[lttng-dev] [PATCH lttng-ust] Improve tracelog handling, reduce exported functions

[Norbert Lange]

Am Do., 20. Mai 2021 um 19:18 Uhr schrieb Mathieu Desnoyers
<mathieu.desnoyers at efficios.com>:
>
>
>
> ----- On May 20, 2021, at 12:51 PM, Norbert Lange nolange79 at gmail.com wrote:
>
> > Am Do., 20. Mai 2021 um 18:25 Uhr schrieb Mathieu Desnoyers
> > <mathieu.desnoyers at efficios.com>:
> >>
> >> ----- On May 20, 2021, at 11:54 AM, Norbert Lange nolange79 at gmail.com wrote:
> >> [...]
> >>
> >> >> What prevents you from linking against lttng-ust.so again ?
> >> >
> >> > I did not poke around enough with Lttng to be confident it wont have
> >> > side effects,
> >> > I really don't want it active in production. It doesn't seem there is
> >> > much public knowledge with Xenomai either.
> >> > lttng-ust.so will spawn threads, lttng-ust-tracepoint.so is mostly passive,
> >>
> >> There is indeed a split between instrumentation and runtime threads done
> >> with lttng-ust-tracepoint.so vs lttng-ust.so.
> >>
> >> I understand that this split is missing for tracelog and tracef, and
> >> would be a good thing to have.
> >>
> >> I would be interested to move the tracelog and tracef implementation
> >> from liblttng-ust.so to liblttng-ust-tracepoint.so, even this late
> >> in the -rc cycle, because all users of tracelog/tracef need to link
> >> against liblttng-ust-tracepoint.so anyway. So moving these symbols
> >> should not affect anyone.
> >>
> >> Can you give it a try and let me know if it works for you ?
> >
> > Will take some time, whats the timeframe you need for feedback?
>
> Here is the tentative commit:
>
> https://review.lttng.org/c/lttng-ust/+/5927 Move tracef/tracelog symbols to liblttng-ust-tracepoint.so

Well... this is certainly an improvement. I am not completely happy
though: "... users now link against
liblttng-ust-tracepoint.so explicitly"

My homecooked solution currently works like this:

*) define the probes from <lttng/lttng-ust-tracelog.h> with
TRACEPOINT_PROBE_DYNAMIC_LINKAGE,
    link them in the application, together with other dynamic probes
*) build a separate library with *other* tracepoints, lets call it
libtracepoint.so
*) don't link the application to any lttng library.

Which means:

1) the application works without lttng libraries. tracepoints are no-ops
2) if available then liblttng-ust-tracepoint.so is loaded (constructor
function from your headers). tracepoints are no-ops
3) if the application dlopen's libtracepoint.so and in turn
liblttng-ust.so then tracepoints work.

I'd lose option 1 compared to reimplementing tracelog using homecooked
lttng-ust-tracelog tracepoints.

So, are there any issues using <lttng/lttng-ust-tracelog.h> that way,
it seems to work fine,
are there mutliple competing instances now?
(I am not re-using any bit from tracelog.h, I am just after using the
tracepoint definition).

I mean I could dlsym all the functions, but tracelog has 1 per
loglevel and really ugly long names ;)

Norbert