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116 lines
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PSCI Performance Measurement
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============================
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TF-A provides two instrumentation tools for performing analysis of the PSCI
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implementation:
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* PSCI STAT
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* Runtime Instrumentation
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This page explains how they may be enabled and used to perform all varieties of
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analysis.
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Performance Measurement Framework
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---------------------------------
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The Performance Measurement Framework :ref:`PMF <firmware_design_pmf>`
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is a framework that provides mechanisms for collecting and retrieving timestamps
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at runtime from the Performance Measurement Unit
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(:ref:`PMU <Performance Monitoring Unit>`).
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The PMU is a generalized abstraction for accessing CPU hardware registers used to
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measure hardware events. This means, for instance, that the PMU might be used to
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place instrumentation points at logical locations in code for tracing purposes.
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TF-A utilises the PMF as a backend for the two instrumentation services it
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provides--PSCI Statistics and Runtime Instrumentation. The PMF is used by
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these services to facilitate collection and retrieval of timestamps. For
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instance, the PSCI Statistics service registers the PMF service
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``psci_svc`` to track its residency statistics.
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This is reserved a unique ID, name, and space in memory by the PMF. The
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framework provides a convenient interface for PSCI Statistics to retrieve
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values from ``psci_svc`` at runtime. Alternatively, the service may be
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configured such that the PMF dumps those values to the console. A platform may
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choose to expose SMCs that allow retrieval of these timestamps from the
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service.
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This feature is enabled with the Boolean flag ``ENABLE_PMF``.
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PSCI Statistics
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---------------
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PSCI Statistics is a runtime service that provides residency statistics for
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power states used by the platform. The service tracks residency time and
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entry count. Residency time is the total time spent in a particular power
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state by a PE. The entry count is the number of times the PE has entered
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the power state. PSCI Statistics implements the optional functions
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``PSCI_STAT_RESIDENCY`` and ``PSCI_STAT_COUNT`` from the `PSCI`_
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specification.
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.. c:macro:: PSCI_STAT_RESIDENCY
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:param target_cpu: Contains copy of affinity fields in the MPIDR register
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for identifying the target core (See section 5.1.4 of `PSCI`_
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specifications for more details).
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:param power_state: identifier for a specific local
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state. Generally, this parameter takes the same form as the power_state
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parameter described for CPU_SUSPEND in section 5.4.2.
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:returns: Time spent in ``power_state``, in microseconds, by ``target_cpu``
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and the highest level expressed in ``power_state``.
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.. c:macro:: PSCI_STAT_COUNT
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:param target_cpu: follows the same format as ``PSCI_STAT_RESIDENCY``.
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:param power_state: follows the same format as ``PSCI_STAT_RESIDENCY``.
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:returns: Number of times the state expressed in ``power_state`` has been
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used by ``target_cpu`` and the highest level expressed in
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``power_state``.
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The implementation provides residency statistics only for low power states,
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and does this regardless of the entry mechanism into those states. The
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statistics it collects are set to 0 during shutdown or reset.
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PSCI Statistics is enabled with the Boolean build flag
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``ENABLE_PSCI_STAT``. All Arm platforms utilise the PMF unless another
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collection backend is provided (``ENABLE_PMF`` is implicitly enabled).
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Runtime Instrumentation
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-----------------------
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The Runtime Instrumentation Service is an instrumentation tool that wraps
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around the PMF to provide timestamp data. Although the service is not
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restricted to PSCI, it is used primarily in TF-A to quantify the total time
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spent in the PSCI implementation. The tool can be used to instrument other
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components in TF-A as well. It is enabled with the Boolean flag
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``ENABLE_RUNTIME_INSTRUMENTATION``, and as with PSCI STAT, requires PMF to
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be enabled.
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In PSCI, this service provides instrumentation points in the
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following code paths:
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* Entry into the PSCI SMC handler
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* Exit from the PSCI SMC handler
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* Entry to low power state
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* Exit from low power state
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* Entry into cache maintenance operations in PSCI
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* Exit from cache maintenance operations in PSCI
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The service captures the cycle count, which allows for the time spent in the
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implementation to be calculated, given the frequency counter.
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PSCI SMC Handler Instrumentation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The timestamp during entry into the handler is captured as early as possible
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during the runtime exception, prior to entry into the handler itself. All
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timestamps are stored in memory for later retrieval. The exit timestamp is
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captured after normal return from the PSCI SMC handler, or, if a low power state
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was requested, it is captured in the warm boot path.
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*Copyright (c) 2023, Arm Limited. All rights reserved.*
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.. _PSCI: https://developer.arm.com/documentation/den0022/latest/
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