Trace hooks and FreeRTOS+Trace - Step-by-Step Execution

Concept Flow - Trace hooks and FreeRTOS+Trace

FreeRTOS Kernel Event

↓

Trace Hook Called?

No→Continue Normal Execution

Yes↓

Trace Hook Function Executes

↓

Event Data Captured

↓

Data Sent to FreeRTOS+Trace Viewer

↓

User Analyzes Trace in Viewer

↓

Optimize or Debug Based on Trace

When FreeRTOS runs, trace hooks capture kernel events, send data to FreeRTOS+Trace, and help users analyze system behavior.

Execution Sample

FreeRTOS

void vApplicationIdleHook(void) {
    // Runs inside idle task
}

void traceTASK_SWITCHED_IN(void) {
    // Record task switch event - called by kernel
}

This code shows the vApplicationIdleHook (runs inside idle task) and traceTASK_SWITCHED_IN (called by kernel on task switches, including to idle task).

Execution Table

Step	Event	Trace Hook Called?	Action	Trace Data Captured
1	Task switch occurs	Yes	traceTASK_SWITCHED_IN() runs	Task switch event recorded
2	Switch to idle task	Yes	traceTASK_SWITCHED_IN() runs	Idle task switch event recorded
3	Tick interrupt	No	Normal tick processing	No trace data
4	Task delay expires	Yes	traceTASK_DELAYED() runs	Task delay event recorded
5	System idle	No	No hook called	No trace data
6	End of trace session	N/A	Data sent to viewer	Trace data complete

💡 Trace hooks run only on specific kernel events; normal execution continues otherwise.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
Trace Data Buffer	Empty	Task switch event	Task switch + Idle task event	Added Task delay event	Complete trace data

Key Moments - 2 Insights

Why doesn't the trace hook run on every kernel event?

Trace hooks are designed to run only on specific important events to avoid slowing down the system, as shown in execution_table rows 3 and 5 where no hook is called.

How does FreeRTOS+Trace get the data from trace hooks?

Trace hooks collect event data into a buffer during execution (see variable_tracker), which is then sent to the FreeRTOS+Trace viewer for analysis.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the idle task event recorded?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Trace Data Captured' column for the idle task event.

According to variable_tracker, what is the state of the Trace Data Buffer after step 4?

A Added Task delay event

B Task switch event only

C Task switch + Idle task event

D Empty

💡 Hint

Look at the 'After Step 4' column for Trace Data Buffer.

If trace hooks were called on every kernel event, what would change in the execution_table?

A Fewer rows overall

B No change in 'Trace Data Captured'

C More 'Yes' in 'Trace Hook Called?' column

D Trace hooks would never run

💡 Hint

Consider how often trace hooks run as shown in the 'Trace Hook Called?' column.

Concept Snapshot

Trace hooks run during key FreeRTOS kernel events.
They capture event data without slowing the system.
Data is buffered and sent to FreeRTOS+Trace viewer.
Users analyze trace to debug and optimize tasks.
Hooks run only on selected events, not all.
Example: traceTASK_SWITCHED_IN() records task switches.

Full Transcript

FreeRTOS uses trace hooks to capture important kernel events like task switches and delays. When such an event happens, the kernel calls a trace hook function that records the event data. This data is stored in a buffer and later sent to the FreeRTOS+Trace viewer software. The viewer helps users see what tasks ran and when, making debugging and optimization easier. Trace hooks do not run on every event to keep the system fast. For example, the traceTASK_SWITCHED_IN hook runs when a task switch occurs, as shown in the execution table. The variable tracker shows how the trace data buffer fills step-by-step. Understanding when hooks run and how data is collected helps users use FreeRTOS+Trace effectively.