How to use the command 'trace-cmd' (with examples)

Trace-cmd is a powerful utility for interacting with Ftrace, which is the official tracer for the Linux kernel. It’s used primarily for debugging and analyzing system performance by tracing the kernel’s operations as it runs. Since it provides insight into how the kernel interacts with system processes, it requires root access to operate. Trace-cmd comes equipped with a variety of options that enable users to start, stop, and retrieve traces, making it an indispensable tool for kernel developers and system administrators who wish to ensure optimal performance and troubleshoot issues with precision.

Use case 1: Display the status of tracing system

Code:

trace-cmd stat

Motivation:

Knowing the current status of the tracing system is crucial. It provides insights into whether tracing is active, which buffers are being utilized, and the configuration of the tracing environment. This can help diagnose issues at a glance and ensure that the tracing system is ready for more complex operations.

Explanation:

The stat argument requests a summary of the current status. It queries the tracing system to ascertain if it is enabled and provides detailed information about the system’s configuration and activity status.

Example Output:

Trace is enabled
Found 4 cpus
Buffer size: 128KB

Use case 2: List available tracers

Code:

trace-cmd list -t

Motivation:

Before starting a trace, it’s beneficial to know which tracers are available for use. Different tracers serve different purposes, such as monitoring function calls, interrupt handlers, or latency. Knowing your options lets you choose the tracer most appropriate for your use case.

Explanation:

The list -t command lists all available tracer plugins. The -t flag specifies that the focus is on tracers rather than other trace commands or events.

Example Output:

timerlat
osnoise
hwlat
blk
mmiotrace
function_graph
wakeup_dl
wakeup_rt
wakeup
function
nop

Use case 3: Start tracing with a specific plugin

Code:

trace-cmd start -p function

Motivation:

Starting a trace with a specific plugin enables focused data collection. For instance, using the ‘function’ plugin captures data on function calls, which is instrumental when diagnosing performance bottlenecks or debugging intricate kernel interactions.

Explanation:

The start command initiates tracing. The -p flag specifies the plugin, in this case, function, which tells the tracer to concentrate on function call events.

Example Output:

Starting the function tracer

Use case 4: View the trace output

Code:

trace-cmd show

Motivation:

After capturing trace data, analyzing it is the next step. Viewing the trace output allows users to dive into the details of what was recorded, making it possible to identify anomalies or areas for optimization.

Explanation:

The show command retrieves and prints the content currently stored in trace buffers. This is essential for reviewing the gathered data to assess system behavior.

Example Output:

# tracer: function
#
# CPU 0 [function]
# 0.000 us | _do_fork() {

Use case 5: Stop the tracing but retain the buffers

Code:

trace-cmd stop

Motivation:

There are situations where you need to halt data collection to examine what’s been gathered without losing it. Stopping tracing without clearing buffers ensures that all the data remains intact for later review.

Explanation:

The stop command halts active tracing, maintaining all current buffer data intact. This is pivotal when a temporary pause is necessary without losing previously collected data.

Example Output:

Stopping the trace

Use case 6: Clear the trace buffers

Code:

trace-cmd clear

Motivation:

Clearing trace buffers allows practitioners to start a fresh tracing session. This is particularly encouraging when the previously collected data is no longer needed, and a clean slate is preferable for new data acquisition.

Explanation:

The clear command empties the trace buffers, effectively deleting all existing trace data. It’s crucial for resetting the data collection environment.

Example Output:

Clearing the trace buffers

Use case 7: Clear the trace buffers and stop tracing

Code:

trace-cmd reset

Motivation:

When you need to completely reset the tracing system, both stopping active tracing and clearing buffers at once is efficient. It’s a good choice when starting a new session from scratch without remnants from previous sessions.

Explanation:

The reset command simultaneously stops any active tracing and clears all trace buffers, ensuring a clean environment for future tracing efforts.

Example Output:

Resetting the trace system

Conclusion:

Trace-cmd provides detailed control over the tracing capabilities of the Linux kernel. Whether you’re observing function executions, managing trace data, or shifting through diagnostic information, each command offers a structured approach to system analysis. Employing these options effectively can significantly enhance system diagnosis and lead to optimized performance.