by Mathieu Desnoyers

LTTng kernel modules are Linux kernel modules which make LTTng kernel tracing possible. They include essential control modules and many probes which instrument numerous interesting parts of Linux. LTTng-modules builds against a vanilla or distribution kernel, with no need for additional patches.

Other notable features:

• Produces CTF (Common Trace Format) natively.
• Tracepoints, function tracer, CPU Performance Monitoring Unit (PMU) counters, kprobes, and kretprobes support.
• Have the ability to attach context information to events in the trace (e.g., any PMU counter, PID, PPID, TID, command name, etc). All the extra information fields to be collected with events are optional, specified on a per-tracing-session basis (except for timestamp and event ID, which are mandatory).

To build and install LTTng-modules, you will need to have your kernel headers available (or access to your full kernel source tree), and do:

make
sudo make modules_install
sudo depmod -a

The above commands will build LTTng-modules against your current kernel. If you need to build LTTng-modules against a custom kernel, do:

make KERNELDIR=/path/to/custom/kernel
sudo make KERNELDIR=/path/to/custom/kernel modules_install
sudo depmod -a kernel_version

It is also possible to build these modules as part of a kernel image. Simply run the built-in.sh script with the path to your kernel source directory as an argument. It will symlink the lttng-modules directory in the kernel sources and add an include in the kernel Makefile.

Then configure your kernel as usual and enable the CONFIG_LTTNG option.

Make sure your target kernel has the following config options enabled:

• CONFIG_MODULES: loadable module support (not strictly required when built into the kernel)
• CONFIG_KALLSYMS: see files in wrapper; this is necessary until the few required missing symbols are exported to GPL modules from mainline
• CONFIG_HIGH_RES_TIMERS: needed for LTTng 2.x clock source
• CONFIG_TRACEPOINTS: kernel tracepoint instrumentation (enabled as a side-effect of any of the perf/ftrace/blktrace instrumentation features)

The following kernel configuration options will affect the features available from LTTng:

• CONFIG_HAVE_SYSCALL_TRACEPOINTS: system call tracing:

  lttng enable-event -k --syscall
  lttng enable-event -k -a
  

• CONFIG_PERF_EVENTS: performance counters:

  lttng add-context -t perf:*
  

• CONFIG_EVENT_TRACING: needed to allow block layer tracing

• CONFIG_KPROBES: dynamic probes:

  lttng enable-event -k --probe ...
  

• CONFIG_KRETPROBES: dynamic function entry/return probes:

  lttng enable-event -k --function ...
  

• CONFIG_KALLSYMS_ALL: state dump of mapping between block device number and name

Use LTTng-tools to control the tracer. The session daemon of LTTng-tools should automatically load the LTTng kernel modules when needed. Use Babeltrace to print traces as a human-readable text log.

Linux kernels >= 2.6.36 are supported.

Each PMU counter has its zero value set when it is attached to a context with add-context. Therefore, it is normal that the same counters attached to both the stream context and event context show different values for a given event; what matters is that they increment at the same rate.