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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 4, 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; i.e. 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.
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 filename 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; i.e. 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. Note that you can also 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.

3.2.2. Systemtap Handler/Body

Consider the following sample script:

Example 3.4. helloworld.stp

probe begin
{
  printf ("hello world\n")
  exit ()
}
In Example 3.4, “helloworld.stp”, the event begin (i.e. the start of the session) triggers the handler enclosed in { }, which simply prints hello world followed by a new-line, then exits.

Note

SystemTap scripts continue to run until the exit() function executes. If the users wants to stop the execution of the script, it can interrupted manually with Ctrl+C.
printf ( ) Statements

The printf () statement is one of the simplest functions for printing data. printf () can also be used to display data using a wide variety of SystemTap functions in the following format: 

		printf ("format string\n", arguments)
The format string specifies how arguments should be printed. The format string of Example 3.4, “helloworld.stp” simply instructs SystemTap to print hello world, and contains no format specifiers.
You can use the format specifiers %s (for strings) and %d (for numbers) in format strings, depending on your list of arguments. Format strings can have multiple format specifiers, each matching a corresponding argument; multiple arguments are delimited by a comma (,).

Note

Semantically, the SystemTap printf function is very similar to its C language counterpart. The aforementioned syntax and format for SystemTap's printf function is identical to that of the C-style printf.
To illustrate this, consider the following probe example:

Example 3.5. variables-in-printf-statements.stp

probe syscall.open
{
  printf ("%s(%d) open\n", execname(), pid())
}
Example 3.5, “variables-in-printf-statements.stp” instructs SystemTap to probe all entries to the system call open; for each event, it prints the current execname() (a string with the executable name) and pid() (the current process ID number), followed by the word open. A snippet of this probe's output would look like: 
vmware-guestd(2206) open
hald(2360) open
hald(2360) open
hald(2360) open
df(3433) open
df(3433) open
df(3433) open
hald(2360) open
SystemTap Functions

SystemTap supports a wide variety of functions that can be used as printf () arguments. Example 3.5, “variables-in-printf-statements.stp” uses the SystemTap functions execname() (name of the process that called a kernel function/performed a system call) and pid() (current process ID).

The following is a list of commonly-used SystemTap functions:
tid()
The ID of the current thread.
uid()
The ID of the current user.
cpu()
The current CPU number.
gettimeofday_s()
The number of seconds since UNIX epoch (January 1, 1970).
ctime()
Convert number of seconds since UNIX epoch to date.
pp()
A string describing the probe point currently being handled.
thread_indent()
This particular function is quite useful, providing you with a way to better organize your print results. The function takes one argument, an indentation delta, which indicates how many spaces to add or remove from a thread's "indentation counter". It then returns a string with some generic trace data along with an appropriate number of indentation spaces.
The generic data included in the returned string includes a timestamp (number of microseconds since the first call to thread_indent() by the thread), a process name, and the thread ID. This allows you to identify what functions were called, who called them, and the duration of each function call.
If call entries and exits immediately precede each other, it is easy to match them. However, in most cases, after a first function call entry is made several other call entries and exits may be made before the first call exits. The indentation counter helps you match an entry with its corresponding exit by indenting the next function call if it is not the exit of the previous one.
Consider the following example on the use of thread_indent():

Example 3.6. thread_indent.stp

probe kernel.function("*@net/socket.c") 
{
  printf ("%s -> %s\n", thread_indent(1), probefunc())
}
probe kernel.function("*@net/socket.c").return 
{
  printf ("%s <- %s\n", thread_indent(-1), probefunc())
}
Example 3.6, “thread_indent.stp” prints out the thread_indent() and probe functions at each event in the following format: 
0 ftp(7223): -> sys_socketcall
1159 ftp(7223):  -> sys_socket
2173 ftp(7223):   -> __sock_create
2286 ftp(7223):    -> sock_alloc_inode
2737 ftp(7223):    <- sock_alloc_inode
3349 ftp(7223):    -> sock_alloc
3389 ftp(7223):    <- sock_alloc
3417 ftp(7223):   <- __sock_create
4117 ftp(7223):   -> sock_create
4160 ftp(7223):   <- sock_create
4301 ftp(7223):   -> sock_map_fd
4644 ftp(7223):    -> sock_map_file
4699 ftp(7223):    <- sock_map_file
4715 ftp(7223):   <- sock_map_fd
4732 ftp(7223):  <- sys_socket
4775 ftp(7223): <- sys_socketcall
This sample output contains the following information:
  • The time (in microseconds) since the initial thread_ident() call for the thread (included in the string from thread_ident()).
  • The process name (and its corresponding ID) that made the function call (included in the string from thread_ident()).
  • An arrow signifying whether the call was an entry (<-) or an exit (->); the indentations help you match specific function call entries with their corresponding exits.
  • The name of the function called by the process.
name
Identifies the name of a specific system call. This variable can only be used in probes that use the event syscall.system_call.
target()
Used in conjunction with stap script -x process ID or stap script -c command. If you want to specify a script to take an argument of a process ID or command, use target() as the variable in the script to refer to it. For example:

Example 3.7. targetexample.stp

probe syscall.* {
  if (pid() == target())
    printf("%s/n", name)
}
When Example 3.7, “targetexample.stp” is run with the argument -x process ID, it watches all system calls (as specified by the event syscall.*) and prints out the name of all system calls made by the specified process.
This has the same effect as specifying if (pid() == process ID) each time you wish to target a specific process. However, using target() makes it easier for you to re-use the script, giving you the ability to simply pass a process ID as an argument each time you wish to run the script (e.g. stap targetexample.stp -x process ID).
For more information about supported SystemTap functions, refer to man stapfuncs.
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