[lttng-dev] Extract lttng trace from kernel coredump

[minyard@acm.org (Corey Minyard)]

On 02/18/2014 03:02 PM, Mathieu Desnoyers wrote:
> ----- Original Message -----
>> From: "Corey Minyard" <minyard@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.

Yes, I'm going through that and the code I have to get things clean.

>> 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).
>
> Thoughts ?

Perfect, that's easy.  I'll do that.

Thanks again.

-corey