Introduction: The Precursor to eBPF

Before eBPF took over the world...

Think of the kernel as a black box running at the heart of your system — it handles every file open, every network packet, every process fork. Normally, you can only observe what the kernel chooses to expose through logs or metrics. SystemTap breaks open that black box: it lets you pause the kernel at almost any point, inspect what's happening inside, and resume — all without rebooting or recompiling the kernel.

Today, everyone talks about eBPF as the ultimate observability tool. But long before eBPF became mainstream, SystemTap (stap) was the king of dynamic tracing. Developed primarily by Red Hat starting around 2005, it gave engineers the first practical way to instrument a running production kernel with custom logic — answering questions like "which process is hammering this file?" or "why does this syscall occasionally take 200ms?"

Why learn it in 2026?

1. Legacy Systems: You will inevitably encounter older kernels (RHEL 6/7, CentOS 7) where modern eBPF tools like bpftrace are not fully supported or require kernel features that simply aren't present. SystemTap works on kernels as old as 2.6.
2. Specific Power: SystemTap has deeply mature tapsets (libraries of pre-built probes) for higher-level runtimes like Java (via USDT probes in the JVM) and Python that sometimes exceed what generic eBPF tooling provides out of the box.
3. Kernel Modules: Unlike eBPF — which runs inside a verified sandbox with strict constraints — SystemTap compiles your script into a full Kernel Module (.ko) and loads it with insmod. This is riskier but historically allowed more aggressive instrumentation, including modifying live kernel variables.
4. Conceptual Foundation: Understanding SystemTap's architecture makes eBPF's design choices (the verifier, CO-RE, BTF) immediately obvious. You understand why eBPF was built the way it was.