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+LTTng-modules
+=============
+
+_by [Mathieu Desnoyers](mailto:mathieu.desnoyers@efficios.com)_
+
+
+LTTng kernel modules are Linux kernel modules which make
+[LTTng](http://lttng.org/) 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](http://www.efficios.com/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).
+
+
+Building
+--------
+
+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
+
+
+### Required kernel config options
+
+Make sure your target kernel has the following config options enabled:
+
+  - `CONFIG_MODULES`: loadable module support
+  - `CONFIG_KALLSYMS`: see files in [`wrapper`](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)
+
+
+### Supported (optional) kernel config options
+
+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
+
+
+Using
+-----
+
+Use [LTTng-tools](https://lttng.org/download) to control the tracer.
+The session daemon of LTTng-tools should automatically load the LTTng
+kernel modules when needed. Use [Babeltrace](https://lttng.org/babeltrace)
+to print traces as a human-readable text log.
+
+
+Support
+-------
+
+So far, it has been tested on various vanilla Linux kernels from 2.6.38
+to 3.16, most of them on IA-32/x86-64 and some of them on ARM and Power-PC
+32-bit. Linux 2.6.32 to 2.6.34 need up to 3 patches applied (refer to
+[`linux-patches`](linux-patches)). It should work fine with newer kernels and
+other architectures, but expect build issues with kernels older than 2.6.36.
+
+
+Notes
+-----
+
+### 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.