[lttng-dev] Correctly using callstack-user context

Genevieve Bastien gbastien at versatic.net
Wed May 13 10:02:04 EDT 2020

Hi Christophe,

On 5/13/20 9:42 AM, Christophe Bédard via lttng-dev wrote:
> On Tue, 12 May 2020 at 08:27, Mathieu Desnoyers
> <mathieu.desnoyers at efficios.com
> <mailto:mathieu.desnoyers at efficios.com>> wrote:
>     How does your test program issue the system call ? Is it directly
>     with syscall() (see syscall(2) man page), or
>     does it call into libc ? Is your entire libc compiled with
>     -fno-omit-frame-pointer ?
> Indirectly, through libc/libstdc++. We use the default system libs
> (which I assume use -O, which in turn implies -fomit-frame-pointer).
>     As soon as one library in the chain to the system call is compiled
>     without frame pointers, this is where the
>     stack walk stops. This is usually libc.
> I see. Thanks for the information. Not sure we want to ship a custom
> libc unfortunately.
> Related question: how feasible would adding the callstack-user context
> to userspace events be? If it is feasible, is such a feature planned?

We had a very very experimental branch with it, using backtrace first,
then unwind. But it is not suggested, not performant. It's far from the
correct desired implementation, for which Mathieu could give more details.

It works, but not to be used in production. Students have used it in the
lab for some specific use case prototyping. I won't share the branch
here, but if you wish to try, let me know.


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