eventcount: extend script with more info and prettier formatting

[systemtap.git] / stapprobes.3stap

.\" t
.TH STAPPROBES 3stap 
.SH NAME
stapprobes \- systemtap probe points

.\" macros
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.SH DESCRIPTION
The following sections enumerate the variety of probe points supported
by the systemtap translator, and some of the additional aliases defined by
standard tapset scripts.  Many are individually documented in the
.IR 3stap
manual section, with the
.IR probe::
prefix.
.PP
The general probe point syntax is a dotted-symbol sequence.  This
allows a breakdown of the event namespace into parts, somewhat like
the Domain Name System does on the Internet.  Each component
identifier may be parametrized by a string or number literal, with a
syntax like a function call.  A component may include a "*" character,
to expand to a set of matching probe points.  It may also include "**"
to match multiple sequential components at once.  Probe aliases likewise
expand to other probe points.  Each and every resulting probe point is
normally resolved to some low-level system instrumentation facility
(e.g., a kprobe address, marker, or a timer configuration), otherwise
the elaboration phase will fail.
.PP
However, a probe point may be followed by a "?" character, to indicate
that it is optional, and that no error should result if it fails to
resolve.  Optionalness passes down through all levels of
alias/wildcard expansion.  Alternately, a probe point may be followed
by a "!" character, to indicate that it is both optional and
sufficient.  (Think vaguely of the Prolog cut operator.) If it does
resolve, then no further probe points in the same comma-separated list
will be resolved.  Therefore, the "!"  sufficiency mark only makes
sense in a list of probe point alternatives.
.PP
Additionally, a probe point may be followed by a "if (expr)" statement, in
order to enable/disable the probe point on-the-fly. With the "if" statement,
if the "expr" is false when the probe point is hit, the whole probe body
including alias's body is skipped. The condition is stacked up through
all levels of alias/wildcard expansion. So the final condition becomes
the logical-and of conditions of all expanded alias/wildcard.

These are all
.B syntactically
valid probe points.  (They are generally
.B semantically
invalid, depending on the contents of the tapsets, and the versions of
kernel/user software installed.)

.SAMPLE
kernel.function("foo").return
process("/bin/vi").statement(0x2222)
end
syscall.*
sys**open
kernel.function("no_such_function") ?
module("awol").function("no_such_function") !
signal.*? if (switch)
kprobe.function("foo")
.ESAMPLE

Probes may be broadly classified into "synchronous" and
"asynchronous".  A "synchronous" event is deemed to occur 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/counters rolling over,
where there is no fixed reference point that is related.  Each probe
point specification may match multiple locations (for example, using
wildcards or aliases), and all them are then probed.  A probe
declaration may also contain several comma-separated specifications,
all of which are probed.

.SH DWARF DEBUGINFO

Resolving some probe points requires DWARF debuginfo or "debug
symbols" for the specific part being instrumented.  For some others,
DWARF is automatically synthesized on the fly from source code header
files.  For others, it is not needed at all.  Since a systemtap script
may use any mixture of probe points together, the union of their DWARF
requirements has to be met on the computer where script compilation
occurs.  (See the \fI\-\-use\-server\fR option and the \fBstap-server\
(8)\fR man page for information about the remote compilation facility,
which allows these requirements to be met on a different machine.)
.PP
The following point lists many of the available probe point families,
to classify them with respect to their need for DWARF debuginfo.

.TS
l l l.
\fBDWARF	AUTO-DWARF	NON-DWARF\fP

kernel.function, .statement	kernel.trace	kernel.mark
module.function, .statement		process.mark
process.function, .statement		begin, end, error, never
process.mark \fI(backup)\fP		timer
		perf
		procfs
		kernel.statement.absolute
		kernel.data
		kprobe.function
		process.statement.absolute
		process.begin, .end, .error
.TE

.SH PROBE POINT FAMILIES

.SS BEGIN/END/ERROR

The probe points
.IR begin " and " end
are defined by the translator to refer to the time of session startup
and shutdown.  All "begin" probe handlers are run, in some sequence,
during the startup of the session.  All global variables will have
been initialized prior to this point.  All "end" probes are run, in
some sequence, during the
.I normal
shutdown of a session, such as in the aftermath of an
.I exit ()
function call, or an interruption from the user.  In the case of an
error-triggered shutdown, "end" probes are not run.  There are no
target variables available in either context.
.PP
If the order of execution among "begin" or "end" probes is significant,
then an optional sequence number may be provided:

.SAMPLE
begin(N)
end(N)
.ESAMPLE

The number N may be positive or negative.  The probe handlers are run in
increasing order, and the order between handlers with the same sequence
number is unspecified.  When "begin" or "end" are given without a
sequence, they are effectively sequence zero.

The
.IR error
probe point is similar to the 
.IR end
probe, except that each such probe handler run when the session ends
after errors have occurred.  In such cases, "end" probes are skipped,
but each "error" probe is still attempted.  This kind of probe can be
used to clean up or emit a "final gasp".  It may also be numerically
parametrized to set a sequence.

.SS NEVER
The probe point
.IR never
is specially defined by the translator to mean "never".  Its probe
handler is never run, though its statements are analyzed for symbol /
type correctness as usual.  This probe point may be useful in
conjunction with optional probes.

.SS SYSCALL

The
.IR syscall.*
aliases define several hundred probes, too many to
summarize here.  They are:

.SAMPLE
syscall.NAME
.br
syscall.NAME.return
.ESAMPLE

Generally, two probes are defined for each normal system call as listed in the
.IR syscalls(2)
manual page, one for entry and one for return.  Those system calls that never
return do not have a corresponding
.IR .return
probe.
.PP
Each probe alias provides a variety of variables. Looking at the tapset source
code is the most reliable way.  Generally, each variable listed in the standard
manual page is made available as a script-level variable, so
.IR syscall.open
exposes
.IR filename ", " flags ", and " mode .
In addition, a standard suite of variables is available at most aliases:
.TP
.IR argstr
A pretty-printed form of the entire argument list, without parentheses.
.TP
.IR name
The name of the system call.
.TP
.IR retstr
For return probes, a pretty-printed form of the system-call result.
.PP
As usual for probe aliases, these variables are all simply initialized
once from the underlying $context variables, so that later changes to
$context variables are not automatically reflected.  Not all probe
aliases obey all of these general guidelines.  Please report any
bothersome ones you encounter as a bug.


.SS TIMERS

Intervals defined by the standard kernel "jiffies" timer may be used
to trigger probe handlers asynchronously.  Two probe point variants
are supported by the translator:

.SAMPLE
timer.jiffies(N)
timer.jiffies(N).randomize(M)
.ESAMPLE

The probe handler is run every N jiffies (a kernel-defined unit of
time, typically between 1 and 60 ms).  If the "randomize" component is
given, a linearly distributed random value in the range [\-M..+M] is
added to N every time the handler is run.  N is restricted to a
reasonable range (1 to around a million), and M is restricted to be
smaller than N.  There are no target variables provided in either
context.  It is possible for such probes to be run concurrently on
a multi-processor computer.
.PP
Alternatively, intervals may be specified in units of time.
There are two probe point variants similar to the jiffies timer:

.SAMPLE
timer.ms(N)
timer.ms(N).randomize(M)
.ESAMPLE

Here, N and M are specified in milliseconds, but the full options for units
are seconds (s/sec), milliseconds (ms/msec), microseconds (us/usec),
nanoseconds (ns/nsec), and hertz (hz).  Randomization is not supported for
hertz timers.

The actual 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 actual
resolution will be arch-dependent.  In either case, if the "randomize"
component is given, then the random value will be added to the interval
before any rounding occurs.
.PP
Profiling timers are also available to provide probes that execute on all
CPUs at the rate of the system tick (CONFIG_HZ).
This probe takes no parameters.

.SAMPLE
timer.profile
.ESAMPLE

Full context information of the interrupted process is available, making
this probe suitable for a time-based sampling profiler.

.SS DWARF

This family of probe points uses symbolic debugging information for
the target kernel/module/program, as may be found in unstripped
executables, or the separate
.I debuginfo
packages.  They allow placement of probes logically into the execution
path of the target program, 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.
.PP
Points in a kernel, which are identified by
module, source file, line number, function name, or some
combination of these.  
.PP
Here is a list of probe point families currently supported.  The
.B .function
variant places a probe near the beginning of the named function, so that
parameters are available as context variables.  The
.B .return
variant places a probe at the moment
.B after
the return from the named function, so the return value is available
as the "$return" context variable.  The
.B .inline
modifier for
.B .function
filters the results to include only instances of inlined functions.
The
.B .call
modifier selects the opposite subset.  Inline functions do not have an
identifiable return point, so
.B .return
is not supported on 
.B .inline
probes. The
.B .statement
variant places a probe at the exact spot, exposing those local variables
that are visible there.

.SAMPLE
kernel.function(PATTERN)
.br
kernel.function(PATTERN).call
.br
kernel.function(PATTERN).return
.br
kernel.function(PATTERN).inline
.br
kernel.function(PATTERN).label(LPATTERN)
.br
module(MPATTERN).function(PATTERN)
.br
module(MPATTERN).function(PATTERN).call
.br
module(MPATTERN).function(PATTERN).return
.br
module(MPATTERN).function(PATTERN).inline
.br
module(MPATTERN).function(PATTERN).label(LPATTERN)
.br
.br
kernel.statement(PATTERN)
.br
kernel.statement(ADDRESS).absolute
.br
module(MPATTERN).statement(PATTERN)
.br
process("PATH").function("NAME")
.br
process("PATH").statement("*@FILE.c:123")
.br
process("PATH").library("PATH").function("NAME")
.br
process("PATH").library("PATH").statement("*@FILE.c:123")
.br
process("PATH").function("*").return
.br
process("PATH").function("myfun").label("foo")
.br
process(PID).statement(ADDRESS).absolute
.ESAMPLE

(See the USER-SPACE section below for more information on the process
probes.)

In the above list, MPATTERN stands for a string literal that aims to
identify the loaded kernel module of interest and LPATTERN stands for
a source program label.  Both MPATTERN and LPATTERN may include the "*"
"[]", and "?" wildcards.  
PATTERN stands for a string literal that
aims to identify a point in the program.  It is made up of three
parts:
.IP \(bu 4
The first part is the name of a function, as would appear in the
.I nm
program's output.  This part may use the "*" and "?" wildcarding
operators to match multiple names.
.IP \(bu 4
The second part is optional and begins with the "@" character. 
It is followed by the path to the source file containing the function,
which may include a wildcard pattern, such as mm/slab*.
If it does not match as is, an implicit "*/" is optionally added
.I before
the pattern, so that a script need only name the last few components
of a possibly long source directory path.
.IP \(bu 4
Finally, the third part is optional if the file name part was given,
and identifies the line number in the source file preceded by a ":"
or a "+".  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".
.PP
As an alternative, PATTERN may be a numeric constant, indicating an
address.  Such an address may be found from symbol tables of the
appropriate kernel / module object file.  It is verified against
known statement code boundaries, and will be relocated for use at
run time.
.PP
In guru mode only, absolute kernel-space addresses may be specified with
the ".absolute" suffix.  Such an address is considered already relocated,
as if it came from 
.BR /proc/kallsyms ,
so it cannot be checked against statement/instruction boundaries.

.SS CONTEXT VARIABLES

.PP
Many of the source-level context variables, such as function parameters,
locals, globals visible in the compilation unit, may be visible to
probe handlers.  They may refer to these variables by prefixing their
name with "$" within the scripts.  In addition, a special syntax
allows limited traversal of structures, pointers, and arrays.  More
syntax allows pretty-printing of individual variables or their groups.
See also 
.BR @cast .

.TP
$var
refers to an in-scope variable "var".  If it's an integer-like type,
it will be cast to a 64-bit int for systemtap script use.  String-like
pointers (char *) may be copied to systemtap string values using the
.IR kernel_string " or " user_string
functions.
.TP
$var\->field traversal via a structure's or a pointer's field.  This
generalized indirection operator may be repeated to follow more
levels.  Note that the
.IR .
operator is not used for plain structure
members, only 
.IR \->
for both purposes.  (This is because "." is reserved for string
concatenation.)
.TP
$return
is available in return probes only for functions that are declared
with a return value.
.TP
$var[N]
indexes into an array.  The index given with a literal number or even
an arbitrary numeric expression.
.PP
A number of operators exist for such basic context variable expressions:
.TP
$$vars
expands to a character string that is equivalent to
.SAMPLE
sprintf("parm1=%x ... parmN=%x var1=%x ... varN=%x",
        parm1, ..., parmN, var1, ..., varN)
.ESAMPLE
for each variable in scope at the probe point.  Some values may be
printed as
.IR =?
if their run-time location cannot be found.
.TP
$$locals
expands to a subset of $$vars for only local variables.
.TP
$$parms
expands to a subset of $$vars for only function parameters.
.TP
$$return
is available in return probes only.  It expands to a string that
is equivalent to sprintf("return=%x", $return)
if the probed function has a return value, or else an empty string.
.TP
& $EXPR
expands to the address of the given context variable expression, if it
is addressable.
.TP
@defined($EXPR)
expands to 1 or 0 iff the given context variable expression is resolvable,
for use in conditionals such as   
.SAMPLE
@defined($foo\->bar) ? $foo\->bar : 0
.ESAMPLE
.TP
$EXPR$
expands to a string with all of $EXPR's members, equivalent to
.SAMPLE
sprintf("{.a=%i, .b=%u, .c={...}, .d=[...]}",
         $EXPR\->a, $EXPR\->b)
.ESAMPLE
.TP
$EXPR$$
expands to a string with all of $var's members and submembers, equivalent to
.SAMPLE
sprintf("{.a=%i, .b=%u, .c={.x=%p, .y=%c}, .d=[%i, ...]}",
        $EXPR\->a, $EXPR\->b, $EXPR\->c\->x, $EXPR\->c\->y, $EXPR\->d[0])
.ESAMPLE

.PP
For ".return" probes, context variables other than the "$return"
value itself are only available for the function call parameters.
The expressions evaluate to the
.IR entry-time
values of those variables, since that is when a snapshot is taken.
Other local variables are not generally accessible, since by the time
a ".return" probe hits, the probed function will have already returned.
.PP
Arbitrary entry-time expressions can also be saved for ".return"
probes using the
.IR @entry(expr)
operator.  For example, one can compute the elapsed time of a function:
.SAMPLE
probe kernel.function("do_filp_open").return {
    println( get_timeofday_us() \- @entry(get_timeofday_us()) )
}
.ESAMPLE


.SS DWARFLESS
In absence of debugging information, entry & exit points of kernel & module
functions can be probed using the "kprobe" family of probes.
However, these do not permit looking up the arguments / local variables
of the function.
Following constructs are supported :
.SAMPLE
kprobe.function(FUNCTION)
kprobe.function(FUNCTION).return
kprobe.module(NAME).function(FUNCTION)
kprobe.module(NAME).function(FUNCTION).return
kprobe.statement.(ADDRESS).absolute
.ESAMPLE
.PP
Probes of type
.B function
are recommended for kernel functions, whereas probes of type
.B module
are recommended for probing functions of the specified module.
In case the absolute address of a kernel or module function is known,
.B statement
probes can be utilized.
.PP
Note that
.I FUNCTION
and
.I MODULE
names
.B must not
contain wildcards, or the probe will not be registered.
Also, statement probes must be run under guru-mode only.


.SS USER-SPACE
Support for user-space probing is available for kernels
that are configured with the utrace extensions.  See
.SAMPLE
http://people.redhat.com/roland/utrace/
.ESAMPLE
.PP
There are several forms.  First, a non-symbolic probe point:
.SAMPLE
process(PID).statement(ADDRESS).absolute
.ESAMPLE
is analogous to 
.IR
kernel.statement(ADDRESS).absolute
in that both use raw (unverified) virtual addresses and provide
no $variables.  The target PID parameter must identify a running
process, and ADDRESS should identify a valid instruction address.
All threads of that process will be probed.
.PP
Second, non-symbolic user-kernel interface events handled by
utrace may be probed:
.SAMPLE
process(PID).begin
process("FULLPATH").begin
process.begin
process(PID).thread.begin
process("FULLPATH").thread.begin
process.thread.begin
process(PID).end
process("FULLPATH").end
process.end
process(PID).thread.end
process("FULLPATH").thread.end
process.thread.end
process(PID).syscall
process("FULLPATH").syscall
process.syscall
process(PID).syscall.return
process("FULLPATH").syscall.return
process.syscall.return
process(PID).insn
process("FULLPATH").insn
process(PID).insn.block
process("FULLPATH").insn.block
.ESAMPLE
.PP
A
.B .begin
probe gets called when new process described by PID or FULLPATH gets created.
A
.B .thread.begin
probe gets called when a new thread described by PID or FULLPATH gets created.
A
.B .end
probe gets called when process described by PID or FULLPATH dies.
A
.B .thread.end
probe gets called when a thread described by PID or FULLPATH dies.
A
.B .syscall
probe gets called when a thread described by PID or FULLPATH makes a
system call.  The system call number is available in the
.BR $syscall
context variable, and the first 6 arguments of the system call
are available in the
.BR $argN
(ex. $arg1, $arg2, ...) context variable.
A
.B .syscall.return
probe gets called when a thread described by PID or FULLPATH returns from a
system call.  The system call number is available in the
.BR $syscall
context variable, and the return value of the system call is available
in the
.BR $return
context variable.
A
.B .insn
probe gets called for every single-stepped instruction of the process described by PID or FULLPATH.
A
.B .insn.block
probe gets called for every block-stepped instruction of the process described by PID or FULLPATH.
.PP
If a process probe is specified without a PID or FULLPATH, all user
threads will be probed.  However, if systemtap was invoked with the
.IR \-c " or " \-x
options, then process probes are restricted to the process
hierarchy associated with the target process.

.PP
Third, symbolic static instrumentation compiled into programs and
shared libraries may be
probed:
.SAMPLE
process("PATH").mark("LABEL")
process("PATH").provider("PROVIDER").mark("LABEL")
.ESAMPLE
.PP
A
.B .mark
probe gets called via a static probe which is defined in the
application by STAP_PROBE1(PROVIDER,LABEL,arg1), which is defined in
sdt.h.  The handle is an application handle, LABEL corresponds to
the .mark argument, and arg1 is the argument.  STAP_PROBE1 is used for
probes with 1 argument, STAP_PROBE2 is used for probes with 2
arguments, and so on.  The arguments of the probe are available in the
context variables $arg1, $arg2, ...  An alternative to using the
STAP_PROBE macros is to use the dtrace script to create custom macros.
Additionally, the variables $$name and $$provider are available as
parts of the probe point name.

.PP
Finally, full symbolic source-level probes in user-space programs
and shared libraries are supported.  These are exactly analogous
to the symbolic DWARF-based kernel/module probes described above,
and expose similar contextual $variables.
.SAMPLE
process("PATH").function("NAME")
process("PATH").statement("*@FILE.c:123")
process("PATH").library("PATH").function("NAME")
process("PATH").library("PATH").statement("*@FILE.c:123")
process("PATH").function("*").return
process("PATH").function("myfun").label("foo")
.ESAMPLE

.PP
Note that for all process probes,
.I PATH
names refer to executables that are searched the same way shells do: relative
to the working directory if they contain a "/" character, otherwise in 
.BR $PATH .
If PATH names refer to scripts, the actual interpreters (specified in the
script in the first line after the #! characters) are probed.
If PATH is a process component parameter referring to shared libraries
then all processes that map it at runtime would be selected for
probing.  If PATH is a library component parameter referring to shared
libraries then the process specified by the process component would be
selected.
If the PATH string contains wildcards as in the MPATTERN case, then
standard globbing is performed to find all matching paths.  In this
case, the 
.BR $PATH
environment variable is not used.

.PP
If systemtap was invoked with the
.IR \-c " or " \-x
options, then process probes are restricted to the process
hierarchy associated with the target process.

.SS PROCFS

These probe points allow procfs "files" in
/proc/systemtap/MODNAME to be created, read and written using a  
permission that may be modified using the proper umask value. Default permissions are 0400 for read
probes, and 0200 for write probes. If both a read and write probe are being 
used on the same file, a default permission of 0600 will be used.
Using procfs.umask(0040).read would
result in a 0404 permission set for the file.
.RI ( MODNAME
is the name of the systemtap module). The
.I proc
filesystem is a pseudo-filesystem which is used an an interface to
kernel data structures. There are several probe point variants supported
by the translator:

.SAMPLE
procfs("PATH").read
procfs("PATH").umask(UMASK).read
procfs("PATH").read.maxsize(MAXSIZE)
procfs("PATH").umask(UMASK).maxsize(MAXSIZE)
procfs("PATH").write
procfs("PATH").umask(UMASK).write
procfs.read
procfs.umask(UMASK).read
procfs.read.maxsize(MAXSIZE)
procfs.umask(UMASK).read.maxsize(MAXSIZE)
procfs.write
procfs.umask(UMASK).write
.ESAMPLE

.I PATH
is the file name (relative to /proc/systemtap/MODNAME) to be created.
If no
.I PATH
is specified (as in the last two variants above),
.I PATH
defaults to "command".
.PP
When a user reads /proc/systemtap/MODNAME/PATH, the corresponding
procfs
.I read
probe is triggered.  The string data to be read should be assigned to
a variable named
.IR $value ,
like this:

.SAMPLE
procfs("PATH").read { $value = "100\\n" }
.ESAMPLE
.PP
When a user writes into /proc/systemtap/MODNAME/PATH, the
corresponding procfs
.I write
probe is triggered.  The data the user wrote is available in the
string variable named
.IR $value ,
like this:

.SAMPLE
procfs("PATH").write { printf("user wrote: %s", $value) }
.ESAMPLE
.PP
.I MAXSIZE
is the size of the procfs read buffer.  Specifying
.I MAXSIZE
allows larger procfs output.  If no
.I MAXSIZE
is specified, the procfs read buffer defaults to
.I STP_PROCFS_BUFSIZE
(which defaults to
.IR MAXSTRINGLEN ,
the maximum length of a string).
If setting the procfs read buffers for more than one file is needed,
it may be easiest to override the
.I STP_PROCFS_BUFSIZE
definition.
Here's an example of using
.IR MAXSIZE :

.SAMPLE
procfs.read.maxsize(1024) {
    $value = "long string..."
    $value .= "another long string..."
    $value .= "another long string..."
    $value .= "another long string..."
}
.ESAMPLE

.SS MARKERS

This family of probe points hooks up to static probing markers
inserted into the kernel or modules.  These markers are special macro
calls inserted by kernel developers to make probing faster and more
reliable than with DWARF-based probes.  Further, DWARF debugging
information is 
.I not
required to probe markers.

Marker probe points begin with 
.BR kernel .
The next part names the marker itself:
.BR mark("name") .
The marker name string, which may contain the usual wildcard characters,
is matched against the names given to the marker macros when the kernel
and/or module was compiled.    Optionally, you can specify
.BR format("format") .
Specifying the marker format string allows differentiation between two
markers with the same name but different marker format strings.

The handler associated with a marker-based probe may read the
optional parameters specified at the macro call site.  These are
named
.BR $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.

The marker format string associated with a marker is available in
.BR $format .
And also the marker name string is available in
.BR $name .

.SS TRACEPOINTS

This family of probe points hooks up to static probing tracepoints
inserted into the kernel or modules.  As with markers, these
tracepoints are special macro calls inserted by kernel developers to
make probing faster and more reliable than with DWARF-based probes,
and DWARF debugging information is not required to probe tracepoints.
Tracepoints have an extra advantage of more strongly-typed parameters
than markers.

Tracepoint probes begin with 
.BR kernel .
The next part names the tracepoint itself:
.BR trace("name") .
The tracepoint name string, which may contain the usual wildcard
characters, is matched against the names defined by the kernel
developers in the tracepoint header files.

The handler associated with a tracepoint-based probe may read the
optional parameters specified at the macro call site.  These are
named according to the declaration by the tracepoint author.  For
example, the tracepoint probe
.BR kernel.trace("sched_switch")
provides the parameters
.BR $rq ", " $prev ", and " $next .
If the parameter is a complex type, as in a struct pointer, then a
script can access fields with the same syntax as DWARF $target
variables.  Also, tracepoint parameters cannot be modified, but in
guru-mode a script may modify fields of parameters.

The name of the tracepoint is available in
.BR $$name ,
and a string of name=value pairs for all parameters of the tracepoint
is available in
.BR $$vars " or " $$parms .

.SS HARDWARE BREAKPOINTS
This family of probes is used to set hardware watchpoints for a given
 (global) kernel symbol. The probes take three components as inputs :

1. The
.BR virtual address / name
of the kernel symbol to be traced is supplied as argument to this class
of probes. ( Probes for only data segment variables are supported. Probing
local variables of a function cannot be done.)

2. Nature of access to be probed :
a.
.I .write
probe gets triggered when a write happens at the specified address/symbol
name.
b.
.I rw
probe is triggered when either a read or write happens.

3.
.BR .length
(optional)
Users have the option of specifying the address interval to be probed
using "length" constructs. The user-specified length gets approximated
to the closest possible address length that the architecture can
support. If the specified length exceeds the limits imposed by
architecture, an error message is flagged and probe registration fails.
Wherever 'length' is not specified, the translator requests a hardware
breakpoint probe of length 1. It should be noted that the "length"
construct is not valid with symbol names.

Following constructs are supported :
.SAMPLE
probe kernel.data(ADDRESS).write
probe kernel.data(ADDRESS).rw
probe kernel.data(ADDRESS).length(LEN).write
probe kernel.data(ADDRESS).length(LEN).rw
probe kernel.data("SYMBOL_NAME").write
probe kernel.data("SYMBOL_NAME").rw
.ESAMPLE

This set of probes make use of the debug registers of the processor,
which is a scarce resource. (4 on x86 , 1 on powerpc ) The script
translation flags a warning if a user requests more hardware breakpoint probes
than the limits set by architecture. For example,a pass-2 warning is flashed
when an input script requests 5 hardware breakpoint probes on an x86
system while x86 architecture supports a maximum of 4 breakpoints.
Users are cautioned to set probes judiciously.

.SH EXAMPLES
.PP
Here are some example probe points, defining the associated events.
.TP
begin, end, end
refers to the startup and normal shutdown of the session.  In this
case, the handler would run once during startup and twice during
shutdown.
.TP
timer.jiffies(1000).randomize(200)
refers to a periodic interrupt, every 1000 +/\- 200 jiffies.
.TP
kernel.function("*init*"), kernel.function("*exit*")
refers to all kernel functions with "init" or "exit" in the name.
.TP
kernel.function("*@kernel/sched.c:240")
refers to any functions within the "kernel/sched.c" file that span
line 240.  
.BR
Note
that this is
.BR not
a probe at the statement at that line number.  Use the
.IR
kernel.statement
probe instead.
.TP
kernel.mark("getuid")
refers to an STAP_MARK(getuid, ...) macro call in the kernel.
.TP
module("usb*").function("*sync*").return
refers to the moment of return from all functions with "sync" in the
name in any of the USB drivers.
.TP
kernel.statement(0xc0044852)
refers to the first byte of the statement whose compiled instructions
include the given address in the kernel.
.TP
kernel.statement("*@kernel/sched.c:2917")
refers to the statement of line 2917 within "kernel/sched.c".
.TP
kernel.statement("bio_init@fs/bio.c+3")
refers to the statement at line bio_init+3 within "fs/bio.c".
.TP
kernel.data("pid_max").write
refers to a hardware preakpoint of type "write" set on pid_max
.TP
syscall.*.return
refers to the group of probe aliases with any name in the third position

.SS PERF

This 
.IR prototype
family of probe points interfaces to the kernel "perf event"
infrasture for controlling hardware performance counters.
The events being attached to are described by the "type",
"config" fields of the 
.IR perf_event_attr
structure, and are sampled at an interval governed by the
"sample_period" field.

These fields are made available to systemtap scripts using
the following syntax:
.SAMPLE
probe perf.type(NN).config(MM).sample(XX)
probe perf.type(NN).config(MM)
.ESAMPLE
The range of valid type/config is described by the 
.IR perf_event_open (2)
system call, and/or the 
.IR linux/perf_event.h
file.  Invalid combinations or exhausted hardware counter resources
result in errors during systemtap script startup.  Systemtap does
not sanity-check the values: it merely passes them through to
the kernel for error- and safety-checking.

.SH SEE ALSO
.IR stap (1),
.IR probe::* (3stap),
.IR tapset::* (3stap)