[lttng-dev] Userspace events not seen by kernel daemon?

[Jérémie Galarneau]

Hi Daniel,

This is the expected behaviour. Only users that belong to the
"tracing" group can interact with the root session daemon as pointed
out in the lttng-tools manpage:

"LTTng provides the use of a tracing group (default: tracing).
Whomever is in that group can interact with the root session daemon
and thus trace the kernel. Session daemons can co-exist meaning that
you can have a session daemon running as Alice that can be used to
trace her applications along side with a root daemon or even a Bob
daemon."

Since your user-space application is running as a non-tracing user, it
can only interact with a session daemon running as that same user.
There is currently no way (that I know of) to create an omniscient
session that would pickup events from the kernel and from every user
without forcing them to be part of the tracing group.

Regards,
Jérémie

On Tue, May 7, 2013 at 10:08 AM, Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
> * Thibault, Daniel (Daniel.Thibault at drdc-rddc.gc.ca) wrote:
>>    I'm running an up-to-date LTTng suite (except for lttng-ust, where I'm using the 5 Mar 2013 20:17:22 commit). I've compiled the lttng-ust/doc/examples/easy-ust/sample application. While I run it (as a user *not* belonging to the tracing group), I notice something unexpected:
>>
>> * The local session daemon lists the sample user-space event ('lttng list -u')
>> * The kernel session daemon does not see the sample user-space event at all ('sudo lttng list -u ')
>
> I guess you mean "per-user" rather than "local", and "system-wide"
> rather than "kernel" here.
>
> I will let Jeremie look into your question,
>
> Thanks,
>
> Mathieu
>
>>
>>    It was my understanding that the kernel daemon should see all kernel and user-space events (for as many user-spaces as there might be).
>>
>>    To test this further, I set up two traces using separate consoles.  The first is a user session:
>>
>> $ lttng create usersession
>> $ lttng enable-event -u --all
>>
>>    The other is a kernel session:
>>
>> $ sudo -H lttng create kernelsession
>> $ sudo -H lttng enable-event -u --all
>> $ sudo -H lttng enable-event -k sched_switch
>>
>>    I start each session (respectively '$ lttng start' and '$ sudo -H lttng start') and the sample application (from a third console, '$ ./sample').  I wait until sample concludes, then shut down both sessions (respectively '$ lttng destroy' and '$ sudo -H lttng destroy').  Use of 'sudo -H' avoids current trace name collisions (contention over the .lttngrc file), as reported earlier on this list (but not filed as a bug...maybe I should do that too).
>>
>>    The kernel trace contains no UST events, as can readily be discerned by the absence of a /ust branch in its folder structure.  This is confirmed by babeltrace, which sees some 245 000 kernel events and nothing else.  There are no missing events (babeltrace does not complain about skipped events, which it did with some other traces generated about the same time).  The System Monitor shows two lttng-sessiond daemons (one for me, another for root) and three lttng-consumerd daemons (one for me, two for root) as expected.
>>
>>    What did I do wrong?
>>
>> Daniel U. Thibault
>> R & D pour la défense Canada - Valcartier (RDDC Valcartier) / Defence R&D Canada - Valcartier (DRDC Valcartier)
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>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com



-- 
Jérémie Galarneau
EfficiOS Inc.
http://www.efficios.com