Understanding USDT in the Context of eBPF

USDT, or Userland Statically Defined Tracing, is a powerful tool that has found its way into the realm of eBPF (Extended Berkeley Packet Filter). By combining the capabilities of USDT with eBPF, you can gain deeper insights into the performance and behavior of your applications. Let’s delve into what USDT is, how it works, and how it can be effectively used with eBPF.

What is USDT?

USDT is a technology introduced by the Solaris operating system. It allows developers to define static probe points within their applications. These probe points are specific locations in the code that can be dynamically attached by tracing tools to collect information at runtime. The beauty of USDT is that these probe points are defined at compile time, but they are disabled by default. This means that the application’s performance is not significantly impacted when USDT is not in use.

How does USDT Work?

USDT works by inserting special macros into the source code of your application. These macros are replaced by the actual probe code during the compilation process. When these probes are activated, they collect and record valuable information about the application’s execution. In C or C++ applications, you can use the DTrace or SystemTap APIs to define USDT probes.

Here’s a simple example of how you can define a USDT probe in a C application:

include void my_function() {    DTRACEPROBE1(my_function, "Starting my_function");    // ... your code here ...    DTRACEPROBE1(my_function, "Exiting my_function");}

In this example, the DTRACEPROBE1 macro is used to define a probe point at the beginning and end of the `my_function` function. When the function is called, the probe will be activated, and the specified information will be collected.

USDT with eBPF

When combined with eBPF, USDT becomes an even more powerful tool for performance analysis and troubleshooting. eBPF is a technology that allows you to run code in the Linux kernel, providing a high-level of visibility into the system’s behavior. By using USDT probes in conjunction with eBPF, you can gain valuable context information that helps you understand the behavior of your applications.

One of the most common use cases for USDT with eBPF is to track the number of times a specific function is called or to collect stack traces when an event occurs. This can be achieved by using BCC (BPF Compiler Collection) to view the USDT probes in your application.

Here’s an example of how you can use BCC to view USDT probes in a C application:

include include include include include include int main() {    struct bpf_program prog;    char filename = "usdt_example.o";    // Load the BPF program    prog = bpf_load_program(filename);    if (prog == NULL) {        perror("Failed to load BPF program");        return -1;    }    // Attach the BPF program to the USDT probe    bpf_set_probe_fd(prog, 0);    bpf_set_probe_fd(prog, 1);    // Run the application    execl("/path/to/your/application", "/path/to/your/application", NULL);    // Handle errors    perror("Failed to execute application");    return -1;}

In this example, the BPF program is loaded and attached to the USDT probe. The application is then executed, and the BPF program collects information about the probe’s activation.

Conclusion

USDT is a valuable tool for developers looking to gain deeper insights into the performance and behavior of their applications. By combining USDT with eBPF, you can achieve even greater visibility into the system’s behavior. With the right tools and techniques, you can effectively use USDT to track function calls, collect stack traces, and much more.