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

[Norbert Lange]

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?

> > So Id want a dynamic tracepoint-provider than i can dlopen (so that
> > the signal masks are inherited,
> > I hope you dont touch them).
>
> The signals are all blocked for lttng-ust listener threads. We don't
> modify the signal masks in the tracepoint probes. Not sure which is
> the target of your question though.

The first one, if i'd preloaded lttng-ust and you dont mask signals,
then those could end up in the realtime threads.
Every Xenomai Thread has a background Linux Thread that's idle when
realtime is active,
ironically making them perfect targets for signal delivery

>
> >
> > Of course I could just remove all lttng libraries on the production
> > system aswell. Still doesnt change that
> > tracelog and tracef doesnt work that way.
>
> Would moving the tracelog/tracef implementation to liblttng-ust-tracepoint.so
> solve your issues ?

Yes, definitely. They should then work identically to other tracepoints
with TRACEPOINT_PROBE_DYNAMIC_LINKAGE.

>
> >
> > I implemented my own tracelog/tracef using the normal lttng
> > tracepoints for now, they totally break on source level with 2.13
> > aswell ;)
> > is it ok if I do this to access them:
> >
> > #define TRACEPOINT_DEFINE
> > #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
> > // 2.12
> > // #include <lttng/lttng-ust-tracelog.h>
> > // #include <lttng/lttng-ust-tracef.h>
> > // 2.13
> > #include <lttng/tp/lttng-ust-tracelog.h>
> > #include <lttng/tp/lttng-ust-tracef.h>
> >
> > ie. I would load lttng-ust.so later and can then use those tracepoints.
>
> Reimplementing the tracelog/tracef providers is not an intended use-case.
> I'd very much prefer if we move their implementation to
> liblttng-ust-tracepoint.so.

FWIW it works, and ill use it for a while (cant just swap out
libraries everywhere now).
Of course Id love a upstream solution.

Norbert