Exploring USDT Probes on Linux

Linux application observability has evolved significantly with the adoption of advanced tracing technologies. Among these, User Statically-Defined Tracing (USDT) stands out as a powerful mechanism for embedding trace points directly into user-space applications. Unlike traditional dynamic tracing tools such as strace or ltrace, which introduce high overhead and limited semantic context, USDT enables low-impact, semantically rich monitoring ideal for production environments.

This guide explores the architecture, implementation, and practical use of USDT probes on Linux, covering core concepts like static vs. dynamic tracing, integration with eBPF and uprobes, and runtime probe generation using tools like libstapsdt. Whether you're debugging performance bottlenecks or analyzing application behavior, understanding USDT unlocks deeper system introspection.

Understanding Linux Tracing Systems

Modern Linux tracing is built on a layered architecture that separates data sources, processing frameworks, and user-facing tools.

Core Components of the Tracing Stack

The Linux tracing ecosystem consists of three primary layers:

• Event Sources: Where trace data originates—kernel functions, system calls, or user-defined markers.
• Tracing Frameworks: Kernel-resident systems that collect, filter, and process events (e.g., ftrace, eBPF).
• Tracing Frontends: User-space tools that interface with frameworks to display, analyze, or act on trace data.

This modular design allows developers to mix and match components based on their observability needs.

Static vs. Dynamic Tracing

Two fundamental approaches define how probes are inserted:

• Static Tracing: Involves hard-coded instrumentation points compiled into binaries. These are stable and low-overhead, ideal for long-term monitoring.
• Dynamic Tracing: Enables real-time instrumentation of any function using techniques like kprobes (kernel) and uprobes (user-space), offering flexibility at the cost of higher runtime impact.

USDT falls under static tracing—it embeds predefined markers in applications during compilation, minimizing runtime disruption while preserving meaningful context.

Key Event Sources in Linux

Among these, USDT is particularly valuable because it allows application developers to define meaningful semantic events—such as "request-start" or "database-query"—that remain consistent across code changes.

👉 Discover how real-time monitoring enhances system observability with advanced tracing tools.

How USDT Works Internally

USDT probes operate through a clever compile-time and runtime interaction between source code and tracing infrastructure.

The Compilation Pipeline

1. Authoring: Developers insert DTRACE_PROBE() macros at strategic locations in C/C++ code.
2. Compilation: The compiler replaces each macro with a nop (no-operation) instruction.
3. Metadata Embedding: Probe details—including provider name, probe name, and argument layout—are stored in the ELF binary’s .note.stapsdt section.

This ensures zero runtime cost until a probe is actively monitored.

Runtime Activation with uprobes

When a tracing tool registers a USDT probe:

1. It reads the .note.stapsdt section to locate the nop instruction.
2. Using uprobe, it replaces the nop with an int3 (breakpoint) instruction.
3. When execution hits the breakpoint, control transfers to the kernel’s uprobe handler, which triggers the associated eBPF program or tracer.
4. Upon deregistration, the original nop is restored, removing all overhead.

This mechanism enables on-demand observability without modifying running applications.

Setting Up USDT: Prerequisites and Examples

Before using USDT, ensure your environment supports static probe instrumentation.

Required Tools and Libraries

On Ubuntu-based systems:

sudo apt-get install systemtap-sdt-dev

This package provides:

• sys/sdt.h: Header file for declaring USDT probes.
• dtrace command wrapper: Needed only if using semaphore-enabled probes.

Basic USDT Probe Without Semaphores

#include "sys/sdt.h"

int main() {
    DTRACE_PROBE("hello_usdt", "enter");
    int reval = 0;
    DTRACE_PROBE1("hello_usdt", "exit", reval);
}

Compile and verify:

gcc hello-usdt.c -o hello-usdt
readelf -n hello-usdt

Output will show entries in .note.stapsdt, confirming probe registration.

Semaphore-Supported Probes for Conditional Tracing

Semaphores allow applications to detect whether a probe is active and conditionally execute expensive operations (e.g., formatting diagnostic data).

Using a DTrace provider definition (tp_provider.d):

provider hello_semaphore_usdt {
    probe enter();
    probe exit(int exit_code);
}

Generate header and object files:

dtrace -G -s tp_provider.d -o tp_provider.o
dtrace -h -s tp_provider.d -o tp_provider.h

Then implement conditional logic:

#include "tp_provider.h"

int main() {
    if (HELLO_SEMAPHORE_USDT_ENTER_ENABLED()) {
        HELLO_SEMAPHORE_USDT_ENTER();
    }
    int reval = 0;
    if (HELLO_SEMAPHORE_USDT_EXIT_ENABLED()) {
        HELLO_SEMAPHORE_USDT_EXIT(reval);
    }
}

This pattern minimizes overhead when no tracer is attached.

Registering USDT Probes via ftrace and BCC

Two primary methods exist for activating USDT probes: using ftrace for direct kernel interaction or leveraging BCC for high-level scripting.

Using ftrace + uprobes (Low-Level Control)

1. Locate the binary’s load address:

   objdump -x ./tick | grep "start address"

2. Extract probe location from ELF notes:

   readelf -n ./tick

3. Register an uprobe:

   echo 'p:/home/ubuntu/labs/hello-usdt/tick:0x579' > /sys/kernel/debug/tracing/uprobe_events

4. Monitor with:

   perf record -e probe:tick:loop1 -a sleep 5

While manual, this method offers deep insight into how probes interact with kernel mechanisms.

Using BCC/eBPF (High-Level Automation)

BCC simplifies USDT interaction through Python-based tooling:

sudo /usr/share/bcc/tools/trace -p $(pgrep myapp) 'u:/path/to/app:my_probe "%d", arg1'

BCC automatically:

• Parses .note.stapsdt
• Attaches eBPF programs to uprobes
• Handles argument extraction via bpf_usdt_readarg()

This abstraction makes USDT accessible even to non-experts.

👉 Learn how modern tracing integrates with performance analysis platforms for faster debugging.

Programmatically Enabling USDT Probes

Standard USDT requires compile-time definitions. However, dynamic languages or runtime-generated code may need runtime probe creation.

Introducing libstapsdt

libstapsdt solves this by dynamically generating shared libraries containing USDT metadata:

1. At runtime, it creates a .so file with embedded .note.stapsdt.
2. The library is loaded via dlopen(), making probes visible to tracers.
3. Existing tools like BCC or perf can immediately detect and trace them.

Example workflow:

readelf -n /tmp/PROVIDER_NAME-XXXXX.so  # Shows embedded probe
sudo tplist -p $PID                     # Lists active probes

This technique enables USDT support in environments where ahead-of-time compilation isn't feasible.

Integrating USDT with Interpreted and JIT Languages

Languages like Node.js can adopt USDT via native bindings to libstapsdt.

Example: Node.js with usdt Module

Install:

npm install usdt

Usage:

const USDT = require("usdt");
const provider = new USDT.USDTProvider("nodeProvider");
const probe1 = provider.addProbe("firstProbe", "int", "char *");

provider.enable();

function fireProbe() {
    probe1.fire(() => [42, "example string"]);
}

setInterval(fireProbe, 1000);

Trace output:

sudo trace.py -p $(pgrep node) 'u::firstProbe "%d - %s", arg1, arg2'

This enables granular observability in JavaScript applications—ideal for monitoring HTTP request lifecycles or garbage collection events.

Frequently Asked Questions

What is the main advantage of USDT over strace?

USDT provides semantic, low-overhead tracing tied to application logic (e.g., "request-start"), whereas strace traces system calls with high overhead and limited context. USDT is safe for production use; strace often is not.

Can USDT probes slow down my application?

Only when actively traced. When inactive, USDT inserts nop instructions—essentially free. With semaphores, applications can avoid costly data formatting unless a probe is enabled.

Is eBPF required to use USDT on Linux?

Not strictly, but highly recommended. While older tools like SystemTap support USDT, modern workflows using BCC or bpftrace leverage eBPF for efficient in-kernel processing and reduced userspace overhead.

How do I list available USDT probes in a binary?

Use:

sudo tplist -p $(pgrep your_app)

Or inspect ELF sections:

readelf -n /path/to/binary | grep -A5 stapsdt

Does Node.js support USDT natively?

Yes. Node.js binaries compiled with --with-dtrace include built-in probes for HTTP, GC, and network events. You can trace them directly using BCC tools without modifying source code.

Can I add USDT probes to running processes?

Yes—via libraries like libstapsdt. It dynamically loads a shared object containing probes into a process using dlopen(), making them immediately available to tracers.

👉 See how developers leverage real-time tracing data to optimize application performance at scale.

Conclusion

USDT represents a mature approach to application observability on Linux, combining the stability of static instrumentation with the flexibility of dynamic analysis. By embedding meaningful trace points directly into code, teams gain production-safe visibility into critical application behaviors.

With robust support from eBPF, BCC, and runtime libraries like libstapsdt, USDT is now accessible across compiled and interpreted languages alike. Whether you're debugging latency issues in microservices or auditing internal state transitions, integrating USDT into your toolchain empowers precise, low-overhead monitoring.

For further exploration, refer to Brendan Gregg’s comprehensive guides on Linux tracing—they remain the definitive resource for mastering this domain.