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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.