| 1 | --- |
| 2 | id: instrumenting-out-of-tree-linux-kernel |
| 3 | --- |
| 4 | |
| 5 | Instrumenting a custom Linux kernel module for LTTng follows the exact |
| 6 | same steps as |
| 7 | [adding instrumentation to the Linux kernel itself](#doc-instrumenting-linux-kernel-itself), |
| 8 | the only difference being that your mainline tracepoint definition |
| 9 | header doesn't reside in the mainline source tree, but in your |
| 10 | kernel module source tree. |
| 11 | |
| 12 | The only reference to this mainline header is in the LTTng custom |
| 13 | probe's source code (`probes/lttng-probe-hello.c` in our example), for |
| 14 | build time verification: |
| 15 | |
| 16 | ~~~ c |
| 17 | /* ... */ |
| 18 | |
| 19 | /* Build time verification of mismatch between mainline TRACE_EVENT() |
| 20 | * arguments and LTTng adaptation layer LTTNG_TRACEPOINT_EVENT() arguments. |
| 21 | */ |
| 22 | #include <trace/events/hello.h> |
| 23 | |
| 24 | /* ... */ |
| 25 | ~~~ |
| 26 | |
| 27 | The preferred, flexible way to include your module's mainline |
| 28 | tracepoint definition header is to put it in a specific directory |
| 29 | relative to your module's root (`tracepoints`, for example) and include it |
| 30 | relative to your module's root directory in the LTTng custom probe's |
| 31 | source: |
| 32 | |
| 33 | ~~~ c |
| 34 | #include <tracepoints/hello.h> |
| 35 | ~~~ |
| 36 | |
| 37 | You may then build LTTng-modules by adding your module's root |
| 38 | directory as an include path to the extra C flags: |
| 39 | |
| 40 | <pre class="term"> |
| 41 | make <strong>ccflags-y=-I/path/to/kernel/module</strong> KERNELDIR=/path/to/custom/linux |
| 42 | </pre> |
| 43 | |
| 44 | Using `ccflags-y` allows you to move your kernel module to another |
| 45 | directory and rebuild the LTTng-modules project with no change to |
| 46 | source files. |