Analyzing CPU Load with cpuload

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In embedded systems development, analyzing CPU load is a critical step for identifying performance bottlenecks, optimizing task scheduling, and managing power consumption. This document details how to configure and use the cpuload feature in the openvela OS and how to perform in-depth performance analysis using advanced tools.

I. cpuload Configuration Methods

openvela provides three different modes for CPU load statistics. Developers can choose the most suitable option based on precision requirements and available hardware resources.

Method 1: System Clock-Based Sampling (Default)

This mode utilizes the system tick timer interrupt to sample the currently running task at each clock tick, thereby estimating CPU usage.

• Principle: Accumulates the execution time of the active task within the system clock's interrupt service routine.

• Pros and Cons:

  • Pros: Simplest to configure and has no dependency on extra hardware timers.
  • Cons: Statistical precision is limited by the system clock frequency and may fail to accurately capture short-running tasks.

• Configuration Option:

  CONFIG_SCHED_CPULOAD_SYSCLK=y
  

Method 2: External High-Precision Timer-Based Sampling (Recommended)

This mode uses a separate hardware timer (External Timer) to sample tasks at a higher frequency, providing more accurate CPU load data than the system clock method.

• Principle: Configures a dedicated hardware timer to trigger interrupts at a frequency higher than the system clock, sampling the active task within the interrupt service routine.

• Pros and Cons:

  • Pros: Higher statistical precision, providing a more accurate reflection of a task's instantaneous CPU usage.
  • Cons: Requires an additional hardware timer and corresponding driver adaptation in the Board Support Package (BSP).

• Configuration Option:

  CONFIG_SCHED_CPULOAD_EXTCLK=y
  

Method 3: High-Precision Calculation Based on Actual Task Execution Time (Recommended)

This mode, the most accurate of the three, uses the SCHED_CRITMONITOR module to precisely record the start and stop timestamps of each task to calculate its exact cumulative execution time.

• Principle: Leverages the Performance Monitor to log the precise moments of context switches, calculating CPU usage by accumulating the actual execution duration of each task.

• Pros and Cons:

  • Pros: Highest statistical precision, independent of sampling frequency, and truly reflects the CPU consumption of each task.
  • Cons: Introduces slight performance overhead due to the extra time-stamping required during context switches.

• Configuration Options:

  Note: Before using this mode, you must ensure that the Board Support Package (BSP) has correctly implemented the performance counter and that it has been initialized by calling the up_perf_init() function.

  CONFIG_SCHED_CRITMONITOR=y
  CONFIG_SCHED_CPULOAD_CRITMONITOR=y
  

II. Viewing and Accessing CPU Load Data

Once any cpuload configuration is enabled, you can retrieve CPU load information in several ways.

1. Using the ps Command

在 shell 终端中执行 ps 命令，可以直接查看到每个线程 (thread) 的 CPU 占用率（CPU 列）。

Executing the ps command in the shell terminal directly displays the CPU usage (CPU column) for each thread.

If you only want to view information for specific threads, you can pass one or more thread IDs (PIDs) to the ps command.

# Example: View information for threads with PIDs 14 and 23
ps 14 23

2. Accessing Through Programming Interfaces

Userspace

Applications can obtain CPU load data by reading virtual files in the /proc filesystem.

• To get the total system load: /proc/cpuload
• To get the load of a specific thread: /proc/${pid}/cpuload

Kernel Space

In kernel-space code, you can directly call the following API function to get CPU load information for a specific thread.

#include <nuttx/clock.h>

int clock_cpuload(int pid, FAR struct cpuload_s *cpuload)

III. Analysis with Advanced Tools

For scenarios requiring more detailed and visual performance analysis, the ps command may not be sufficient. In such cases, professional system analysis tools can be used.

1. Using SEGGER SystemView

SystemView is a powerful visual trace and diagnostics tool. Through a J-Link debugger, it can capture and display detailed openvela kernel scheduling events in real time, including context switches, interrupts, and API calls.

Compared to the ps command, SystemView provides higher time resolution and richer contextual information, enabling you to:

• Precisely measure the execution time of each thread's individual time slice.
• Visually observe interactions and preemption relationships between tasks.
• Analyze the overall system load within specific time frames.