LTTng 2.x modules

Mathieu Desnoyers
March 29, 2013

LTTng 2.x kernel modules build against a vanilla or distribution kernel, without
need for additional patches. Other features:

- Produces CTF (Common Trace Format) natively,
  (http://www.efficios.com/ctf)
- Tracepoints, Function tracer, CPU Performance Monitoring Unit (PMU)
  counters, kprobes, and kretprobes support,
- Integrated interface for both kernel and userspace tracing,
- Have the ability to attach "context" information to events in the
  trace (e.g. any PMU counter, pid, ppid, tid, comm 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, you will need to have your kernel headers available (or
access to your full kernel source tree), and use:

% make
# make modules_install
# depmod -a

If you need to specify the target directory to the kernel you want to build
against, use:

% KERNELDIR=path_to_kernel_dir make
# KERNELDIR=path_to_kernel_dir make modules_install
# depmod -a kernel_version

Use lttng-tools to control the tracer. LTTng tools should automatically load
the kernel modules when needed. Use Babeltrace to print traces as a
human-readable text log. These tools are available at the following URL:
http://lttng.org/lttng2.0

So far, it has been tested on vanilla Linux kernels 2.6.38, 2.6.39, 3.0,
3.1, 3.2, 3.3 (on x86 32/64-bit, and powerpc 32-bit at the moment, build
tested on ARM), 3.4, 3.5, 3.8, 3.9-rc on x86 64-bit. Kernels 2.6.32 to
2.6.34 need up to 3 patches applied (refer to linux-patches within the
lttng-modules tree). It should work fine with newer kernels and other
architectures, but expect build issues with kernels older than 2.6.36.
The clock source currently used is the standard gettimeofday (slower,
less scalable and less precise than the LTTng 0.x clocks).  Support for
LTTng 0.x clocks will be added back soon into LTTng 2.0.  Please note
that lttng-modules 2.0 can build on a Linux kernel patched with the
LTTng 0.x patchset, but the lttng-modules 2.0 replace the lttng-modules
0.x, so both tracers cannot be installed at the same time for a given
kernel version.


* Kernel config options required

CONFIG_MODULES: required
  * Kernel modules support.
CONFIG_KALLSYMS: required
  * See wrapper/ files. This is necessary until the few required missing
    symbols are exported to GPL modules from mainline.
CONFIG_HIGH_RES_TIMERS: required
  * Needed for LTTng 2.0 clock source.
CONFIG_TRACEPOINTS: required
    kernel tracepoint instrumentation
    * Enabled as side-effect of any of the perf/ftrace/blktrace
      instrumentation features.


* Kernel config options supported (optional)

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 probe.
               lttng enable-event -k --probe ...
CONFIG_KRETPROBES:
            Dynamic function entry/return probe.
               lttng enable-event -k --function ...


* Note about Perf PMU counters support

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.