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3.2. SystemTap Scripts
Note
SystemTap scripts use the .stp
file extension and contains probes written in the following format:
probe event {statements}
,
). If multiple events are specified in a single probe, SystemTap executes the handler when any of the specified events occurs.
{ }
) 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
function function_name(arguments){statements} probe event {function_name(arguments)}
Important
3.2.1. Event
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.
- 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 theclose
system call, usesyscall.close
andsyscall.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 thefile_operation
operation. - kernel.function("function")
- The entry to the
function
kernel function. For example,kernel.function("sys_open")
refers to the event that occurs when thesys_open
kernel function is called by any thread in the system. To specify the return of thesys_open
kernel function, append thereturn
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 thenet/socket.c
kernel source file. 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 theext3
module. The second probe points to the exits of all functions for that same module; the use of the.return
suffix is similar tokernel.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 are not tied to a particular instruction or location in code. This family of probe points consists mainly of counters, timers, and similar constructs.
- 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 printshello world
every four 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 periodic updates and see how that information changes over time.
Important
3.2.2. Systemtap Handler/Body
Example 3.4. helloworld.stp
probe begin { printf ("hello world\n") exit () }
begin
event (the start of the session) triggers the handler enclosed in { }
, which simply prints hello world
followed by a new line, then exits.
Note
exit()
function executes. If the users wants to stop the execution of the script, it can interrupted manually with Ctrl+C.
The printf()
statement is one of the simplest functions for printing data. printf()
can also be used to display data using many SystemTap functions in the following format:
printf ("format string\n", arguments)
hello world
and contains no 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
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
.
Example 3.5. variables-in-printf-statements.stp
probe syscall.open { printf ("%s(%d) open\n", execname(), pid()) }
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 supports many 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).
- 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 ofthread_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 thethread_indent()
andprobe
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_indent()
call for the thread. - The process name (and its corresponding ID) that made the function call.
- 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 either of the following two commands:
stap script -x process ID
stap script -c command
If you want to specify a script to take an argument of a process ID or command, usetarget()
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 thesyscall.*
event) and prints out the name of all system calls made by the specified process.This has the same effect as specifyingif (pid() == process ID)
each time you wish to target a specific process. However, usingtarget()
makes it easier 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. For example:stap targetexample.stp -x process ID