[lttng-dev] Some questions about Lttng

[Zheng.Chang at emc.com]

> -----Original Message-----
> From: Mathieu Desnoyers [mailto:mathieu.desnoyers at efficios.com] 
> Sent: Wednesday, June 20, 2012 0:37 AM
> To: Francis Giraldeau
> Cc: lttng-dev at lists.lttng.org
> Subject: Re: [lttng-dev] Some questions about Lttng

> * 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 ?


My tsetbed is x86 32bit and lttng's version is 2.0.1. You're right. I can see the arguments now.


> > 
> > > 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


And here are some subsequent questions about lttng:

6. Does lttng-ust support marker? Marker is easier to be compatible with the APIs like printf if we don't care its performance issue.
7. What's supposed to show with 'lttng list -u'? It's empty now. Is that possible to show the events defined in an application?
8. What does disable-event command of lttng do? With the example(hello) provided by lttng-ust, I enabled all events with '-a -u' and then disabled them again.  I launched the example with gdb and dumped the tracepoint's structure and then found its state was active. It's supposed to be inactive here, right? BTW: I didn't see any trace generated here with view command.

Thanks all for your useful info!

Best regards
Zheng

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

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