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SystemTap Language Reference
Red Hat Enterprise Linux 5
A guide to the constructs and syntax used in SystemTap scripts
Edition 1
IBM Documentation
Abstract
The SystemTap Language Reference is a comprehensive reference of the language constructs and syntax used in SystemTap scripts. It is suitable for users who have intermediate to advanced knowledge of SystemTap.
Chapter 1. SystemTap overview
1.1. About this guide
This guide is a comprehensive reference of SystemTap's language constructs and syntax. The contents borrow heavily from existing SystemTap documentation found in manual pages and the tutorial. The presentation of information here provides the reader with a single place to find language syntax and recommended usage. In order to successfully use this guide, you should be familiar with the general theory and operation of SystemTap. If you are new to SystemTap, you will find the tutorial to be an excellent place to start learning. For detailed information about tapsets, see the manual pages provided with the distribution. For information about the entire collection of SystemTap reference material, see Section Chapter 10, For Further Reference
1.2. Reasons to use SystemTap
SystemTap provides infrastructure to simplify the gathering of information about a running Linux kernel so that it may be further analyzed. This analysis assists in identifying the underlying cause of a performance or functional problem. SystemTap was designed to eliminate the need for a developer to go through the tedious instrument, recompile, install, and reboot sequence normally required to collect this kind of data. To do this, it provides a simple command-line interface and scripting language for writing kernel instrumentation. With SystemTap, developers, system administrators, and users can easily write scripts that gather and manipulate kernel data that is not otherwise available using standard Linux tools. Users of SystemTap will find it to be a significant improvement over older methods.
1.3. Event-action language
SystemTap's language is strictly typed, declaration free, procedural, and inspired by dtrace and awk. Source code points or events in the kernel are associated with handlers, which are subroutines that are executed synchronously. These probes are conceptually similar to "breakpoint command lists" in the GDB debugger.
There are two main outermost constructs: probes and functions. Within these, statements and expressions use C-like operator syntax and precedence.
1.4. Sample SystemTap scripts
Following are some example scripts that illustrate the basic operation of SystemTap. For more examples, see the examples/small_demos/ directory in the source directory, the SystemTap wiki at http://sourceware.org/systemtap/wiki/HomePage, or the SystemTap War Stories at http://sourceware.org/systemtap/wiki/WarStories page.
1.4.1. Basic SystemTap syntax and control structures
The following code examples demonstrate SystemTap syntax and control structures.
global odds, evens
probe begin {
# "no" and "ne" are local integers
for (i = 0; i < 10; i++) {
if (i % 2) odds [no++] = i
else evens [ne++] = i
}
delete odds[2]
delete evens[3]
exit()
}
probe end {
foreach (x+ in odds)
printf ("odds[%d] = %d", x, odds[x])
foreach (x in evens-)
printf ("evens[%d] = %d", x, evens[x])
}
This prints:
odds[0] = 1 odds[1] = 3 odds[3] = 7 odds[4] = 9 evens[4] = 8 evens[2] = 4 evens[1] = 2 evens[0] = 0
Note that all variable types are inferred, and that all locals and globals are initialized.
1.4.2. Primes between 0 and 49
function isprime (x) {
if (x < 2) return 0
for (i = 2; i < x; i++) {
if (x % i == 0) return 0
if (i * i > x) break
}
return 1
}
probe begin {
for (i = 0; i < 50; i++)
if (isprime (i)) printf("%d\n", i)
exit()
}
This prints:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
1.4.3. Recursive functions
function fibonacci(i) {
if (i < 1) error ("bad number")
if (i == 1) return 1
if (i == 2) return 2
return fibonacci (i-1) + fibonacci (i-2)
}
probe begin {
printf ("11th fibonacci number: %d", fibonacci (11))
exit ()
}
This prints:
11th fibonacci number: 118
Any larger number input to the function may exceed the MAXACTION or MAXNESTING limits, which will be caught at run time and result in an error. For more about limits see Section Section 1.6, “Safety and security”.
1.5. The stap command
The stap program is the front-end to the SystemTap tool. It accepts probing instructions written in its scripting language, translates those instructions into C code, compiles this C code, and loads the resulting kernel module into a running Linux kernel to perform the requested system trace or probe functions. You can supply the script in a named file, from standard input, or from the command line. The program runs until it is interrupted by the user or a sufficient number of soft errors, or if the script voluntarily invokes the exit() function.
The stap command does the following:
- Translates the script
- Generates and compiles a kernel module
- Inserts the module; output to stap's stdout
- CTRL-C unloads the module and terminates stap
For a full list of options to the stap command, see the stap(1) manual page.
1.6. Safety and security
SystemTap is an administrative tool. It exposes kernel internal data structures and potentially private user information. It requires root privileges to actually run the kernel objects it builds using the
sudo command, applied to the staprun program.
staprun is a part of the SystemTap package, dedicated to module loading and unloading and kernel-to-user data transfer. Since staprun does not perform any additional security checks on the kernel objects it is given, do not give elevated privileges via sudo to untrusted users.
The translator asserts certain safety constraints. It ensures that no handler routine can run for too long, allocate memory, perform unsafe operations, or unintentionally interfere with the kernel. Use of script global variables is locked to protect against manipulation by concurrent probe handlers. Use of guru mode constructs such as embedded C (see Section Section 3.5, “Embedded C”) can violate these constraints, leading to a kernel crash or data corruption.
The resource use limits are set by macros in the generated C code. These may be overridden with the -D flag. The following list describes a selection of these macros:
MAXNESTING – The maximum number of recursive function call levels. The default is 10.
MAXSTRINGLEN – The maximum length of strings. The default is 128.
MAXTRYLOCK – The maximum number of iterations to wait for locks on global variables before declaring possible deadlock and skipping the probe. The default is 1000.
MAXACTION – The maximum number of statements to execute during any single probe hit. The default is 1000.
MAXMAPENTRIES – The maximum number of rows in an array if the array size is not specified explicitly when declared. The default is 2048.
MAXERRORS – The maximum number of soft errors before an exit is triggered. The default is 0.
MAXSKIPPED – The maximum number of skipped reentrant probes before an exit is triggered. The default is 100.
MINSTACKSPACE -- The minimum number of free kernel stack bytes required in order to run a probe handler. This number should be large enough for the probe handler's own needs, plus a safety margin. The default is 1024.
If something goes wrong with stap or staprun after a probe has started running, you may safely kill both user processes, and remove the active probe kernel module with the rmmod command. Any pending trace messages may be lost.
Chapter 2. Types of SystemTap scripts
2.1. Probe scripts
Probe scripts are analogous to programs; these scripts identify probe points and associated handlers.
2.2. Tapset scripts
Tapset scripts are libraries of probe aliases and auxiliary functions.
The /usr/share/systemtap/tapset directory contains tapset scripts. While these scripts look like regular SystemTap scripts, they cannot be run directly.
Chapter 3. Components of a SystemTap script
The main construct in the scripting language identifies probes. Probes associate abstract events with a statement block, or probe handler, that is to be executed when any of those events occur.
The following example shows how to trace entry and exit from a function using two probes.
probe kernel.function("sys_mkdir") { log ("enter") }
probe kernel.function("sys_mkdir").return { log ("exit") }
To list the probe-able functions in the kernel, use the last-pass option to the translator. The output needs to be filtered because each inlined function instance is listed separately. The following statement is an example.
# stap -p2 -e 'probe kernel.function("*") {}' | sort | uniq
3.1. Probe definitions
The general syntax is as follows.
probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
Events are specified in a special syntax called probe points. There are several varieties of probe points defined by the translator, and tapset scripts may define others using aliases. The provided probe points are listed in the stapprobes(5) man pages.
The probe handler is interpreted relative to the context of each event. For events associated with kernel code, this context may include variables defined in the source code at that location. These target variables are presented to the script as variables whose names are prefixed with a dollar sign ($). They may be accessed only if the compiler used to compile the kernel preserved them, despite optimization. This is the same constraint imposed by a debugger when working with optimized code. Other events may have very little context.
3.2. Probe aliases
The general syntax is as follows.
probe <alias> = <probepoint> { <prologue_stmts> }
probe <alias> += <probepoint> { <epilogue_stmts> }
New probe points may be defined using aliases. A probe point alias looks similar to probe definitions, but instead of activating a probe at the given point, it defines a new probe point name as an alias to an existing one. New probe aliases may refer to one or more existing probe aliases. The following is an example.
probe socket.sendmsg = kernel.function ("sock_sendmsg") { ... }
probe socket.do_write = kernel.function ("do_sock_write") { ... }
probe socket.send = socket.sendmsg, socket.do_write { ... }
There are two types of aliases, the prologue style and the epilogue style which are identified by the equal sign (
=) and "+=" respectively.
A probe that names the new probe point will create an actual probe, with the handler of the alias pre-pended.
This pre-pending behavior serves several purposes. It allows the alias definition to pre-process the context of the probe before passing control to the handler specified by the user. This has several possible uses, demonstrated as follows.
# Skip probe unless given condition is met: if ($flag1 != $flag2) next # Supply values describing probes: name = "foo" # Extract the target variable to a plain local variable: var = $var
3.2.1. Prologue-style aliases (=)
For a prologue style alias, the statement block that follows an alias definition is implicitly added as a prologue to any probe that refers to the alias. The following is an example.
# Defines a new probe point syscall.read, which expands to
# kernel.function("sys_read"), with the given statement as
# a prologue.
#
probe syscall.read = kernel.function("sys_read") {
fildes = $fd
}
3.2.2. Epilogue-style aliases (+=)
The statement block that follows an alias definition is implicitly added as an epilogue to any probe that refers to the alias. It is not useful to define new variable there (since no subsequent code will see it), but rather the code can take action based upon variables left set by the prologue or by the user code. The following is an example:
# Defines a new probe point with the given statement as an
# epilogue.
#
probe syscall.read += kernel.function("sys_read") {
if (traceme) println ("tracing me")
}
3.2.3. Probe alias usage
A probe alias is used the same way as any built-in probe type, by naming it:
probe syscall.read {
printf("reading fd=%d\n", fildes)
}
3.3. Variables
Identifiers for variables and functions are alphanumeric sequences, and may include the underscore (_) and the dollar sign ($) characters. They may not start with a plain digit. Each variable is by default local to the probe or function statement block where it is mentioned, and therefore its scope and lifetime is limited to a particular probe or function invocation. Scalar variables are implicitly typed as either string or integer. Associative arrays also have a string or integer value, and a tuple of strings or integers serves as a key. Arrays must be declared as global. Local arrays are not allowed.
The translator performs type inference on all identifiers, including array indexes and function parameters. Inconsistent type-related use of identifiers results in an error.
Variables may be declared global. Global variables are shared among all probes and remain instantiated as long as the SystemTap session. There is one namespace for all global variables, regardless of the script file in which they are found. Because of possible concurrency limits, such as multiple probe handlers, each global variable used by a probe is automatically read- or write-locked while the handler is running. A global declaration may be written at the outermost level anywhere in a script file, not just within a block of code. Global variables which are written but never read will be displayed automatically at session shutdown. The following declaration marks
var1 and var2 as global. The translator will infer a value type for each, and if the variable is used as an array, its key types.
global var1[=<value>], var2[=<value>]
3.4. Auxiliary functions
General syntax:
function <name>[:<type>] ( <arg1>[:<type>], ... ) { <stmts> }
SystemTap scripts may define subroutines to factor out common work. Functions may take any number of scalar arguments, and must return a single scalar value. Scalars in this context are integers or strings. For more information on scalars, see Section Section 3.3, “Variables” and Section Section 5.2, “Data types”
. The following is an example function declaration.
function thisfn (arg1, arg2) {
return arg1 + arg2
}
Note the general absence of type declarations, which are inferred by the translator. If desired, a function definition may include explicit type declarations for its return value, its arguments, or both. This is helpful for embedded-C functions. In the following example, the type inference engine need only infer the type of arg2, a string.
function thatfn:string(arg1:long, arg2) {
return sprintf("%d%s", arg1, arg2)
}
Functions may call others or themselves recursively, up to a fixed nesting limit. See Section Section 1.6, “Safety and security”.
3.5. Embedded C
SystemTap supports a guru mode where script safety features such as code and data memory reference protection are removed. Guru mode is set by passing the ”-g” flag to the stap command. When in guru mode, the translator accepts embedded code enclosed between “%{” and “%}” markers in the script file. Embedded code is transcribed verbatim, without analysis, in sequence, into generated C code. At the outermost level of a script, guru mode may be useful to add #include instructions, or any auxiliary definitions for use by other embedded code.
3.6. Embedded C functions
General syntax:
function <name>:<type> ( <arg1>:<type>, ... ) %{ <C_stmts> %}
Embedded code is permitted in a function body. In that case, the script language body is replaced entirely by a piece of C code enclosed between %{ and %} markers. The enclosed code may do anything reasonable and safe as allowed by the parser.
There are a number of undocumented but complex safety constraints on concurrency, resource consumption and runtime limits that are applied to code written in the SystemTap language. These constraints are not applied to embedded C code, so use such code with caution as it is used verbatim. Be especially careful when dereferencing pointers. Use the kread() macro to dereference any pointers that could potentially be invalid or dangerous. If you are unsure, err on the side of caution and use kread(). The kread() macro is one of the safety mechanisms used in code generated by embedded C. It protects against pointer accesses that could crash the system.
For example, to access the pointer chain
name = skb->dev->name in embedded C, use the following code.
struct net_device *dev; char *name; dev = kread(&(skb->dev)); name = kread(&(dev->name));
The memory locations reserved for input and output values are provided to a function using a macro named
THIS . The following are examples.
function add_one (val) %{
THIS->__retvalue = THIS->val + 1;
}
function add_one_str (val) %{
strlcpy (THIS->__retvalue, THIS->val, MAXSTRINGLEN);
strlcat (THIS->__retvalue, "one", MAXSTRINGLEN);
}
The function argument and return value types must be inferred by the translator from the call sites in order for this method to work. You should examine C code generated for ordinary script language functions to write compatible embedded-C. Note that all SystemTap functions and probes run with interrupts disabled, thus you cannot call functions that might sleep from within embedded C.
Chapter 4. Probe points
4.1. General syntax
The general probe point syntax is a dotted-symbol sequence. This divides the event namespace into parts, analogous to the style of the Domain Name System. Each component identifier is parameterized by a string or number literal, with a syntax analogous to a function call.
The following are all syntactically valid probe points.
kernel.function("foo")
kernel.function("foo").return
module{"ext3"}.function("ext3_*")
kernel.function("no_such_function") ?
syscall.*
end
timer.ms(5000)
Probes may be broadly classified into synchronous or asynchronous. A synchronous event occurs when any processor executes an instruction matched by the specification. This gives these probes a reference point (instruction address) from which more contextual data may be available. Other families of probe points refer to asynchronous events such as timers, where no fixed reference point is related. Each probe point specification may match multiple locations, such as by using wildcards or aliases, and all are probed. A probe declaration may contain several specifications separated by commas, which are all probed.
4.1.2. Suffixes
Suffixes further qualify the point to probe, such as
.return for the exit point of a probed function. The absence of a suffix implies the function entry point.
4.1.3. Wildcarded file names, function names
A component may include an asterisk (*) character, which expands to other matching probe points. An example follows.
kernel.syscall.*
kernel.function("sys_*)
4.1.4. Optional probe points
A probe point may be followed by a question mark (?) character, to indicate that it is optional, and that no error should result if it fails to expand. This effect passes down through all levels of alias or wildcard expansion.
The following is the general syntax.
kernel.function("no_such_function") ?
4.2. Built-in probe point types (DWARF probes)
This family of probe points uses symbolic debugging information for the target kernel or module, as may be found in executables that have not been stripped, or in the separate
debuginfo packages. They allow logical placement of probes into the execution path of the target by specifying a set of points in the source or object code. When a matching statement executes on any processor, the probe handler is run in that context.
Points in a kernel are identified by module, source file, line number, function name or some combination of these.
Here is a list of probe point specifications currently supported:
kernel.function(PATTERN) kernel.function(PATTERN).call kernel.function(PATTERN).return kernel.function(PATTERN).return.maxactive(VALUE) kernel.function(PATTERN).inline kernel.function(PATTERN).label(LPATTERN) module(MPATTERN).function(PATTERN) module(MPATTERN).function(PATTERN).call module(MPATTERN).function(PATTERN).return.maxactive(VALUE) module(MPATTERN).function(PATTERN).inline kernel.statement(PATTERN) kernel.statement(ADDRESS).absolute module(MPATTERN).statement(PATTERN)
The
.function variant places a probe near the beginning of the named function, so that parameters are available as context variables.
The
.return variant places a probe at the moment of return from the named function, so the return value is available as the $return context variable. The entry parameters are also available, though the function may have changed their values. Return probes may be further qualified with .maxactive, which specifies how many instances of the specified function can be probed simultaneously. You can leave off .maxactive in most cases, as the default should be sufficient. However, if you notice an excessive number of skipped probes, try setting .maxactive to incrementally higher values to see if the number of skipped probes decreases.
The
.inline modifier for .function filters the results to include only instances of inlined functions. The .call modifier selects the opposite subset. Inline functions do not have an identifiable return point, so .return is not supported on .inline probes.
The
.statement variant places a probe at the exact spot, exposing those local variables that are visible there.
In the above probe descriptions, MPATTERN stands for a string literal that identifies the loaded kernel module of interest and LPATTERN stands for a source program label. Both MPATTERN and LPATTERN may include asterisk (*), square brackets "[]", and question mark (?) wildcards.
PATTERN stands for a string literal that identifies a point in the program. It is composed of three parts:
- The first part is the name of a function, as would appear in the nm program's output. This part may use the asterisk and question mark wildcard operators to match multiple names.
- The second part is optional, and begins with the ampersand (@) character. It is followed by the path to the source file containing the function, which may include a wildcard pattern, such as mm/slab*. In most cases, the path should be relative to the top of the linux source directory, although an absolute path may be necessary for some kernels. If a relative pathname doesn't work, try absolute.
- The third part is optional if the file name part was given. It identifies the line number in the source file, preceded by a “:” or “+”. The line number is assumed to be an absolute line number if preceded by a “:”, or relative to the entry of the function if preceded by a “+”. All the lines in the function can be matched with “:*”. A range of lines x through y can be matched with “:x-y”.
Alternately, specify PATTERN as a numeric constant to indicate a relative module address or an absolute kernel address.
Some of the source-level variables, such as function parameters, locals, or globals visible in the compilation unit, are visible to probe handlers. Refer to these variables by prefixing their name with a dollar sign within the scripts. In addition, a special syntax allows limited traversal of structures, pointers, and arrays.
$
var refers to an in-scope variable var. If it is a type similar to an integer, it will be cast to a 64-bit integer for script use. Pointers similar to a string (char *) are copied to SystemTap string values by the kernel_string() or user_string() functions.
$
var->field traverses a structure's field. The indirection operator may be repeated to follow additional levels of pointers.
$
var[N] indexes into an array. The index is given with a literal number.
$$
vars expands to a character string that is equivalent to sprintf("parm1=%x ... parmN=%x var1=%x ... varN=%x", $parm1, ..., $parmN, $var1, ..., $varN)
$$
locals expands to a character string that is equivalent to sprintf("var1=%x ... varN=%x", $var1, ..., $varN)
$$
parms expands to a character string that is equivalent to sprintf("parm1=%x ... parmN=%x", $parm1, ..., $parmN)
4.2.1. kernel.function, module().function
The
.function variant places a probe near the beginning of the named function, so that parameters are available as context variables.
General syntax:
kernel.function("func[@file]"
module("modname").function("func[@file]"
Examples:
# Refers to all kernel functions with "init" or "exit"
# in the name:
kernel.function("*init*"), kernel.function("*exit*")
# Refers to any functions within the "kernel/sched.c"
# file that span line 240:
kernel.function("*@kernel/sched.c:240")
# Refers to all functions in the ext3 module:
module("ext3").function("*")
4.2.2. kernel.statement, module().statement
The
.statement variant places a probe at the exact spot, exposing those local variables that are visible there.
General syntax:
kernel.statement("func@file:linenumber")
module("modname").statement("func@file:linenumber")
Example:
# Refers to the statement at line 2917 within the
# kernel/sched.c file:
kernel.statement("*@kernel/sched.c:2917")
# Refers to the statement at line bio_init+3 within the fs/bio.c file:
kernel.statement("bio_init@fs/bio.c+3")
4.3. DWARF-less probing
In the absence of debugging information, you can still use the kprobe family of probes to examine the entry and exit points of kernel and module functions. You cannot look up the arguments or local variables of a function using these probes. However, you can access the parameters by following this procedure:
When you're stopped at the entry to a function, you can refer to the function's arguments by number. For example, when probing the function declared:
asmlinkage ssize_t sys_read(unsigned int fd, char __user * buf, size_t count)
You can obtain the values of
fd, buf, and count, respectively, as uint_arg(1), pointer_arg(2), and ulong_arg(3). In this case, your probe code must first call asmlinkage(), because on some architectures the asmlinkage attribute affects how the function's arguments are passed.
When you're in a return probe, $
return isn't supported without DWARF, but you can call returnval() to get the value of the register in which the function value is typically returned, or call returnstr() to get a string version of that value.
And at any code probepoint, you can call
register("regname") to get the value of the specified CPU register when the probe point was hit. u_register("regname") is like register("regname"), but interprets the value as an unsigned integer.
SystemTap supports the following constructs:
kprobe.function(FUNCTION) kprobe.function(FUNCTION).return kprobe.module(NAME).function(FUNCTION) kprobe.module(NAME).function(FUNCTION).return kprobe.statement(ADDRESS).absolute
Use
.function probes for kernel functions and .module probes for probing functions of a specified module. If you do not know the absolute address of a kernel or module function, use .statement probes. Do not use wildcards in FUNCTION and MODULE names. Wildcards cause the probe to not register. Also, run statement probes in guru mode only.
4.4. Marker probes
This family of probe points connects to static probe markers inserted into the kernel or a module. These markers are special macro calls in the kernel that make probing faster and more reliable than with DWARF-based probes. DWARF debugging information is not required to use probe markers.
Marker probe points begin with a kernel or module("name") prefix, the same as DWARF probes. This prefix identifies the source of the symbol table used for finding markers. The suffix names the marker itself: mark("name"). The marker name string, which may contain wildcard characters, is matched against the names given to the marker macros when the kernel or module was compiled.
The handler associated with a marker probe reads any optional parameters specified at the macro call site named $arg1 through $argNN, where NN is the number of parameters supplied by the macro. Number and string parameters are passed in a type-safe manner.
4.5. Timer probes
You can use intervals defined by the standard kernel jiffies timer to trigger probe handlers asynchronously. A jiffy is a kernel-defined unit of time typically between 1 and 60 msec. Two probe point variants are supported by the translator:
timer.jiffies(N) timer.jiffies(N).randomize(M)
The probe handler runs every N jiffies. If the
randomize component is given, a linearly distributed random value in the range [-M … +M] is added to N every time the handler executes. N is restricted to a reasonable range (1 to approximately 1,000,000), and M is restricted to be less than N. There are no target variables provided in either context. Probes can be run concurrently on multiple processors.
Intervals may be specified in units of time. There are two probe point variants similar to the jiffies timer:
timer.ms(N) timer.ms(N).randomize(M)
Here, N and M are specified in milliseconds, but the full options for units are seconds (s or sec), milliseconds (ms or msec), microseconds (us or usec), nanoseconds (ns or nsec), and hertz (hz). Randomization is not supported for hertz timers.
The resolution of the timers depends on the target kernel. For kernels prior to 2.6.17, timers are limited to jiffies resolution, so intervals are rounded up to the nearest jiffies interval. After 2.6.17, the implementation uses hrtimers for tighter precision, though the resulting resolution will be dependent upon architecture. In either case, if the randomize component is given, then the random value will be added to the interval before any rounding occurs.
Profiling timers are available to provide probes that execute on all CPUs at each system tick. This probe takes no parameters, as follows.
timer.profile
Full context information of the interrupted process is available, making this probe suitable for implementing a time-based sampling profiler.
The following is an example of timer usage.
# Refers to a periodic interrupt, every 1000 jiffies: timer.jiffies(1000) # Fires every 5 seconds: timer.sec(5) # Refers to a periodic interrupt, every 1000 +/- 200 jiffies: timer.jiffies(1000).randomize(200)
4.6. Return probes
The
.return variant places a probe at the moment of return from the named function, so that the return value is available as the $return context variable. The entry parameters are also accessible in the context of the return probe, though their values may have been changed by the function. Inline functions do not have an identifiable return point, so .return is not supported on .inline probes.
4.7. Special probe points
The probe points
begin and end are defined by the translator to refer to the time of session startup and shutdown. There are no target variables available in either context.
4.7.1. begin
The
begin probe is the start of the SystemTap session. All begin probe handlers are run during the startup of the session. All global variables must be declared prior to this point.
4.7.2. end
The
end probe is the end of the SystemTap session. All end probes are run during the normal shutdown of a session, such as in the aftermath of an exit function call, or an interruption from the user. In the case of an shutdown triggered by error, end probes are not run.
4.7.3. begin and end probe sequence
begin and end probes are specified with an optional sequence number that controls the order in which they are run. If no sequence number is provided, the sequence number defaults to zero and probes are run in the order that they occur in the script file. Sequence numbers may be either positive or negative, and are especially useful for tapset writers who want to do initialization in a begin probe. The following are examples.
# In a tapset file:
probe begin(-1000) { ... }
# In a user script:
probe begin { ... }
The user script
begin probe defaults to sequence number zero, so the tapset begin probe will run first.
4.7.4. never
The
never probe point is defined by the translator to mean never. Its statements are analyzed for symbol and type correctness, but its probe handler is never run. This probe point may be useful in conjunction with optional probes. See Section Section 4.1.4, “Optional probe points”.
4.8. Probes to monitor performance
The perfmon family of probe points is used to access the performance monitoring hardware available in modern processors. These probe points require perfmon2 support in the kernel to access the hardware.
Performance monitor hardware points have a
perfmon prefix. The suffix names the event being counted, for example counter(event). The event names are specific to the processor implementation, except for generic cycle and instructions events, which are available on all processors. The probe perfmon.counter(event) starts a counter on the processor which counts the number of events that occur on that processor. For more details about the performance monitoring events available on a specific processor, see the help text returned by typing the perfmon2 command pfmon -l.
4.8.1. $counter
$counter is a handle used in the body of a probe for operations involving the counter associated with the probe.
4.8.2. read_counter
read_counter is a function passed to the handle for a perfmon probe. It returns the current count for the event.
Chapter 5. Language elements
5.1. Identifiers
Identifiers are used to name variables and functions. They are an alphanumeric sequence that may include the underscore (_) and dollar sign ($) characters. They have the same syntax as C identifiers, except that the dollar sign is also a legal character. Identifiers that begin with a dollar sign are interpreted as references to variables in the target software, rather than to SystemTap script variables. Identifiers may not start with a plain digit.
5.2. Data types
The SystemTap language includes a small number of data types, but no type declarations. A variable's type is inferred from its use. To support this inference, the translator enforces consistent typing of function arguments and return values, array indices and values. There are no implicit type conversions between strings and numbers. Inconsistent type-related use of identifiers signals an error.
5.2.1. Literals
Literals are either strings or integers. Literals can be expressed as decimal, octal, or hexadecimal, using C notation. Type suffixes (e.g., L or U) are not used.
5.2.2. Integers
Integers are decimal, hexadecimal, or octal, and use the same notation as in C. Integers are 64-bit signed quantities, although the parser also accepts (and wraps around) values above positive
263 but below 264.
5.2.3. Strings
Strings are enclosed in quotation marks (“string”), and pass through standard C escape codes with backslashes. Strings are limited in length to MAXSTRINGLEN. For more information about this and other limits, see Section Section 1.6, “Safety and security”.
5.2.4. Associative arrays
See Section Chapter 7, Associative arrays
5.2.5. Statistics
See Section Chapter 8, Statistics (aggregates)
5.3. Semicolons
The semicolon is the null statement, or do nothing statement. It is optional, and useful as a separator between statements to improve detection of syntax errors and to reduce ambiguities in grammar.
5.4. Comments
Three forms of comments are supported, as follows.
# ... shell style, to the end of line // ... C++ style, to the end of line /* ... C style ... */
5.5. Whitespace
As in C, spaces, tabs, returns, newlines, and comments are treated as whitespace. Whitespace is ignored by the parser.
5.6. Expressions
SystemTap supports a number of operators that use the same general syntax, semantics, and precedence as in C and awk. Arithmetic is performed per C rules for signed integers. If the parser detects division by zero or an overflow, it generates an error. The following subsections list these operators.
5.6.4. String assignment operators
= .=
5.6.11. <value> in <array_name>
This expression evaluates to true if the array contains an element with the specified index.
5.6.12. [ <value>, … ] in <array_name>
The number of index values must match the number of indexes previously specified.
5.7. Literals passed in from the stap command line
Literals are either strings enclosed in double quotes (” ”) or integers. For information about integers, see Section Section 5.2.2, “Integers”. For information about strings, see Section Section 5.2.3, “Strings”.
Script arguments at the end of a command line are expanded as literals. You can use these in all contexts where literals are accepted. A reference to a nonexistent argument number is an error.
5.7.2. @1 … @<NN> for strings
Use
@1 … @<NN> for casting as a string literal.
5.7.3. Examples
For example, if the following script named example.stp
probe begin { printf("%d, %s\n", $1, @2) }
is invoked as follows
# stap example.stp 10 mystring
then 10 is substituted for $1 and "mystring" for @2. The output will be
10, mystring
5.8. Conditional compilation
5.8.1. Conditions
One of the steps of parsing is a simple conditional preprocessing stage. The general form of this is similar to the ternary operator (Section Section 5.6.7, “Ternary operator”).
%( CONDITION %? TRUE-TOKENS %) %( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)
The CONDITION is a limited expression whose format is determined by its first keyword. The following is the general syntax.
%( <condition> %? <code> [ %: <code> ] %)
5.8.2. Conditions based on kernel version: kernel_v, kernel_vr
If the first part of a conditional expression is the identifier
kernel_v or kernel_vr, the second part must be one of six standard numeric comparison operators “<”, “<=”, “==”, “!=”, “>”, or “>=”, and the third part must be a string literal that contains an RPM-style version-release value. The condition returns true if the version of the target kernel (as optionally overridden by the -r option) matches the given version string. The comparison is performed by the glibc function strverscmp.
kernel_v refers to the kernel version number only, such as “2.6.13".
kernel_vr refers to the kernel version number including the release code suffix, such as “2.6.13-1.322FC3smp”.
5.8.3. Conditions based on architecture: arch
If the first part of the conditional expression is the identifier
arch which refers to the processor architecture, then the second part is a string comparison operator ”==” or ”!=”, and the third part is a string literal for matching it. This comparison is a simple string equality or inequality. The currently supported architecture strings are i386, i686, x86_64, ia64, s390x and ppc64.
5.8.4. True and False Tokens
TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens, possibly including nested preprocessor conditionals, that are pasted into the input stream if the condition is true or false. For example, the following code induces a parse error unless the target kernel version is newer than 2.6.5.
%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence
The following code adapts to hypothetical kernel version drift.
probe kernel.function (
%( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
%( kernel_vr == "2.6.13-1.8273FC3smp" %? "do_page_fault" %: UNSUPPORTED %)
%)) { /* ... */ }
%( arch == "ia64" %?
probe syscall.vliw = kernel.function("vliw_widget") {}
%)
Chapter 6. Statement types
Statements enable procedural control flow within functions and probe handlers. The total number of statements executed in response to any single probe event is limited to MAXACTION, which defaults to 1000. See Section Section 1.6, “Safety and security”.
6.1. break and continue
Use
break or continue to exit or iterate the innermost nesting loop statement, such as within a while, for, or foreach statement. The syntax and semantics are the same as those used in C.
6.2. delete
delete removes an element.
The following statement removes from ARRAY the element specified by the index tuple. The value will no longer be available, and subsequent iterations will not report the element. It is not an error to delete an element that does not exist.
delete ARRAY[INDEX1, INDEX2, ...]
The following syntax removes all elements from ARRAY:
delete ARRAY
The following statement removes the value of SCALAR. Integers and strings are cleared to zero and null ("") respectively, while statistics are reset to their initial empty state.
delete SCALAR
6.4. EXP (expression)
An
expression executes a string- or integer-valued expression and discards the value.
6.5. for
General syntax:
for (EXP1; EXP2; EXP3) STMT
The
for statement is similar to the for statement in C. The for expression executes EXP1 as initialization. While EXP2 is non-zero, it executes STMT, then the iteration expression EXP3.
6.6. foreach
General syntax:
foreach (VAR in ARRAY) STMT
The
foreach statement loops over each element of a named global array, assigning the current key to VAR. The array must not be modified within the statement. If you add a single plus (+) or minus (-) operator after the VAR or the ARRAY identifier, the iteration order will be sorted by the ascending or descending index or value.
The following statement behaves the same as the first example, except it is used when an array is indexed with a tuple of keys. Use a sorting suffix on at most one VAR or ARRAY identifier.
foreach ([VAR1, VAR2, ...] in ARRAY) STMT
The following statement is the same as the first example, except that the
limit keyword limits the number of loop iterations to EXP times. EXP is evaluated once at the beginning of the loop.
foreach (VAR in ARRAY limit EXP) STMT
6.7. if
General syntax:
if (EXP) STMT1 [ else STMT2 ]
The
if statement compares an integer-valued EXP to zero. It executes the first STMT if non-zero, or the second STMT if zero.
The
if command has the same syntax and semantics as used in C.
6.9. ; (null statement)
General syntax:
statement1 ; statement2
The semicolon represents the null statement, or do nothing. It is useful as an optional separator between statements to improve syntax error detection and to handle certain grammar ambiguities.
6.10. return
General syntax:
return EXP
The
return statement returns the EXP value from the enclosing function. If the value of the function is not returned, then a return statement is not needed, and the function will have a special unknown type with no return value.
6.11. { } (statement block)
This is the statement block with zero or more statements enclosed within brackets. The following is the general syntax:
{ STMT1 STMT2 ... }
The statement block executes each statement in sequence in the block. Separators or terminators are generally not necessary between statements. The statement block uses the same syntax and semantics as in C.
Chapter 7. Associative arrays
Associative arrays are implemented as hash tables with a maximum size set at startup. Associative arrays are too large to be created dynamically for individual probe handler runs, so they must be declared as global. The basic operations for arrays are setting and looking up elements. These operations are expressed in awk syntax: the array name followed by an opening bracket ([), a comma-separated list of up to five index index expressions, and a closing bracket (]). Each index expression may be a string or a number, as long as it is consistently typed throughout the script.
7.1. Examples
# Increment the named array slot: foo [4,"hello"] ++ # Update a statistic: processusage [uid(),execname()] ++ # Set a timestamp reference point: times [tid()] = get_cycles() # Compute a timestamp delta: delta = get_cycles() - times [tid()]
7.2. Types of values
Array elements may be set to a number or a string. The type must be consistent throughout the use of the array. The first assignment to the array defines the type of the elements. Unset array elements may be fetched and return a null value (zero or empty string) as appropriate, but they are not seen by a membership test.
7.3. Array capacity
Array sizes can be specified explicitly or allowed to default to the maximum size as defined by MAXMAPENTRIES. See Section Section 1.6, “Safety and security” for details on changing MAXMAPENTRIES.
You can explicitly specify the size of an array as follows:
global ARRAY[<size>]
If you do not specify the size parameter, then the array is created to hold MAXMAPENTRIES number of elements
7.4. Iteration, foreach
Like awk, SystemTap's foreach creates a loop that iterates over key tuples of an array, not only values. The iteration may be sorted by any single key or a value by adding an extra plus symbol (+) or minus symbol (-) to the code. The following are examples.
# Simple loop in arbitrary sequence:
foreach ([a,b] in foo)
fuss_with(foo[a,b])
# Loop in increasing sequence of value:
foreach ([a,b] in foo+) { ... }
# Loop in decreasing sequence of first key:
foreach ([a-,b] in foo) { ... }
The
break and continue statements also work inside foreach loops. Since arrays can be large but probe handlers must execute quickly, you should write scripts that exit iteration early, if possible. For simplicity, SystemTap forbids any modification of an array during iteration with a foreach.
Chapter 8. Statistics (aggregates)
Aggregate instances are used to collect statistics on numerical values, when it is important to accumulate new data quickly and in large volume. These instances operate without exclusive locks, and store only aggregated stream statistics. Aggregates make sense only for global variables. They are stored individually or as elements of an array.
8.1. The aggregation (<<<) operator
The aggregation operator is “<<<”, and its effect is similar to an assignment or a C++ output streaming operation. The left operand specifies a scalar or array-index l-value, which must be declared global. The right operand is a numeric expression. The meaning is intuitive: add the given number to the set of numbers to compute their statistics. The specific list of statistics to gather is given separately by the extraction functions. The following is an example.
a <<< delta_timestamp writes[execname()] <<< count
8.2. Extraction functions
For each instance of a distinct extraction function operating on a given identifier, the translator computes a set of statistics. With each execution of an extraction function, the aggregation is computed for that moment across all processors. The first argument of each function is the same style of l-value as used on the left side of the aggregation operation.
8.3. Integer extractors
The following functions provide methods to extract information about integer values.
8.4. Histogram extractors
The following functions provide methods to extract histogram information. Printing a histogram with the print family of functions renders a histogram object as a tabular "ASCII art" bar chart.
8.4.1. @hist_linear
The statement
@hist_linear(v,L,H,W) represents a linear histogram v, where L and H represent the lower and upper end of a range of values and W represents the width (or size) of each bucket within the range. The low and high values can be negative, but the overall difference (high minus low) must be positive. The width parameter must also be positive.
In the output, a range of consecutive empty buckets may be replaced with a tilde (~) character. This can be controlled on the command line with -DHIST_ELISION=<num>, where <num> specifies how many empty buckets at the top and bottom of the range to print. The default is 2. A <num> of 0 removes all empty buckets. A negative <num> turns off bucket removal all together.
For example, if you specify -DHIST_ELISION=3 and the histogram has 10 consecutive empty buckets, the first 3 and last 3 empty buckets will be printed and the middle 4 empty buckets will be represented by a tilde (~).
The following is an example.
global reads
probe netdev.receive {
reads <<< length
}
probe end {
print(@hist_linear(reads, 0, 10240, 200))
}
This generates the following output.
value |-------------------------------------------------- count 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1650 200 | 8 400 | 0 600 | 0 ~ 1000 | 0 1200 | 0 1400 | 1 1600 | 0 1800 | 0
This shows that 1650 network reads were of a size between 0 and 200 bytes, 8 reads were between 200 and 400 bytes, and 1 read was between 1200 and 1400 bytes. The tilde (~) character indicates buckets 700, 800 and 900 were removed because they were empty. Empty buckets at the upper end were also removed.
8.4.2. @hist_log
The statement
@hist_log(v) represents a base-2 logarithmic histogram. Empty buckets are replaced with a tilde (~) character in the same way as @hist_linear() (see above).
The following is an example.
global reads
probe netdev.receive {
reads <<< length
}
probe end {
print(@hist_log(reads))
}
This generates the following output.
value |-------------------------------------------------- count 8 | 0 16 | 0 32 | 254 64 | 3 128 | 2 256 | 2 512 | 4 1024 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 16689 2048 | 0 4096 | 0
Chapter 9. Predefined functions
Unlike built-in functions, predefined functions are implemented in tapsets.
9.1. Output functions
The following sections describe the functions you can use to output data.
9.1.1. error
General syntax:
error:unknown (msg:string)
This function logs the given string to the error stream. It appends an implicit end-of-line. It blocks any further execution of statements in this probe. If the number of errors exceeds the MAXERRORS parameter, it triggers an
exit.
9.1.2. log
General syntax:
log:unknown (msg:string) log (const char *fmt, )
This function logs data.
log sends the message immediately to staprun and to the bulk transport (relayfs) if it is being used. If the last character given is not a newline, then one is added.
This function is not as efficient as printf and should only be used for urgent messages.
9.1.4. printf
General syntax:
printf:unknown (fmt:string, )
The printf function takes a formatting string as an argument, and a number of values of corresponding types, and prints them all. The format must be a literal string constant. The printf formatting directives are similar to those of C, except that they are fully checked for type by the translator.
The formatting string can contain tags that are defined as follows:
%[flags][width][.precision][length]specifier
Where
specifier is required and defines the type and the interpretation of the value of the corresponding argument. The following table shows the details of the specifier parameter:
Table 9.1. printf specifier values
Specifier | Output | Example |
| d or i | Signed decimal | 392 |
| o | Unsigned octal | 610 |
| s | String | sample |
| u | Unsigned decimal | 7235 |
| x | Unsigned hexadecimal (lowercase letters) | 7fa |
| X | Unsigned hexadecimal (uppercase letters) | 7FA |
| p | Pointer address | 0x0000000000bc614e |
| b | Writes a binary value as text. The field width specifies the number of bytes to write. Valid specifications are %b, %1b, %2b, %4b and %8b. The default width is 8 (64-bits). | See below |
| % | A % followed by another % character will write % to stdout. | % |
The tag can also contain
flags, width, .precision and modifiers sub-specifiers, which are optional and follow these specifications:
Table 9.2. printf flag values
Flags | Description | |
| - (minus sign) | Left-justify within the given field width. Right justification is the default (see width sub-specifier). | |
| + (plus sign) | Precede the result with a plus or minus sign even for positive numbers. By default, only negative numbers are preceded with a minus sign. | |
| (space) | If no sign is going to be written, a blank space is inserted before the value. | |
| # | Used with o, x or X specifiers the value is preceded with 0, 0x or 0X respectively for non-zero values. | |
| 0 | Left-pads the number with zeroes instead of spaces, where padding is specified (see width sub-specifier). |
Table 9.3. printf width values
Width | Description | |
| (number) | Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger. |
Table 9.4. printf precision values
Precision | Description | |
| .number | For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. When no precision is specified, the default is 1. If the period is specified without an explicit value for precision, 0 is assumed. |
Binary Write Examples
The following is an example of using the binary write functions:
probe begin {
for (i = 97; i < 110; i++)
printf("%3d: %1b%1b%1b\n", i, i, i-32, i-64)
exit()
}
This prints:
97: aA! 98: bB" 99: cC# 100: dD$ 101: eE% 102: fF& 103: gG' 104: hH( 105: iI) 106: jJ* 107: kK+ 108: lL, 109: mM-
Another example:
stap -e 'probe begin{printf("%b%b", 0xc0dedbad, \
0x12345678);exit()}' | hexdump -C
This prints:
00000000 ad db de c0 00 00 00 00 78 56 34 12 00 00 00 00 |........xV4.....| 00000010
Another example:
probe begin{
printf("%1b%1b%1blo %1b%1brld\n", 72,101,108,87,111)
exit()
}
This prints:
Hello World
9.1.5. printd
General syntax:
printd:unknown (delimiter:string, )
This function takes a string delimiter and two or more values of any type, then prints the values with the delimiter interposed. The delimiter must be a literal string constant.
For example:
printd("/", "one", "two", "three", 4, 5, 6)
prints:
one/two/three/4/5/6
9.1.6. printdln
General syntax:
printdln:unknown ()
This function operates like
printd, but also appends a newline.
9.1.7. println
General syntax:
println:unknown ()
This function operates like
print, but also appends a newline.
9.1.8. sprint
General syntax:
sprint:unknown ()
This function operates like
print, but returns the string rather than printing it.
9.1.9. sprintf
General syntax:
sprintf:unknown (fmt:string, )
This function operates like
printf, but returns the formatted string rather than printing it.
9.2. Context at the probe point
The following functions provide ways to access the current task context at a probe point. Note that these may not return correct values when a probe is hit in interrupt context.
9.2.1. backtrace
General syntax:
backtrace:string ()
Returns a string of hex addresses that are a backtrace of the stack. The output is truncated to MAXSTRINGLEN.
9.2.2. caller
General syntax:
caller:string()
Returns the address and name of the calling function. It works only for return probes.
9.2.3. caller_addr
General syntax:
caller_addr:long ()
Returns the address of the calling function. It works only for return probes.
9.2.9. is_return
General syntax:
is_return:long ()
Returns 1 if the probe point is a return probe, else it returns zero.
Deprecated.
9.2.15. print_backtrace
General syntax:
print_backtrace:unknown ()
This function is equivalent to
print_stack(backtrace()), except that deeper stack nesting is supported. The function does not return a value.
9.2.17. print_stack
General syntax:
print_stack:unknown (stk:string)
This function performs a symbolic lookup of the addresses in the given string, which is assumed to be the result of a prior call to
backtrace(). It prints one line per address. Each printed line includes the address, the name of the function containing the address, and an estimate of its position within that function. The function does not return a value.
9.2.19. stack_unused
General syntax:
stack_unused:long ()
Returns how many bytes are currently unused in the stack.
9.2.20. stack_used
General syntax:
stack_used:long ()
Returns how many bytes are currently used in the stack.
9.2.22. target
General syntax:
target:long ()
Returns the process ID of the target process. This is useful in conjunction with the -x PID or -c CMD command-line options to stap. An example of its use is to create scripts that filter on a specific process.
-x <pid>
target() returns the pid specified by -x
-c <command>
target() returns the pid for the executed command specified by -c.
9.3. Task data
These functions return data about a task. They all require a task handle as input, such as the value return by task_current() or the variables prev_task and next_task in the scheduler.ctxswitch probe alias.
9.3.1. task_cpu
General syntax:
task_cpu:long (task:long)
Returns the scheduled cpu for the given task.
9.3.2. task_current
General syntax:
task_current:long ()
Returns the address of the task_struct representing the current process. This address can be passed to the various task_*() functions to extract more task-specific data.
9.3.3. task_egid
General syntax:
task_egid:long (task:long)
Returns the effective group ID of the given task.
9.3.4. task_execname
General syntax:
task_execname:string (task:long)
Returns the name of the given task.
9.3.5. task_euid
General syntax:
task_euid:long (task:long)
Returns the effective user ID of the given task.
9.3.7. task_nice
General syntax:
task_nice:long (task:long)
Returns the nice value of the given task.
9.3.8. task_parent
General syntax:
task_parent:long (task:long)
Returns the address of the parent task_struct of the given task. This address can be passed to the various task_*() functions to extract more task-specific data.
9.3.10. task_prio
General syntax:
task_prio:long (task:long)
Returns the priority value of the given task.
9.3.11. task_state
General syntax:
task_state:long (task:long)
Returns the state of the given task. Possible states are:
TASK_RUNNING 0 TASK_INTERRUPTIBLE 1 TASK_UNINTERRUPTIBLE 2 TASK_STOPPED 4 TASK_TRACED 8 EXIT_ZOMBIE 16 EXIT_DEAD 32
9.4. Accessing string data at a probe point
The following functions provide methods to access string data at a probe point.
9.4.1. kernel_string
General syntax:
kernel_string:string (addr:long)
Copies a string from kernel space at a given address. The validation of this address is only partial.
9.4.2. user_string
General syntax:
user_string:string (addr:long)
This function copies a string from user space at a given address. The validation of this address is only partial. In rare cases when userspace data is not accessible, this function returns the string
<unknown>.
9.4.3. user_string2
General syntax:
user_string2:string (addr:long, err_msg:string)
This function is similar to
user_string, (Section Section 9.4.2, “user_string”) but allows passing an error message as an argument to be returned if userspace data is not available.
9.5. Initializing queue statistics
The queue_stats tapset provides functions that, when given notification of queuing events like wait, run, or done, track averages such as queue length, service and wait times, and utilization. Call the following three functions from appropriate probes, in sequence.
9.5.1. qs_wait
General syntax:
qs_wait:unknown (qname:string)
This function records that a new request was enqueued for the given queue name.
9.6. Using queue statistics
Functions with the qsq_ prefix query the statistics averaged since the first queue operation or when qsq_start was called. Since statistics are often fractional, a scale parameter multiplies the result to a more useful scale. For some fractions, a scale of 100 returns percentage numbers.
9.6.1. qsq_blocked
General syntax:
qsq_blocked:long (qname:string, scale:long)
This function returns the fraction of elapsed time during which one or more requests were on the wait queue.
9.6.2. qsq_print
General syntax:
qsq_print:unknown (qname:string)
This function prints a line containing the following statistics for the given queue:
- queue name
- average rate of requests per second
- average wait queue length
- average time on the wait queue
- average time to service a request
- percentage of time the wait queue was used
- percentage of time any request was being serviced
9.6.3. qsq_service_time
General syntax:
qsq_service_time:long (qname:string, scale:long)
This function returns the average time in microseconds required to service a request once it is removed from the wait queue.
9.6.4. qsq_start
General syntax:
qsq_start:unknown (qname:string)
This function resets the statistics counters for the given queue, and restarts tracking from the moment the function was called. This command is used to create a queue.
9.6.5. qsq_throughput
General syntax:
qsq_throughput:long (qname:string, scale:long)
This function returns the average number of requests served per microsecond.
9.6.6. qsq_utilization
General syntax:
qsq_utilization:long (qname:string, scale:long)
This function returns the average time in microseconds that at least one request was being serviced.
9.6.7. qsq_wait_queue_length
General syntax:
qsq_wait_queue_length:long (qname:string, scale:long)
This function returns the average length of the wait queue.
9.6.8. qsq_wait_time
General syntax:
qsq_wait_time:long (qname:string, scale:long)
This function returns the average time in microseconds that it took for a request to be serviced (qs_wait() to qs_done()).
9.6.9. A queue example
What follows is an example from src/testsuite/systemtap.samples/queue_demo.stp. It uses the randomize feature of the timer probe to simulate queuing activity.
probe begin {
qsq_start ("block-read")
qsq_start ("block-write")
}
probe timer.ms(3500), end {
qsq_print ("block-read")
qsq_start ("block-read")
qsq_print ("block-write")
qsq_start ("block-write")
}
probe timer.ms(10000) {
exit ()
}
# synthesize queue work/service using three randomized "threads" for each queue.
global tc
function qs_doit (thread, name) {
n = tc[thread] = (tc[thread]+1) % 3 # per-thread state counter
if (n==1) qs_wait (name)
else if (n==2) qs_run (name)
else if (n==0) qs_done (name)
}
probe timer.ms(100).randomize(100) { qs_doit (0, "block-read") }
probe timer.ms(100).randomize(100) { qs_doit (1, "block-read") }
probe timer.ms(100).randomize(100) { qs_doit (2, "block-read") }
probe timer.ms(100).randomize(100) { qs_doit (3, "block-write") }
probe timer.ms(100).randomize(100) { qs_doit (4, "block-write") }
probe timer.ms(100).randomize(100) { qs_doit (5, "block-write") }
This prints:
block-read: 9 ops/s, 1.090 qlen, 215749 await, 96382 svctm, 69% wait, 64% util block-write: 9 ops/s, 0.992 qlen, 208485 await, 103150 svctm, 69% wait, 61% util block-read: 9 ops/s, 0.968 qlen, 197411 await, 97762 svctm, 63% wait, 63% util block-write: 8 ops/s, 0.930 qlen, 202414 await, 93870 svctm, 60% wait, 56% util block-read: 8 ops/s, 0.774 qlen, 192957 await, 99995 svctm, 58% wait, 62% util block-write: 9 ops/s, 0.861 qlen, 193857 await, 101573 svctm, 56% wait, 64% util
9.7. Probe point identification
The following functions help you identify probe points.
9.7.1. pp
General syntax:
pp:string ()
This function returns the probe point associated with a currently running probe handler, including alias and wild-card expansion effects.
9.8. Formatting functions
The following functions help you format output.
9.8.1. ctime
General syntax:
ctime:string(epochsecs:long)
This function accepts an argument of seconds since the epoch as returned by
gettimeofday_s(). It returns a date string in UTC of the form:
"Wed Jun 30 21:49:008 2006"
This function does not adjust for timezones. The returned time is always in GMT. Your script must manually adjust epochsecs before passing it to ctime() if you want to print local time.
9.8.2. errno_str
General syntax:
errno_str:string (err:long)
This function returns the symbolic string associated with the given error code, such as ENOENT for the number 2, or E#3333 for an out-of-range value such as 3333.
9.8.3. returnstr
General syntax:
returnstr:string (returnp:long)
This function is used by the syscall tapset, and returns a string. Set returnp equal to 1 for decimal, or 2 for hex.
9.8.4. thread_indent
General syntax:
thread_indent:string (delta:long)
This function returns a string with appropriate indentation for a thread. Call it with a small positive or matching negative delta. If this is the outermost, initial level of indentation, then the function resets the relative timestamp base to zero.
The following example uses thread_indent() to trace the functions called in the drivers/usb/core kernel source. It prints a relative timestamp and the name and ID of the current process, followed by the appropriate indent and the function name. Note that "swapper(0)" indicates the kernel is running in interrupt context and there is no valid current process.
probe kernel.function("*@drivers/usb/core/*") {
printf ("%s -> %s\n", thread_indent(1), probefunc())
}
probe kernel.function("*@drivers/usb/core/*").return {
printf ("%s <- %s\n", thread_indent(-1), probefunc())
}
This prints:
0 swapper(0): -> usb_hcd_irq 8 swapper(0): <- usb_hcd_irq 0 swapper(0): -> usb_hcd_irq 10 swapper(0): -> usb_hcd_giveback_urb 16 swapper(0): -> urb_unlink 22 swapper(0): <- urb_unlink 29 swapper(0): -> usb_free_urb 35 swapper(0): <- usb_free_urb 39 swapper(0): <- usb_hcd_giveback_urb 45 swapper(0): <- usb_hcd_irq 0 usb-storage(1338): -> usb_submit_urb 6 usb-storage(1338): -> usb_hcd_submit_urb 12 usb-storage(1338): -> usb_get_urb 18 usb-storage(1338): <- usb_get_urb 25 usb-storage(1338): <- usb_hcd_submit_urb 29 usb-storage(1338): <- usb_submit_urb 0 swapper(0): -> usb_hcd_irq 7 swapper(0): <- usb_hcd_irq
9.9. String functions
The following are string functions you can use.
9.9.1. isinstr
General syntax:
isinstr:long (s1:string, s2:string)
This function returns 1 if string s1 contains string s2, otherwise zero.
9.9.2. strlen
General syntax:
strlen:long (str:string)
This function returns the number of characters in str.
9.9.3. strtol
General syntax:
strtol:long (str:string, base:long)
This function converts the string representation of a number to an integer. The base parameter indicates the number base to assume for the string (e.g. 16 for hex, 8 for octal, 2 for binary).
9.9.4. substr
General syntax:
substr:string (str:string, start:long, stop:long)
This function returns the substring of
str starting from character position start and ending at character position stop.
9.9.5. text_str
General syntax:
text_str:string (input:string)
This function accepts a string argument. Any ASCII characters in the string that are not printable are replaced by a corresponding escape sequence in the returned string.
9.9.6. text_strn
General syntax:
text_strn:string (input:string, len:long, quoted:long)
This function accepts a string of length
len. Any ASCII characters that are not printable are replaced by a corresponding escape sequence in the returned string. If quoted is not null, the function adds a backslash character to the output.
9.9.7. tokenize
General syntax:
tokenize:string (input:string, delim:string)
This function returns the next non-empty token in the given input string, where the tokens are delimited by characters in the delim string. If the input string is non-NULL, it returns the first token. If the input string is NULL, it returns the next token in the string passed in the previous call to tokenize. If no delimiter is found, the entire remaining input string is returned. It returns NULL when no more tokens are available.
9.10. Timestamps
The following functions provide methods to extract time data.
9.10.1. get_cycles
General syntax:
get_cycles:long ()
This function returns the processor cycle counter value if available, else it returns zero.
9.10.2. gettimeofday_ms
General syntax:
gettimeofday_ms:long ()
This function returns the number of milliseconds since the UNIX epoch.
9.10.3. gettimeofday_ns
General syntax:
gettimeofday_ns:long ()
This function returns the number of nanoseconds since the UNIX epoch.
9.11. Miscellaneous tapset functions
The following are miscellaneous functions.
9.11.1. addr_to_node
General syntax:
addr_to_node:long (addr:long)
This function accepts an address, and returns the node that the given address belongs to in a NUMA system.
Chapter 10. For Further Reference
For more information, see:
- The SystemTap tutorial at http://sourceware.org/systemtap/tutorial/
- The SystemTap wiki at http://sourceware.org/systemtap/wiki
- The SystemTap documentation page at http://sourceware.org/systemtap/documentation.html
- From an unpacked source tarball or GIT directory, the examples in in the src/examples directory, the tapsets in the src/tapset directory, and the test scripts in the src/testsuite directory.
- The man pages for tapsets. For a list, run the command
``man -k stapprobes”.
Index
Symbols
A
- addr_to_node, addr_to_node
- aggregates, Statistics (aggregates)
- arch, Conditions based on architecture: arch
- associative arrays, Associative arrays
- asynchronous, General syntax
- auxiliary functions, Auxiliary functions
- avg, @avg(s)
B
- backtrace, backtrace
- begin, begin
- binary, Binary numeric operators, Binary string operators
- break, break and continue
- built-in probes, Built-in probe point types (DWARF probes)
C
- caller, caller
- caller_addr, caller_addr
- comments, Comments
- comparison, Binary numeric or string comparison operators
- conditions, Conditions
- constraints, Safety and security
- continue, break and continue
- count, @count(s)
- cpu, cpu
- ctime, ctime
D
- data types, Data types
- delete, delete
- do, do
- dwarf probes, Built-in probe point types (DWARF probes)
- DWARF-less probing, DWARF-less probing
E
- egid, egid
- embedded C, Embedded C
- end, end
- epilogue-style aliases, Epilogue-style aliases (+=)
- errno_str, errno_str
- error, error
- euid, euid
- example scripts, Sample SystemTap scripts
- execname, execname
- exit, exit
- expression, EXP (expression)
- expressions, Expressions
- extraction, Extraction functions
F
- fn, Function call
- for, for
- foreach, foreach, Iteration, foreach
- formatting, Formatting functions
G
- gettimeofday_ s, gettimeofday_s
- gettimeofday_ms, gettimeofday_ms
- gettimeofday_ns, gettimeofday_ns
- gettimeofday_us, gettimeofday_us
- get_cycles, get_cycles
- gid, gid
- grouping, Grouping operator
- guru mode, Embedded C
H
- histograms, Histogram extractors
- hist_linear, @hist_linear
- hist_log, @hist_log
I
- identifiers, Identifiers
- if, if
- index, <value> in <array_name>
- inference, Data types
- integers, Integers
- isinstr, isinstr
- is_return, is_return
J
- jiffies, Timer probes
K
- kernel version, Conditions based on kernel version: kernel_v, kernel_vr
- kernel.function, kernel.function, module().function
- kernel.statement, kernel.statement, module().statement
- kernel_string, kernel_string
- kernel_v, Conditions based on kernel version: kernel_v, kernel_vr
- kernel_vr, Conditions based on kernel version: kernel_v, kernel_vr
L
- language, Event-action language
- limits, Safety and security
- literals, Literals, Literals passed in from the stap command line
- local arrays, Variables
- log, log
M
- max, @max(s)
- milliseconds, Timer probes
- min, @min(s)
- module().function, kernel.function, module().function
- module().statement, kernel.statement, module().statement
N
- never, never
- next, next
- null statement, ; (null statement)
- numbers, Integers
- numeric, Numeric assignment operators
P
- pexecname, pexecname
- pid, pid
- pointer, $ptr->member
- pp, pp
- ppid, ppid
- prefixes, Prefixes
- print, print
- printd, printd
- printdln, printdln
- printf, printf
- println, println
- print_backtrace, print_backtrace
- print_regs, print_regs
- print_stack, print_stack
- probe aliases, Probe aliases
- probe points, Probe points
- probe syntax, General syntax
- probefunc, probefunc, probemod
- prologue-style aliases, Prologue-style aliases (=)
Q
- qsq wait_queue_length, qsq_wait_queue_length
- qsq_blocked, qsq_blocked
- qsq_print, qsq_print
- qsq_service_time, qsq_service_time
- qsq_start, qsq_start
- qsq_throughput, qsq_throughput
- qsq_utilization, qsq_utilization
- qsq_wait_time, qsq_wait_time
- qs_done, qs_done
- qs_run, qs_run
- qs_wait, qs_wait
- queue statistics, Initializing queue statistics
R
- randomize, Timer probes
- recursion, Recursive functions
- return, return
- return probes, Return probes
- returnstr, returnstr
S
- sequence, begin and end probe sequence
- sprint, sprint
- sprintf, sprintf
- stack_size, stack_size
- stack_unused, stack_unused
- stack_used, stack_used
- stap, The stap command
- statement block, { } (statement block)
- stp_pid, stp_pid
- string, String functions
- strings, Strings
- strlen, strlen
- substr, substr
- suffixes, Suffixes
- sum, @sum(s)
- synchronous, General syntax
- system, system, system
T
- target, target
- target variables, Probe definitions
- task_cpu, task_cpu
- task_current, task_current
- task_egid, task_egid
- task_euid, task_euid
- task_execname, task_execname
- task_gid, task_gid
- task_max_file_handles, task_max_file_handles
- task_nice, task_nice
- task_open_file_handles, task_open_file_handles
- task_parent, task_parent
- task_pid, task_pid
- task_prio, task_prio
- task_state, task_state
- task_tid, task_tid
- task_uid, task_uid
- text_str, text_str
- text_strn, text_strn
- THIS, Embedded C functions
- thread_indent, thread_indent
- thread_timestamp, thread_timestamp
- tid, tid
- timer probes, Timer probes
- timestamps, Timestamps
- tokens, True and False Tokens
U
- uid, uid
- unary, Unary numeric operators
- unused variables, Unused alias variables
- user_string, user_string
- user_string2, user_string2
- user_string_quoted, user_string_quoted
- user_string_warn, user_string_warn
V
- variables, Variables
W
- warn, warn
- while, while
- whitespace, Whitespace
- wildcards, Wildcarded file names, function names
Legal Notice
Copyright © 2009 Red Hat Inc., Copyright © 2009 IBM Corp., Copyright © 2009 Intel Corporation.
This document was derived from other documents contributed to the SystemTap project by employees of Red Hat, IBM and Intel.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
The GNU Free Documentation License is available from http://www.gnu.org/licenses/fdl.html or by writing to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
Red Hat, Red Hat Enterprise Linux, and the Shadowman logo are trademarks of Red Hat, Inc., registered in the U.S. and other countries. Linux® is the registered trademark of Linus Torvalds in the U.S. and other countries.
