user mode backtrace

[Frank Ch. Eigler]

David Boreham <david_list@boreham.org> writes:

> I'd like to get a stack trace for the process that made the system
> call I'm probing [...]

Yes, that's an understandable need.

> But I'm worried that [calling gdb] might deadlock since the process
> is inside a system call.

Indeed, that would be several orders of magnitude too slow and invasive.

We don't yet have a good answer to user-space stuff in general.  We
may adopt a mixture of two approaches: one where the compiled
systemtap probes get some extract of backtrace-enabling unwind
information for selected user-space programs and libraries (so the
backtrace processing can be done instantly right there); and another
one where the probe may snapshot only an approximation (say a vector
of probable PCs from the thread's stack frame), and rely on a
user-space helper to correct/complete it for output later, off-line.

- FChE