[lttng-dev] Extract lttng trace from kernel coredump

[Mathieu Desnoyers]

----- Original Message -----
> From: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> To: minyard at acm.org
> Cc: lttng-dev at lists.lttng.org
> Sent: Tuesday, February 18, 2014 4:42:06 PM
> Subject: Re: [lttng-dev] Extract lttng trace from kernel coredump
> 
> ----- Original Message -----
> > From: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> > To: minyard at acm.org
> > Cc: lttng-dev at lists.lttng.org
> > Sent: Tuesday, February 18, 2014 4:02:15 PM
> > Subject: Re: [lttng-dev] Extract lttng trace from kernel coredump
> > 
> > ----- Original Message -----
> > > From: "Corey Minyard" <minyard at acm.org>
> > > To: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> > > Cc: "David Goulet" <dgoulet at efficios.com>, lttng-dev at lists.lttng.org
> > > Sent: Tuesday, February 18, 2014 2:29:23 PM
> > > Subject: Re: [lttng-dev] Extract lttng trace from kernel coredump
> > > 
> > > I have attached some gdb macros for dumping LTT buffers from a kernel
> > > coredump.  I haven't done a lot of testing, though.
> > 
> > Very cool! I'll have a look when things get less busy here. I hope my
> > earlier reply helps clearing things out.
> > 
> > > 
> > > I do have one question.  I've been getting the metadata by taking a
> > > snapshot right when the trace starts and saving that to stick into the
> > > trace output from the coredump.  Is that the best way, or is there some
> > > other way I can extract it?  There doesn't seem to be a metadata channel
> > > running in snapshot mode.
> > 
> > Actually, it's better if you grab the metadata near when you grab the
> > coredump. Grabbing the metadata at the beginning of your tracing session
> > will not have the info about other modules that might be coming and going
> > while tracing is active (which might contain tracepoints).
> > 
> > You could grab the metadata from the coredump actually, this would be the
> > best approach. Looking at lttng modules 2.4 rc, you will want to look at
> > 
> >  /lttng-events.h
> > 
> > struct lttng_session {
> >   ...
> >   struct lttng_metadata_cache *metadata_cache;
> >   ...
> > };
> > 
> > Which has the metadata string:
> > 
> > struct lttng_metadata_cache {
> >         char *data;                     /* Metadata cache */
> >         unsigned int cache_alloc;       /* Metadata allocated size (bytes)
> >         */
> >         unsigned int metadata_written;  /* Number of bytes written in
> >         metadata cache */
> >         struct kref refcount;           /* Metadata cache usage */
> >         struct list_head metadata_stream;       /* Metadata stream list */
> > };
> > 
> > You simply need to dump this metadata string into a "metadata" file, add a
> > "/* CTF 1.8 */\n line at the beginning, and this will generate a "text only
> > CTF metadata", which can be read by babeltrace.
> > 
> > If you happen to core dump while the metadata cache is being resized, you
> > might
> > encounter a corrupted metadata. It should not happen frequently, but it's
> > possible.
> > We could eventually change lttng_metadata_printf() so it makes sure to
> > always
> > keep a readable metadata around (by e.g. using kcalloc rather than krealloc
> > and
> > dealing carefully with cache_alloc vs data fields updates).
> 
> Oops, I meant "kzalloc".
> 
> I'm attaching a patch that should take care of making sure it is always in
> a valid state when a core dump occurs. You should be able to safely read
> "metadata_flushed" bytes of data at any time. Whatever has been flushed
> should be parseable by Babeltrace.

Updated patch (fixing poll).

Thanks,

Mathieu

> 
> Thoughts ?
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Thoughts ?
> > 
> > Thanks,
> > 
> > Mathieu
> > 
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com
> > 
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev at lists.lttng.org
> > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> > 
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
> 
> _______________________________________________
> lttng-dev mailing list
> lttng-dev at lists.lttng.org
> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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