[lttng-dev] Some questions about Lttng

Francis Giraldeau francis.giraldeau at gmail.com
Tue Jun 19 05:28:01 EDT 2012

Le 2012-06-19 05:50, Zheng.Chang at emc.com a écrit :
> Hi folks,
> I'm studying how to use Lttng now. I built a kernel which version is
> 2.6.38 and ran with lttng 2.0.
> I got some confused when I started to use it. Here are my questions:
> 1. I didn't see kernel patches for kernel 3.x. Does it mean kernel 3.x
> support it already?

You don't need a kernel patch with lttng 2.0, only modules are required.
It uses the tracepoints already present in the kernel, and trace system
calls, and uses perf performance counters and kprobes.

> 2. I tried to do something like, dump the arguments of system call, or
> dump a backtrace in a specified function. But the output of lttng is
> very limited. Is there a way to do that with lttng?

If system calls are enabled, arguments are supposed to be dumped (option
--syscall to lttng enable-event). If it's not the case, then are you
sure you are using lttng 2.0 and not 0.12? ;) Because the older version
has a different behavior. One additional note: few system calls do not
have all their arguments decoded in lttng 2, but there are only a few.

> 3. I looked into some UST examples and found here are three header
> files:  tracepoint.h, tracepoint-event.h and ust-tracepoint-event.h.
> They have some duplicated macro definitions like TRACEPOINT_EVENT.

The macros are quite complicated. Some includes files are included more
than once to generate various portion of the tracepoint code. So, my
advice here is to take a working example and adapt it to your needs.

> And the examples includes all of these three header files despite no
> conflict here. Could someone help to explain the intention?
> 4. Once I defined a tracepoint in my code, seems some initializations
> would register default probe into the hook point.  How to disable the
> default probe and  register my self-defined probes?

You mean, call a custom function when tracepoint is executed? Maybe
somebody else has an answer, but AFAIK this is not possible. But you
could make a wrapper to your tracepoint and then call your additional
function there.

> 5. Does lttng-ust support dynamic traceing like kprobe?

AFAIK, the kernel tracer supports kprobe, but not UST. Maybe somebody
else can confirm/infirm the dynamic tracepoint feature in user-space?

You can use a feature of GCC to regiter callback on function entry and
exit, but since it executes for all functions, then the overhead is very
high. You can have a look here:



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