Hi Chris,
On 2023-10-04 12:56, Christopher Harvey via lttng-dev wrote:
Hi all,
I'm just getting started with lttng and have a couple of questions. My target
is getting userspace tracepoints to work in a c++ application I control. I
don't need kernel or system tracing. The target is running in a docker
container I have control over as well. Running with --privileged is possible in
my setup. I could run lttng inside the target container, outside on the host,
or in another container on the same host. It's all the same to me. I don't know
which setup is best, or if it even matters given that I don't need system or
kernel traces.
I am using:
lttng-tools-2.13.11/ and lttng-ust-2.13.6/
with userspace-rcu-0.14.0/
There is a slight version mismatch there, I just downloaded the "-lastest" from
the links in the guide.
Here is my tracepoint provider header:
-----
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider
#undef LTTNG_UST_TRACEPOINT_INCLUDE
#define LTTNG_UST_TRACEPOINT_INCLUDE "./myLttngUstProvider.hpp"
#if !defined(_TP_H) || defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H
#include "lttng/tracepoint.h"
LTTNG_UST_TRACEPOINT_EVENT(
/* Tracepoint provider name */
my_provider,
/* Tracepoint name */
test,
/* Input arguments */
LTTNG_UST_TP_ARGS(
int, arg1
),
/* Output event fields */
LTTNG_UST_TP_FIELDS(
lttng_ust_field_integer(int, arg1, arg1)
)
)
#endif /* _TP_H */
#include "lttng/tracepoint-event.h"
--------
and the cpp:
-------
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TRACEPOINT_DEFINE
#include "myLttngUstProvider.hpp"
#include "lttng/ust-sigbus.h"
DEFINE_LTTNG_UST_SIGBUS_STATE();
----
I'm suspicious of a couple of things in my setup.
First, if I omit the DEFINE_LTTNG_UST_SIGBUS_STATE(); line, my program fails to
compile with the following error:
liblttng-ust-ctl.so: undefined reference to `lttng_ust_sigbus_state'
I wasn't able to reproduce the compilation error you are seeing. Could
you describe in more detail how you are compiling and linking your
tracepoint provider and application?
The getting started guide doesn't say anything about needing
DEFINE_LTTNG_UST_SIGBUS_STATE so it makes me wonder if I have done something
else wrong somewhere else. I ran a find *.so | xargs nm -D | grep
lttng_ust_sigbus_state and nothing in lttng-ust provides that symbol, so it
looks like the target application has to define it. Is that normal?
If the application is actually using liblttng-ust-ctl, then it would be
normal to do the define. The header file explains that this is done to
avoid unnecessarily pre-allocating space in the thread-local storage.
https://github.com/lttng/lttng-ust/blob/master/include/lttng/ust-sigbus.h#L39
Traced applications typically don't link against liblttng-ust-ctl.
If I leave DEFINE_LTTNG_UST_SIGBUS_STATE in then my program compiles. If I run
lttng-sessiond in the target container, then run lttng list --userspace I see
no tracepoints in the list.
Is your application actively running while you run `lttng list --userspace`?
`lttng list --userspace` will only list the userspace tracepoints of
applications which are currently registered to the running session
daemon. If an application disconnects, the tracepoints it registered are
unloaded.
I tried looking at the lttng source code but there is a lot there. I'm looking
for some next steps for debugging this setup. What are some things I could
check to make sure the lttng daemon is running properly? What is the mechanism
for advertising tracepoints to the lttng daemon? Is it possible that docker has
somehow interfered with the lttng ipc mechanism? It would be a tiny bit more
convenient for me to simply deploy lttng-tools as a separate container, can I
consider doing that instead? I'm not running anything as root, just a normal
user.
I appreciate any and all guidance. Looking forward to using this neat looking
tool.
-Chris
thanks,
kienan
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