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3.2. SystemTap Scripts

For the most part, SystemTap scripts are the foundation of each SystemTap session. SystemTap scripts instruct SystemTap on what type of information to collect, and what to do once that information is collected.
As stated in Chapter 3, Understanding How SystemTap Works, SystemTap scripts are made up of two components: events and handlers. Once a SystemTap session is underway, SystemTap monitors the operating system for the specified events and executes the handlers as they occur.

Note

An event and its corresponding handler is collectively called a probe. A SystemTap script can have multiple probes.
A probe's handler is commonly referred to as a probe body.
In terms of application development, using events and handlers is similar to instrumenting the code by inserting diagnostic print statements in a program's sequence of commands. These diagnostic print statements allow you to view a history of commands executed once the program is run.
SystemTap scripts allow insertion of the instrumentation code without recompilation of the code and allows more flexibility with regard to handlers. Events serve as the triggers for handlers to run; handlers can be specified to record specified data and print it in a certain manner.
Format
SystemTap scripts use the file extension .stp, and contains probes written in the following format:
probe	event {statements}
SystemTap supports multiple events per probe; multiple events are delimited by a comma (,). If multiple events are specified in a single probe, SystemTap will execute the handler when any of the specified events occur.
Each probe has a corresponding statement block. This statement block is enclosed in braces ({ }) and contains the statements to be executed per event. SystemTap executes these statements in sequence; special separators or terminators are generally not necessary between multiple statements.

Note

Statement blocks in SystemTap scripts follow the same syntax and semantics as the C programming language. A statement block can be nested within another statement block.
SystemTap allows you to write functions to factor out code to be used by a number of probes. Thus, rather than repeatedly writing the same series of statements in multiple probes, you can just place the instructions in a function, as in:
function function_name(arguments) {statements}
probe event {function_name(arguments)}
The statements in function_name are executed when the probe for event executes. The arguments are optional values passed into the function.

Important

Section 3.2, “SystemTap Scripts” is designed to introduce readers to the basics of SystemTap scripts. To understand SystemTap scripts better, it is advisable that you refer to Chapter 5, Useful SystemTap Scripts; each section therein provides a detailed explanation of the script, its events, handlers, and expected output.

3.2.1. Event

SystemTap events can be broadly classified into two types: synchronous and asynchronous.
Synchronous Events
A synchronous event occurs when any process executes an instruction at a particular location in kernel code. This gives other events a reference point from which more contextual data may be available.
Examples of synchronous events include:
syscall.system_call
The entry to the system call system_call. If the exit from a syscall is desired, appending a .return to the event monitor the exit of the system call instead. For example, to specify the entry and exit of the system call close, use syscall.close and syscall.close.return respectively.
vfs.file_operation
The entry to the file_operation event for Virtual File System (VFS). Similar to syscall event, appending a .return to the event monitors the exit of the file_operation operation.
kernel.function("function")
The entry to the kernel function function. For example, kernel.function("sys_open") refers to the "event" that occurs when the kernel function sys_open is called by any thread in the system. To specify the return of the kernel function sys_open, append the return string to the event statement; that is, kernel.function("sys_open").return.
When defining probe events, you can use asterisk (*) for wildcards. You can also trace the entry or exit of a function in a kernel source file. Consider the following example:

Example 3.1. wildcards.stp

probe kernel.function("*@net/socket.c") { }
probe kernel.function("*@net/socket.c").return { }

In the previous example, the first probe's event specifies the entry of ALL functions in the kernel source file net/socket.c. The second probe specifies the exit of all those functions. Note that in this example, there are no statements in the handler; as such, no information will be collected or displayed.
kernel.trace("tracepoint")
The static probe for tracepoint. Recent kernels (2.6.30 and newer) include instrumentation for specific events in the kernel. These events are statically marked with tracepoints. One example of a tracepoint available in systemtap is kernel.trace("kfree_skb") which indicates each time a network buffer is freed in the kernel.
module("module").function("function")
Allows you to probe functions within modules. For example:

Example 3.2. moduleprobe.stp

probe module("ext3").function("*") { }
probe module("ext3").function("*").return { }

The first probe in Example 3.2, “moduleprobe.stp” points to the entry of all functions for the ext3 module. The second probe points to the exits of all functions for that same module; the use of the .return suffix is similar to kernel.function(). Note that the probes in Example 3.2, “moduleprobe.stp” do not contain statements in the probe handlers, and as such will not print any useful data (as in Example 3.1, “wildcards.stp”).
A system's kernel modules are typically located in /lib/modules/kernel_version, where kernel_version refers to the currently loaded kernel version. Modules use the file name extension .ko.
Asynchronous Events
Asynchronous events are not tied to a particular instruction or location in code. This family of probe points consists mainly of counters, timers, and similar constructs.
Examples of asynchronous events include:
begin
The startup of a SystemTap session; that is, as soon as the SystemTap script is run.
end
The end of a SystemTap session.
timer events
An event that specifies a handler to be executed periodically. For example:

Example 3.3. timer-s.stp

probe timer.s(4)
{
  printf("hello world\n")
}

Example 3.3, “timer-s.stp” is an example of a probe that prints hello world every 4 seconds. It is also possible to use the following timer events:
  • timer.ms(milliseconds)
  • timer.us(microseconds)
  • timer.ns(nanoseconds)
  • timer.hz(hertz)
  • timer.jiffies(jiffies)
When used in conjunction with other probes that collect information, timer events allows you to print out get periodic updates and see how that information changes over time.

Important

SystemTap supports the use of a large collection of probe events. For more information about supported events, refer to man stapprobes. The SEE ALSO section of man stapprobes also contains links to other man pages that discuss supported events for specific subsystems and components.