path: root/stapprobes.5.in

.\" -*- nroff -*-
.TH STAPPROBES 5 @DATE@ "Red Hat"
.SH NAME
stapprobes \- systemtap probe points

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.SH DESCRIPTION
The following sections enumerate the variety of probe points supported
by the systemtap translator, and additional aliases defined by
standard tapset scripts.
.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.  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 syntactically valid probe points:

.SAMPLE
kernel.function("foo").return
syscall(22)
user.inode("/bin/vi").statement(0x2222)
end
syscall.*
kernel.function("no_such_function") ?
module("awol").function("no_such_function") !
signal.*? if (switch)
.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.

.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" prober 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 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.  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 of 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
module(MPATTERN).function(PATTERN)
.br
module(MPATTERN).function(PATTERN).call
.br
module(MPATTERN).function(PATTERN).return
.br
module(MPATTERN).function(PATTERN).inline
.br
.br
kernel.statement(PATTERN)
.br
kernel.statement(ADDRESS).absolute
.br
module(MPATTERN).statement(PATTERN)
.ESAMPLE

In the above list, MPATTERN stands for a string literal that aims to
identify the loaded kernel module of interest.  It may include "*", "[]",
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*.
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.
.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 ":".
.PP
As an alternative, PATTERN may be a numeric constant, indicating an
(module-relative or kernel-_stext-relative) address.  In guru mode
only, absolute kernel addresses may be specified with the ".absolute"
suffix.
.PP
Some of the source-level 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.
.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 to a structure's field.  The indirection operator
may be repeated to follow more levels of pointers.
.TP
$var[N]
indexes into an array.  The index is given with a
literal number.

.SS PROCFS

These probe points allow procfs "files" in
/proc/systemtap/MODNAME to be created, read and written
.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 four probe point variants supported
by the translator:

.SAMPLE
procfs("PATH").read
procfs("PATH").write
procfs.read
procfs.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

.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.

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.

.SS PERFORMANCE MONITORING HARDWARE

The perfmon family of probe points is used to access the performance
monitoring hardware available in modern processors. This family of
probes points needs the perfmon2 support in the kernel to access the
performance monitoring hardware.
.PP
Performance monitor hardware points begin with a 
.BR perfmon ". "
The next part of the names the event being counted
.BR counter("event") .
The event names are processor implementation specific with the
execption of the generic
.BR cycles " and " instructions
events, which are available on all processors. This sets up a counter
on the processor to count the number of events occuring on the
processor. For more details on the performance monitoring events
available on a specific processor use the command perfmon2 command:

.SAMPLE
pfmon \-l
.ESAMPLE
.TP
$counter
is a handle used in the body of the probe for operations
involving the counter associated with the probe.
.TP
read_counter
is a function that is passed the handle for the perfmon probe and returns
the current count for the event.

.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.
.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 the "kernel/sched.c".
.TP
syscall.*.return
refers to the group of probe aliases with any name in the third position

.SH SEE ALSO
.IR stap (1),
.IR stapprobes.iosched (5),
.IR stapprobes.netdev (5),
.IR stapprobes.nfs (5),
.IR stapprobes.nfsd (5),
.IR stapprobes.pagefault (5),
.IR stapprobes.process (5),
.IR stapprobes.rpc (5),
.IR stapprobes.scsi (5),
.IR stapprobes.signal (5),
.IR stapprobes.socket (5),
.IR stapprobes.tcp (5),
.IR stapprobes.udp (5),
.IR proc (5)