SystemTap
In computing, SystemTap (stap) is a scripting language and tool for dynamically instrumenting running production Linux-based operating systems. System administrators can use SystemTap to extract, filter and summarize data in order to enable diagnosis of complex performance or functional problems.

SystemTap consists of free and open-source software and includes contributions from Red Hat, IBM, Intel, Hitachi, Oracle, the University of Wisconsin-Madison and other community members.

History
SystemTap debuted in 2005 in Red Hat Enterprise Linux 4 Update 2 as a technology preview.

After four years in development, SystemTap 1.0 was released in 2009.

, SystemTap runs fully supported in all Linux distributions including RHEL / CentOS 5 since update 2, SLES 10, Fedora, Debian and Ubuntu.

Tracepoints in the CPython VM and JVM were added in SystemTap 1.2 in 2009.

In November 2019, SystemTap 4.2 included prometheus exporter.

Usage
SystemTap files are written in the SystemTap language (saved as .stp files) and run with the stap command-line.

The system carries out a number of analysis passes on the script before allowing it to run. Scripts may be executed with one of three backends selected by the --runtime= option. The default is a loadable kernel module, which has the fullest capability to inspect and manipulate any part of the system, and therefore requires most privilege. Another backend is based on the dynamic program analysis library DynInst to instrument the user's own user-space programs only, and requires least privilege. The newest backend is based on eBPF byte-code, is limited to the Linux kernel interpreter's capabilities, and requires an intermediate level of privilege. In each case, the module is unloaded when the script has finished running.

Scripts generally focus on events (such as starting or finishing a script), compiled-in probe points such as Linux "tracepoints", or the execution of functions or statements in the kernel or user-space.

Some "guru mode" scripts may also have embedded C, which may run with the -g command-line option. However, use of guru mode is discouraged, and each SystemTap release includes more probe points designed to remove the need for guru-mode scripts. Guru mode is required in order to permit scripts to modify state in the instrumented software, such as to apply some types of emergency security fixes.

As of SystemTap version 1.7, the software implements the new stapsys group and privilege level.

Simple examples
The following script shows all applications setting TCP socket options on the system, what options are being set, and whether the option is set successfully or not.

# Show sockets setting options

# Return enabled or disabled based on value of optval
function getstatus(optval)
{
    if ( optval == 1 )
        return "enabling"
    else
        return "disabling"
}

probe begin
{
	print ("\nChecking for apps setting socket options\n")
}

    # Set a socket option
    probe tcp.setsockopt
    {
        status = getstatus(user_int($optval))
    	printf ("  App '%s' (PID %d) is %s socket option %s... ", execname(), pid(), status, optstr)
    }
    
        # Check setting the socket option worked
        probe tcp.setsockopt.return
        {
            if ( ret == 0 )
                printf ("success")
            else
                printf ("failed")
            printf ("\n")
        }

probe end
{
	print ("\nClosing down\n")
}

Many other examples are shipped with SystemTap. There are also real-world examples of SystemTap use at the War Stories page.

Importing scripts from other tracing technologies
SystemTap can attach to DTrace markers when they are compiled into an application using macros from the sys/sdt.h header file.

See also

 Kernel marker
 DProbes
 LTTng
 strace
 ProbeVue

References

External links
 
 SystemTap Wiki
 SystemTap Language Reference (PDF)
 SystemTap project analysis by Ohloh
 Dynamic Tracing with DTrace & SystemTap
 Systemtap tutorial Frank Ch. Eigler
 SystemTap Beginner's Guide Don Domingo, William Cohen
 Problem Solving With Systemtap, Ottawa Linux Symposium 2006
 Problem Solving with SystemTap, Eugene Teo, presented at the Red Hat Summit 2007 (scripts)
 Dynamic Tracing and Performance Analysis Using SystemTap, Joshua Stone, presented at the LinuxWorld Conference 2008
 SystemTap Tutorial, Jim Keniston, presented at the Linux Plumbers Conference 2008

Free software programmed in C++
Free system software
Debuggers
Linux kernel features
Linux-only free software