Table of Contents
DTrace provides a facility for developers to define customized
probes in kernel modules. These static probes appear as additional
probes of the sdt provider and impose little to
no overhead if the sdt module is not loaded.
For example, for x86_64, the overhead is a single-byte NOP,
followed by a 4-byte NOP. This chapter provides a full example of
how to define and use static probes in a kernel module.
The general principles for naming probes and choosing insertion
points are the same for kernel modules as they are for user-space
applications. You should define probes that have a semantic meaning
that is readily understood by your DTrace user community. Typically,
you might name probes for the routine in which you place them and
their position in that routine. For example, if your probes provide
information about data values on entry to or return from a routine
named foo, you might name them
foo-entry and foo-return. The
data values that are returned by such probes could present the
routine as a black box, rather than return
intermediate values from the internal implementation of the module.
To gather data from deeper within a module, you might insert
additional probes with names such as foo-stage1
or foo-post-hardware-init.
In one respect, using static probes with kernel modules can be
simpler than for user-space applications. You do not need to modify
the build files unless you want to conditionally compile a module to
include the probes. Inserting the probes in the source code is
slightly more complex, as you cannot use the dtrace
-h command to generate the probe macros. However, using a
DTRACE_PROBE macro to insert a probe is a
relatively simple change to make to the source code.
You can insert sdt static probes in any Oracle Linux
kernel module for which you have the source files and the necessary
build infrastructure, but note that DTrace supports statically
defined tracing of 64-bit kernel modules only.
For more information about the sdt provider, see
Section 11.5, “sdt provider”.
For an introduction to the concepts of statically defined tracing as applied to user-space applications, see Chapter 13, Statically Defined Tracing of User Applications.