[lttng-dev] Trying to understand use of lttng enable-event --kernel --userspace-probe=

[Brian Hutchinson]

On Thu, May 18, 2023 at 11:00 AM Brian Hutchinson <b.hutchman at gmail.com> wrote:
>
> On Thu, May 18, 2023 at 10:45 AM Mathieu Desnoyers
> <mathieu.desnoyers at efficios.com> wrote:
> >
> > On 2023-05-18 10:10, Brian Hutchinson wrote:
> > [...]
> > > I updated my hello world to have a function I'd like to use the
> > > --userspace-probe method on with the very original name of
> > > 'probe_function':
> > >
> > > #include <stdio.h>
> > > #include <lttng/tracef.h>
> > >
> > > void probe_function(int i);
> > >
> > > int main(int argc, char *argv[])
> > > {
> > >     unsigned int i;
> > >     puts("Hello, World!\nPress Enter to continue...");
> > >     /*
> > >      * The following getchar() call only exists for the purpose of this
> > >      * demonstration, to pause the application in order for you to have
> > >      * time to list its tracepoints. You don't need it otherwise.
> > >      */
> > >     getchar();
> > >
> > >     lttng_ust_tracef("Number %d, string %s", 23, "hi there!");
> > >     printf("Number %d, string %s", 23, "hi there!");
> > >
> > >     for (i = 0; i < argc; i++) {
> > >         lttng_ust_tracef("Number %d, argv %s", i, argv[i]);
> > >         printf("Number %d, argv %s", i, argv[i]);
> > >     }
> > >
> > >     puts("Quitting now!");
> > >
> > >     probe_function(i);
> > >
> > >     return 0;
> > > }
> > >
> > > void probe_function(int i) {
> > >
> > >     lttng_ust_tracef("Number %d, string %s", i * i, "i^2");
> > >     printf("Number %d, string %s", i * i, "i^2");
> > >
> > > }
> > >
> > > ... and I get the same error as before when I try to enable the probe:
> > > # lttng enable-event --kernel
> > > --userspace-probe=/usr/local/bin/hello:probe_function
> > > Error: Missing event name(s).
> >
> > As the error states, you are missing the event name. See
> >
> > man 1 lttng-enable-event
> >
> >         lttng [GENERAL OPTIONS] enable-event --kernel
> >               [--probe=SOURCE | --function=SOURCE | --syscall |
> >                --userspace-probe=SOURCE]
> >               [--filter=EXPR] [--session=SESSION]
> >               [--channel=CHANNEL] EVENT[,EVENT]...
> >
> >
> > You will want something like:
> >
> > lttng enable-event --kernel --userspace-probe=/usr/local/bin/hello:probe_function my_probe_function
> >
> > Where "my_probe_function" is the event name that will appear in the collected traces.
>
> Wow!  I must not have woken up this morning ha, ha.  Thanks for that!
> The event is enabled now.  Hope to actually get tracing data now.

Well, I guess we just have the app that thwarts all attempts at tracing.

I did a dynamic probe on several functions that should be getting
called like crazy and again I get no tracing data.

Tried it with my hello world example above after Mathieu set me
straight on the event syntax and it works.

I saw this comment in the documentation "As of this version, only USDT
probes that are not surrounded by a reference counter (semaphore) are
supported."

I don't know that I can say that this function I'm probing isn't
"surrounded" by a reference counter, it's in a large multi-threaded
application so I guess it's possible.

Sigh, I'm striking out every which way.

No offense (since this is lttng list - please don't flame me ... I
want/need lttng), but I think I'm going to try just straight kprobes
and uprobes and see if trace compass can show those traces in an
attempt to get "something/anything" working.

Regards,

Brian