[ltt-dev] LTTng kernel integration roadmap, update

Mathieu Desnoyers compudj at krystal.dyndns.org
Mon Nov 24 15:46:22 EST 2008

* Steven Rostedt (rostedt at goodmis.org) wrote:
> On Mon, 24 Nov 2008, Mathieu Desnoyers wrote:
> > 
> > The key idea behind this is to answer to Thomas Gleixner concerns, who
> > supports that a tracer should output data in text-format only so it can
> > be used with tools kernel developers have on their system, like "cat".
> > 
> > However, getting data out of the kernel efficiently simply cannot be
> > done with such approach. Therefore, LTTng needs its own userspace tools
> > to splice the data out of the kernel efficiently. Another tool is used
> > to pretty-print the binary data into text.
> > 
> > Then the problem becomes : we have to make the userspace tool easy
> > enough to deploy so even Linus can find and use it. ;)
> > 
> > But indeed, the trace buffers are versioned, so if the format changes
> > between kernel versions, the userspace tools will detect it and the user
> > will know it must update its tools. So it's not really a problem there.
> > 
> > The question that prevails is therefore : should we ship userspace
> > binary with the kernel tree at all ? And if yes, how should the resuting
> > executables be packaged and deployed ? Should it be installed in the
> > system along with kernel modules or should it be populated into a
> > filesystem populated by kernelspace ?
> > 
> > Or is it better to do as we have always done and keep the userspace
> > tools separated from the kernel tree ?
> I say keep the user space tools separate as much as possible.

I'd be in favor of that too. We should just document and package it so
it's easy to find.

> What about having a meta-data file for all binary files. This meta-data 
> could explain the format that is read. Big endian, little endian, the 
> fields and offsets, the event ids etc.  This way we will not need a 
> "version" file, which means absolutely nothing if you do not know what 
> comes with that version. Any tool could look at the meta-data file and 
> figure out what is in the buffers.
> -- Steve

This is exactly what I do in LTTng, modulo the fact that I repeat this
information also in other buffer headers, but only use the information
located in the metadata buffer header. I duplicated the information to
make sure all subbuffer headers looks the same, but I could easily
change that.

I would however keep a small subbuffer header with a version number for
each subbuffers though, just so the parser can "know" what file this is
and what metadata should be expected with it. I think about the poor
user who lost its metadata file and wonders what tool could open the
other tracefiles he has... without a header containing at least a magic
number and a version, those files won't be identified. But we can keep
this information as minimalistic as possible.

Thanks for the feedback.


Mathieu Desnoyers
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