Bug #629: Cannot enable events after a stop command - LTTng-tools - LTTng bugs repository

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Bug #629

closed

Cannot enable events after a stop command

Added by Yannick Brosseau over 11 years ago. Updated almost 11 years ago.

Status:

Won't fix

Priority:

Normal

Assignee:

Target version:

Start date:

09/09/2013

Due date:

% Done:

Estimated time:

Description

Using the latest release (2.3), I cannot enable events after I issue a start and stop commands.

Related issues 2 (1 open — 1 closed)

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Updated by Yannick Brosseau over 11 years ago

See the command line output:

root@atwood:~# lttng create
Session auto-20130909-123758 created.
Traces will be written in /root/lttng-traces/auto-20130909-123758
root@atwood:~# lttng enable-event -u -a
All UST events are enabled in channel channel0
root@atwood:~# lttng start
Tracing started for session auto-20130909-123758
root@atwood:~# lttng stop
Waiting for data availability
Tracing stopped for session auto-20130909-123758
(reverse-i-search)`enabl': lttng ^Cable-event -u -a
root@atwood:~# lttng enable-event -k -a --syscall
Error: Events: Tracing already started (channel channel0, session auto-20130909-123758)
root@atwood:~# lttng start
Tracing started for session auto-20130909-123758
root@atwood:~# lttng stop
Waiting for data availability
Tracing stopped for session auto-20130909-123758

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Updated by Yannick Brosseau over 11 years ago

output of the session daemon:
http://paste.debian.net/37508/

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Updated by Yannick Brosseau over 11 years ago

Subject changed from Can enable events after a stop command to Cannot enable events after a stop command

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Updated by David Goulet over 11 years ago

Status changed from New to Confirmed
Priority changed from High to Normal

So yes this is a limitation of the tracer (both kernel and ust). Once a session has started, you can't enable any more channel. In this case, there is no existing kernel channel after the stop thus doing "-a -k" fails because it tries to create "channel0".

You have to enable a channel prior to the start and then you'll be able to enable/disable whatever you like in it.

Now, what I'm realizing is that there is NO documentation of that limitation. So, what I propose is to rename that ticket so it becomes a documentation bug. I think we should document that in the tracer man page but also in lttng.1 since people will most likely encounter this issue with the lttng command line.

Thoughts?

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Updated by David Goulet over 11 years ago

Status changed from Confirmed to Feedback

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Updated by Yannick Brosseau over 11 years ago

I might be wrong, but I think it was possible to do this before?

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Updated by Yannick Brosseau over 11 years ago

I strongly think we should get rid of this limitation, not just document it.

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Updated by David Goulet over 11 years ago

It's possible but maybe changes in 2.1 or 2.2 changed that behavior.
There is now an explicit check done for that use case in the session
daemon in order to avoid to send the request to the tracer.

I can't be sure on before but for now it's certain you can't. No idea on
the specifics though. You'll have to ask Mathieu for that.

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