the Boolean flag ENABLE_PMF.

8.1.2. PSCI Statistics

PSCI Statistics is a runtime service that provides residency statistics for power states used by the platform. The service tracks residency time and entry count. Residency time is the total time spent in a particular power state by a PE. The entry count is the number of times the PE has entered the power state. PSCI Statistics implements the optional functions PSCI_STAT_RESIDENCY and PSCI_STAT_COUNT from the PSCI specification.

PSCI_STAT_RESIDENCY

Parameters:

target_cpu – Contains copy of affinity fields in the MPIDR register for identifying the target core (See section 5.1.4 of PSCI specifications for more details).
power_state – identifier for a specific local state. Generally, this parameter takes the same form as the power_state parameter described for CPU_SUSPEND in section 5.4.2.

Returns:

Time spent in power_state, in microseconds, by target_cpu and the highest level expressed in power_state.

PSCI_STAT_COUNT

Parameters:

target_cpu – follows the same format as PSCI_STAT_RESIDENCY.
power_state – follows the same format as PSCI_STAT_RESIDENCY.

Returns:

Number of times the state expressed in power_state has been used by target_cpu and the highest level expressed in power_state.

The implementation provides residency statistics only for low power states, and does this regardless of the entry mechanism into those states. The statistics it collects are set to 0 during shutdown or reset.

PSCI Statistics is enabled with the Boolean build flag ENABLE_PSCI_STAT. All Arm platforms utilise the PMF unless another collection backend is provided (ENABLE_PMF is implicitly enabled).

8.1.3. Runtime Instrumentation

The Runtime Instrumentation Service is an instrumentation tool that wraps around the PMF to provide timestamp data. Although the service is not restricted to PSCI, it is used primarily in TF-A to quantify the total time spent in the PSCI implementation. The tool can be used to instrument other components in TF-A as well. It is enabled with the Boolean flag ENABLE_RUNTIME_INSTRUMENTATION, and as with PSCI STAT, requires PMF to be enabled.

In PSCI, this service provides instrumentation points in the following code paths:

Entry into the PSCI SMC handler
Exit from the PSCI SMC handler
Entry to low power state
Exit from low power state
Entry into cache maintenance operations in PSCI
Exit from cache maintenance operations in PSCI

The service captures the cycle count, which allows for the time spent in the implementation to be calculated, given the frequency counter.

8.1.3.1. PSCI SMC Handler Instrumentation

The timestamp during entry into the handler is captured as early as possible during the runtime exception, prior to entry into the handler itself. All timestamps are stored in memory for later retrieval. The exit timestamp is captured after normal return from the PSCI SMC handler, or, if a low power state was requested, it is captured in the warm boot path.