Exploring USDT Probes on Linux

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Modern software systems demand deep observability to ensure performance, reliability, and rapid troubleshooting—especially in production environments. One powerful technique for achieving this is User Statically Defined Tracing (USDT), which allows developers to embed trace points directly into user-space applications. When combined with modern Linux tracing frameworks like eBPF and uprobes, USDT provides a low-overhead, high-fidelity method for monitoring application behavior in real time.

This guide explores the fundamentals of USDT probes on Linux, from core concepts and implementation details to practical usage with tools like bcc, bpftrace, and perf. Whether you're debugging latency issues or analyzing application flow, mastering USDT can significantly enhance your observability toolkit.

Understanding Linux Tracing Systems

Linux offers a rich ecosystem of tracing technologies designed to help developers and system administrators observe kernel and user-space behavior. At its core, the Linux tracing architecture consists of three main components:

👉 Discover how real-time tracing can boost system observability with advanced tools.

Key Terminology

Before diving deeper, it’s essential to understand key tracing concepts:

Evolution of Linux Tracing

Historically, tools like strace and ltrace provided visibility into system calls and library functions but introduced significant overhead. The modern Linux tracing stack has evolved to support efficient, programmable, and safe instrumentation through:

These advancements paved the way for robust support of USDT, a feature originally developed for Solaris’ DTrace.

What Are USDT Probes?

User Statically Defined Tracing (USDT) allows developers to define named trace points within their applications using macros like DTRACE_PROBE(). These probes are compiled into NOP (no-operation) instructions and annotated in the ELF binary's .note.stapsdt section.

At runtime, tracing tools detect these probes and dynamically replace NOPs with breakpoints (int3 on x86), activating them only when needed. This ensures minimal runtime overhead when probes are inactive.

How USDT Works Internally

  1. Authoring Time: A developer inserts DTRACE_PROBE() in source code.
  2. Compilation: The compiler emits a NOP instruction and records probe metadata in the ELF file.
  3. Runtime Activation: A tracing tool reads the .note.stapsdt section and uses uprobe to patch the NOP with a breakpoint.
  4. Event Handling: When the probe is hit, control transfers to the kernel tracer (e.g., eBPF program), which processes the event.
  5. Deactivation: The breakpoint is reverted to NOP, restoring normal execution.

This mechanism enables fine-grained, semantically meaningful instrumentation—such as "request-start" or "gc-begin"—without requiring knowledge of internal function names.

Setting Up USDT on Linux

To use USDT, your system must support:

Installation (Ubuntu)

sudo apt-get install systemtap-sdt-dev

This installs:

Implementing USDT Probes

Basic Probe Without Semaphores

#include "sys/sdt.h"
int main() {
    DTRACE_PROBE("hello_usdt", "enter");
    int reval = 0;
    DTRACE_PROBE1("hello_usdt", "exit", reval);
}

Compile:

gcc hello-usdt.c -o hello-usdt

Verify probe presence:

readelf -n ./hello-usdt

Output will show .note.stapsdt entries with provider, name, location, and arguments.

Probes with Semaphores

Semaphores allow conditional execution of expensive operations only when a probe is active.

  1. Define provider in .d file:

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

  2. Generate header and object:

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

  3. Use in code:

    #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 avoids argument preparation unless the probe is actively traced.

Registering USDT Probes

Using ftrace + uprobe

Although not the most user-friendly method, using ftrace reveals how uprobes work under the hood.

  1. Find binary load address:

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

  2. Extract probe location from ELF notes:

    readelf -n ./tick

  3. Register uprobe:

    echo 'p:loop1 /path/to/tick:0x579' > /sys/kernel/debug/tracing/uprobe_events

  4. Enable and monitor:

    echo 1 > /sys/kernel/debug/tracing/events/uprobes/loop1/enable
cat /sys/kernel/debug/tracing/trace_pipe

While educational, this approach is error-prone. Prefer higher-level tools.

Using bcc or bpftrace (Recommended)

With bcc:

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

With bpftrace:

bpftrace -e 'usdt:/path/to/app:probe_name { printf("Hit: %s\\n", str(arg0)); }'

These tools automatically parse .note.stapsdt, attach uprobes, and handle argument extraction.

👉 See how modern observability stacks integrate with advanced tracing systems.

Dynamic USDT with libstapsdt

Traditional USDT requires compile-time definitions. However, libstapsdt enables runtime probe creation by generating shared libraries on-the-fly with embedded .note.stapsdt sections.

This is particularly useful for dynamic languages like Node.js, Python, or Java via JNI wrappers.

Example: Node.js with USDT

Using the usdt npm package:

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

setInterval(() => {
    probe1.fire(() => [42, "hello"]);
}, 1000);

List active probes:

sudo tplist -p $(pgrep node)

Trace events:

sudo trace 'u::firstProbe "%d %s", arg1, arg2'

This enables deep introspection into interpreted or JIT-compiled runtimes without modifying the VM itself.

Frequently Asked Questions

Q: What’s the difference between static and dynamic tracing?
A: Static tracing uses predefined probes (like USDT), while dynamic tracing instruments any function at runtime (e.g., uprobes). Static probes are safer and more stable; dynamic ones offer broader coverage.

Q: Do USDT probes slow down my app?
A: When inactive, USDT probes are just NOPs—virtually zero overhead. Only when traced do they introduce minor latency due to breakpoint handling.

Q: Can I use USDT without recompiling my app?
A: No—probes must be compiled in. However, tools like libstapsdt allow injecting probes at runtime via shared libraries.

Q: Which tools support USDT on Linux?
A: bcc, bpftrace, perf, and SystemTap all support USDT. Node.js, PostgreSQL, and Java (via DTrace agents) include built-in probes.

Q: Are USDT probes safe in production?
A: Yes—they’re designed for production use. Since they’re inactive by default and only activated during diagnosis, they pose minimal risk.

Q: How do I list available USDT probes in a binary?
A: Use tplist from bcc:

sudo tplist -p $(pgid myapp)

Practical Applications

USDT shines in real-world scenarios:

By correlating user-defined events with system metrics, teams gain unprecedented insight into application behavior—without resorting to logging or debug builds.

Conclusion

USDT brings enterprise-grade observability to Linux user-space applications. Combined with eBPF-based tooling like bcc and bpftrace, it offers a powerful, low-overhead way to monitor and troubleshoot complex software systems in production.

From simple NOP-based markers to dynamic probe injection in interpreted languages, USDT continues to evolve as a cornerstone of modern observability practices. As more runtimes adopt built-in probes, the ability to trace semantic events—rather than raw function calls—will become increasingly vital for maintaining resilient, performant systems.

Whether you're building high-frequency trading platforms or cloud-native services, integrating USDT into your development workflow empowers faster debugging, deeper insights, and better software overall.