[Ltrace-devel] Support for systemtap probes in ltrace

[Steve Fink]

On Mon, Aug 8, 2011 at 3:54 AM, Petr Machata <pmachata at redhat.com> wrote:
> Steve Fink <sphink at gmail.com> writes:
>
>> On Sat, Aug 6, 2011 at 6:58 PM, Petr Machata <pmachata at redhat.com> wrote:
>>> Hi there,
>>>
>>> SDT probes are used by some application--typically, they are placed to
>>> "strategic" places, such as when MySQL parses a query, or when OpenJDK
>>> compiles a method.  I believe it would be useful for ltrace to be able
>>> to put breakpoints on these.
>>
>> I will soon be adding Firefox to that list (well, really just the
>> Javascript interpreter within Firefox). But that makes me curious --
>> how are you planning to use this functionality?
>
> I don't plan adding SDT probes to ltrace, I want ltrace to be able to
> "see" and trace the probes.  Is that what you had in mind?

Yes. I've added SDT probes to Firefox, and hope to enable them in the
default build soon. As I understand it, your work would allow running
ltrace on an application containing SDT probes and see the shared
library calls plus the static probes. I can see how that would be
theoretically useful (eg to only consider library calls after a MySQL
query was parsed to avoid startup cruft, or only look between start
and end markers of a garbage collection run in Firefox.) But I'm
curious if you have specific use cases in mind. I'm planning to
collect a set of (hopefully) compelling examples of when this sort of
tracing stuff is useful, and it'd be great to add an ltrace example to
the list. Especially since it would show that inserting these probes
is more generally useful than just being able to write systemtap
scripts that make use of them. But I don't want to argue that in the
abstract; I want concrete examples of where it would be useful for
actual problems/analyses.

Thanks,
Steve