SDT(9) FreeBSD Kernel Developer's Manual SDT(9)
NAME
SDT - a DTrace framework for adding statically-defined tracing probes
SYNOPSIS
#include <sys/param.h>
#include <sys/queue.h>
#include <sys/sdt.h>
SDT_PROVIDER_DECLARE(prov);
SDT_PROVIDER_DEFINE(prov);
SDT_PROBE_DECLARE(prov, mod, func, name);
SDT_PROBE_DEFINE(prov, mod, func, name);
SDT_PROBE_DEFINE0(prov, mod, func, name);
SDT_PROBE_DEFINE1(prov, mod, func, name, arg0);
SDT_PROBE_DEFINE2(prov, mod, func, name, arg0, arg1);
SDT_PROBE_DEFINE3(prov, mod, func, name, arg0, arg1, arg2);
SDT_PROBE_DEFINE4(prov, mod, func, name, arg0, arg1, arg2, arg3);
SDT_PROBE_DEFINE5(prov, mod, func, name, arg0, arg1, arg2, arg3, arg4);
SDT_PROBE_DEFINE6(prov, mod, func, name, arg0, arg1, arg2, arg3, arg4,
arg5);
SDT_PROBE_DEFINE7(prov, mod, func, name, arg0, arg1, arg2, arg3, arg4,
arg5, arg6);
SDT_PROBE_DEFINE0_XLATE(prov, mod, func, name);
SDT_PROBE_DEFINE1_XLATE(prov, mod, func, name, arg0, xarg0);
SDT_PROBE_DEFINE2_XLATE(prov, mod, func, name, arg0, xarg0, arg1, xarg1);
SDT_PROBE_DEFINE3_XLATE(prov, mod, func, name, arg0, xarg0, arg1, xarg1,
arg2, xarg2);
SDT_PROBE_DEFINE4_XLATE(prov, mod, func, name, arg0, xarg0, arg1, xarg1,
arg2, xarg2, arg3, xarg3);
SDT_PROBE_DEFINE5_XLATE(prov, mod, func, name, arg0, xarg0, arg1, xarg1,
arg2, xarg2, arg3, xarg3, arg4, xarg4);
SDT_PROBE_DEFINE6_XLATE(prov, mod, func, name, arg0, xarg0, arg1, xarg1,
arg2, xarg2, arg3, xarg3, arg4, xarg4, arg5, xarg5);
SDT_PROBE_DEFINE7_XLATE(prov, mod, func, name, arg0, xarg0, arg1, xarg1,
arg2, xarg2, arg3, xarg3, arg4, xarg4, arg5, xarg5, arg6, xarg6);
SDT_PROBE0(prov, mod, func, name);
SDT_PROBE1(prov, mod, func, name, arg0);
SDT_PROBE2(prov, mod, func, name, arg0, arg1);
SDT_PROBE3(prov, mod, func, name, arg0, arg1, arg2);
SDT_PROBE4(prov, mod, func, name, arg0, arg1, arg2, arg3);
SDT_PROBE5(prov, mod, func, name, arg0, arg1, arg2, arg3, arg4);
SDT_PROBE6(prov, mod, func, name, arg0, arg1, arg2, arg3, arg4, arg5);
SDT_PROBE7(prov, mod, func, name, arg0, arg1, arg2, arg3, arg4, arg5,
arg6);
DESCRIPTION
The SDT macros allow programmers to define static trace points in kernel
code. These trace points are used by the SDT framework to create DTrace
probes, allowing the code to be instrumented using dtrace(1). By
default, SDT trace points are disabled and have no effect on the
surrounding code. When a DTrace probe corresponding to a given trace
point is enabled, threads that execute the trace point will call a
handler and cause the probe to fire. Moreover, trace points can take
arguments, making it possible to pass data to the DTrace framework when
an enabled probe fires.
Multiple trace points may correspond to a single DTrace probe, allowing
programmers to create DTrace probes that correspond to logical system
events rather than tying probes to specific code execution paths. For
instance, a DTrace probe corresponding to the arrival of an IP packet
into the network stack may be defined using two SDT trace points: one for
IPv4 packets and one for IPv6 packets.
In addition to defining DTrace probes, the SDT macros allow programmers
to define new DTrace providers, making it possible to namespace
logically-related probes. An example is FreeBSD's sctp provider, which
contains SDT probes for FreeBSD's sctp(4) implementation.
The SDT_PROVIDER_DECLARE() and SDT_PROVIDER_DEFINE() macros are used
respectively to declare and define a DTrace provider named prov with the
SDT framework. A provider need only be defined once; however, the
provider must be declared before defining any SDT probes belonging to
that provider.
Similarly, the SDT_PROBE_DECLARE() and SDT_PROBE_DEFINE*() macros are
used to declare and define DTrace probes using the SDT framework. Once a
probe has been defined, trace points for that probe may be added to
kernel code. DTrace probe identifiers consist of a provider, module,
function and name, all of which may be specified in the SDT probe
definition. Note that probes should not specify a module name: the
module name of a probe is used to determine whether or not it should be
destroyed when a kernel module is unloaded. See the BUGS section. Note
in particular that probes must not be defined across multiple kernel
modules.
If `-' character (dash) is wanted in a probe name, then it should be
represented as `__' (double underscore) in the probe name parameter
passed to various SDT_*() macros, because of technical reasons (a dash is
not valid in C identifiers).
The SDT_PROBE_DEFINE*() macros also allow programmers to declare the
types of the arguments that are passed to probes. This is optional; if
the argument types are omitted (through use of the SDT_PROBE_DEFINE()
macro), users wishing to make use of the arguments will have to manually
cast them to the correct types in their D scripts. It is strongly
recommended that probe definitions include a declaration of their
argument types.
The SDT_PROBE_DEFINE*_XLATE() macros are used for probes whose argument
types are to be dynamically translated to the types specified by the
corresponding xarg arguments. This is mainly useful when porting probe
definitions from other operating systems. As seen by dtrace(1), the
arguments of a probe defined using these macros will have types which
match the xarg types in the probe definition. However, the arguments
passed in at the trace point will have types matching the native argument
types in the probe definition, and thus the native type is dynamically
translated to the translated type. So long as an appropriate translator
is defined in /usr/lib/dtrace, scripts making use of the probe need not
concern themselves with the underlying type of a given SDT probe
argument.
The SDT_PROBE*() macros are used to create SDT trace points. They are
meant to be added to executable code and can be used to instrument the
code in which they are called.
PROVIDERS
A number of kernel DTrace providers are available. In general, these
providers define stable interfaces and should be treated as such:
existing D scripts may be broken if a probe is renamed or its arguments
are modified. However, it is often useful to define ad-hoc SDT probes
for debugging a subsystem or driver. Similarly, a developer may wish to
provide a group of SDT probes without committing to their future
stability. Such probes should be added to the `sdt' provider instead of
defining a new provider.
EXAMPLES
The DTrace providers available on the current system can be listed with
dtrace -l | sed 1d | awk '{print $2}' | sort -u
A detailed list of the probes offered by a given provider can be obtained
by specifying the provider using the -P flag. For example, to view the
probes and argument types for the `sched' provider, run
dtrace -lv -P sched
The following probe definition will create a DTrace probe called
`icmp:::receive-unreachable', which would hypothetically be triggered
when the kernel receives an ICMP packet of type Destination Unreachable:
SDT_PROVIDER_DECLARE(icmp);
SDT_PROBE_DEFINE1(icmp, , , receive__unreachable,
"struct icmp *");
This particular probe would take a single argument: a pointer to the
struct containing the ICMP header for the packet. Note that the module
name of this probe is not specified.
Consider a DTrace probe which fires when the network stack receives an IP
packet. Such a probe would be defined by multiple tracepoints:
SDT_PROBE_DEFINE3(ip, , , receive, "struct ifnet *",
"struct ip *", "struct ip6_hdr *");
int
ip_input(struct mbuf *m)
{
struct ip *ip;
...
ip = mtod(m, struct ip *);
SDT_PROBE3(ip, , , receive, m->m_pkthdr.rcvif, ip, NULL);
...
}
int
ip6_input(struct mbuf *m)
{
struct ip6_hdr *ip6;
...
ip6 = mtod(m, struct ip6_hdr *);
SDT_PROBE3(ip, , , receive, m->m_pkthdr.rcvif, NULL, ip6);
...
}
In particular, the probe should fire when the kernel receives either an
IPv4 packet or an IPv6 packet.
Consider the ICMP probe discussed above. We note that its second
argument is of type struct icmp, which is a type defined in the FreeBSD
kernel to represent the ICMP header of an ICMP packet, defined in RFC
792. Linux has a corresponding type, struct icmphdr, for the same
purpose, but its field names differ from FreeBSD's struct icmp.
Similarly, illumos defines the icmph_t type, again with different field
names. Even with the `icmp:::pkt-receive' probes defined in all three
operating systems, one would still have to write OS-specific scripts to
extract a given field out of the ICMP header argument. Dynamically-
translated types solve this problem: one can define an OS-independent
c(7) struct to represent an ICMP header, say struct icmp_hdr_dt, and
define translators from each of the three OS-specific types to struct
icmp_hdr_dt, all in the dtrace(1) library path. Then the FreeBSD probe
above can be defined with:
SDT_PROBE_DEFINE1_XLATE(ip, , , receive, "struct icmp *",
"struct icmp_hdr_dt *");
SEE ALSO
dtrace(1), dtrace_io(4), dtrace_ip(4), dtrace_proc(4), dtrace_sched(4),
dtrace_tcp(4), dtrace_udp(4)
AUTHORS
DTrace and the SDT framework were originally ported to FreeBSD from
Solaris by John Birrell <jb@FreeBSD.org>. This manual page was written
by Mark Johnston <markj@FreeBSD.org>.
BUGS
The SDT macros allow the module and function names of a probe to be
specified as part of a probe definition. The DTrace framework uses the
module name of probes to determine which probes should be destroyed when
a kernel module is unloaded, so the module name of a probe should match
the name of the module in which its defined. SDT will set the module
name properly if it is left unspecified in the probe definition; see the
EXAMPLES section.
One of the goals of the original SDT implementation (and by extension, of
FreeBSD's port) is that inactive SDT probes should have no performance
impact. This is unfortunately not the case; SDT trace points will add a
small but non-zero amount of latency to the code in which they are
defined. A more sophisticated implementation of the probes will help
alleviate this problem.
FreeBSD 13.1-RELEASE-p6 April 18, 2015 FreeBSD 13.1-RELEASE-p6
man2web Home...