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
Kernel built-in support
It is also possible to build these modules as part of a kernel image. Simply
scripts/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
Then configure your kernel as usual and enable the
Required kernel config options
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).
CONFIG_KPROBES(5.7+): use kallsyms for kernel 5.7 and newer.
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
The lttng-modules source includes definitions for the actual callback functions that will be attached to the kernel tracepoints by lttng. The lttng-modules project implements its own macros generating these callbacks: the LTTNG_TRACEPOINT_EVENT macro family found in instrumentation/events/lttng-module/. In order to show up in a lttng-modules trace, a kernel tracepoint must be defined within the kernel tree, and also defined within lttng-modules with the LTTNG_TRACEPOINT_EVENT macro family. Customizations or extensions must be done by modifying instances of these macros within the lttng-modules source.
Linux kernels >= 3.0 are supported.
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.