[lttng-dev] Userspace Tracing and Backtraces

Brian Robbins brianrob at microsoft.com
Mon Mar 9 19:17:51 EDT 2015

Thanks Francis.

This is what I expected to have to do.  I do agree though that adding this to lttng-ust would be a good way to go.

Should we end up on this path, it certainly seems like it might we worth our time to investigate what it would take to add it to lttng-ust.  Do you know who is the right person to talk to about this?  I’d want to make sure that this would not be a non-starter.


From: Francis Giraldeau [mailto:francis.giraldeau at gmail.com]
Sent: Monday, March 9, 2015 12:39 PM
To: Brian Robbins
Cc: lttng-dev at lists.lttng.org
Subject: Re: [lttng-dev] Userspace Tracing and Backtraces

2015-03-06 13:51 GMT-05:00 Brian Robbins <brianrob at microsoft.com<mailto:brianrob at microsoft.com>>:
Thanks Francis.

Is it accurate to say then that the array of addresses would need to be captured by app code by writing a stack walker by hand

Yes, the callstack can be recorded in userspace. You would need a tracepoint with a varying length field:

TRACEPOINT_EVENT(myprovider, callstack,
            TP_ARGS(unsigned long *, buf, size_t, depth),
                        ctf_sequence(unsigned long, addr, buf, size_t, depth)

In the app code, use libunwind [1] to get the addresses, then call the tracepoint:

  do_unwind() // use libunwind here
  tracepoint(myprovider, buf, depth);

However, the unwind will be done whether or not the tracepoint is active (~10us-100us in steady state, so it's may become expansive if called often). I know there was discussion about tp_code() for such use case (some code to call before the tracepoint only if it is enabled). Or you can cheat:

if (__builtin_expect(!!(__tracepoint_myprovider___callstack.state), 0)) {
    tracepoint(myprovider, buf, depth);

That said, instead of having a callstack tracepoint, IMHO the best solution would be instead extending lttng-ust to add callstack event context (itself linked to libunwind). Then, recording the callstack would be simple like that:

  $ lttng add-context -u -t callstack

or using the perf capture mechanism that you describe below?

Perf is peeking at the userspace from kernel space, it's another story. I guess that libunwind was not ported to the kernel because it is a large chunk of complicated code that performs a lot of I/O and computation, while copying a portion of the stack is really about KISS and low runtime overhead.


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