[lttng-dev] Some questions about Lttng

[Mathieu Desnoyers]

* Francis Giraldeau (francis.giraldeau at gmail.com) wrote:
> 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.

There is no backtrace dump feature in lttng 2.0.

Arguments of system calls are almost all there on x86 32/64 and ARM.
What architecture are you using ?

> 
> > 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.

Good advice.

> 
> > 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.

Yep, not possible. You'd have to wrap the tracepoint.

> 
> > 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?

This is correct.

Thanks,

Mathieu

> 
> 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:
> 
> https://github.com/giraldeau/workload-kit/blob/master/ust/cyg.c
> 
> Cheers,
> 
> Francis



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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com