[lttng-dev] Extract lttng trace from kernel coredump

Corey Minyard minyard at acm.org
Sat Jan 18 13:29:52 EST 2014

On 01/18/2014 10:47 AM, Mathieu Desnoyers wrote:
> ----- Original Message -----
>> From: "Corey Minyard" <minyard at acm.org>
>> To: "David Goulet" <dgoulet at efficios.com>
>> Cc: lttng-dev at lists.lttng.org
>> Sent: Tuesday, January 14, 2014 3:39:43 PM
>> Subject: Re: [lttng-dev] Extract lttng trace from kernel coredump
>> On 01/14/2014 12:42 PM, David Goulet wrote:
>>> On 13 Jan (16:39:47), Corey Minyard wrote:
>>>> I'm working on a feature to allow an lttng trace to be extracted from a
>>>> kernel coredump, and I'm wondering the best way to proceed.  I've looked
>>>> at extracting from the existing data structures, but that would require
>>>> something to always have a trace file open and it looks more than a
>>>> little complicated.  I was thinking instead of having a different trace
>>>> channel for configuring this coredump and some globals to point to the
>>>> various data structures.  Would that be the best way to pursue this?  Is
>>>> there a better way, perhaps?
>>> Oh yes we do actually already have a way to do that :)
>>> Please see:
>>> https://git.lttng.org/?p=lttng-tools.git;a=tree;f=extras/core-handler;h=29ded89a71fd36e159d671ca1f405cea394f8683;hb=HEAD
>>> With the snapshot feature, it's quite easy to do that. Hope that helps
>>> you with what you are trying to achieve.
>> I did see that, but it seems to be for an application coredump.  I was
>> looking for something that would work for a kernel coredump.  So that if
>> the kernel crashes, you can use kdump to extract the kernel core and
>> then extract the LTT buffers from that.  The idea is that if the kernel
>> crashes, you can get some idea of what was happening before the crash.
> Something along these lines have been proposed a while back on the ML,
> but it was against an old LTTng version (0.x):
> http://lists.lttng.org/pipermail/lttng-dev/2010-August/014239.html
> It might be interesting to adapt it to the newer LTTng 2.x. Anyone interested
> to look into this ?

That was done by a colleague of mine.  However, things appear to have
changed so much since then that it doesn't seem simple to extract a
trace from memory.  It could be done, but I'm currently looking at
adding a separate transport that writes to a simpler per-core circular
data structure and doesn't actually ever generate any output.  Since it
doesn't generate any output, it can be much simpler.

But I am certainly open to suggestions on how to do this, and happy to
have anything included back into the mainline.

And I'm still learning about the internals of LTT.



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