[lttng-dev] tracing program flow

[Mathieu Desnoyers]

* Alexandre Montplaisir (alexandre.montplaisir at polymtl.ca) wrote:
> On 12-05-06 01:17 AM, Chuck Tuffli wrote:
> > Hi -
> >
> > I'm investigating the feasibility of replacing some home grown tracing
> > code with LTTng and am looking for advice from others who have used
> > LTTng to trace program flow (i.e. trace function entry and exit
> > points). One of the strengths of LTTng seems to be the ability to
> > selectively enable tracepoints of interest (much more useful than our
> > all-or-nothing tracing). To get this type of capability, would I need
> > to implement an entry and exit tracepoint for each function or is
> > there a better approach? Any other best practices people would be
> > willing to share? TIA.
> 
> Hi,
> 
> For now you'd have to create a custom probe for each function entry and
> exit.
> 
> There's a feature on the wishlist to use GCC hooks to automatically
> generate those probes [1]. The end result would be the same, but it
> would be much less tedious to do the instrumentation part. A Clang
> plugin was also mentioned previously [2], but I don't think anybody has
> tried it yet.

Looking at how the Linux kernel implements function entry and exit
tracing is relevant here:

- kretprobes: using breakpoints, and boosted breakpoints.
- function tracer: based on gcc mcount instrumentation, replaced at
  build time and runtime by nops/function call.

Ideally at some point, we might want to reimplement those in user-space
as a LTTng-UST library.

Best regards,

Mathieu


> 
> 
> [1] https://bugs.lttng.org/issues/39
> [2] http://lists.lttng.org/pipermail/lttng-dev/2012-April/017787.html
> 
> 
> Hope this helps,
> 
> -- 
> Alexandre Montplaisir
> DORSAL lab,
> École Polytechnique de Montréal
> 
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com