[lttng-dev] Userspace Tracing and Backtraces

[Francis Giraldeau]

2015-03-06 13:51 GMT-05:00 Brian Robbins <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),
TP_FIELDS(
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)) {
    do_unwind(...)
    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.

Cheers,

Francis
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