[lttng-dev] How to check is tracepoint is currently enabled (UST)

[Thibault, Daniel]

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Date: Sun, 22 Jun 2014 19:27:31 +0400
From: Dmitri Shubin <sbn at tbricks.com>

> In dtrace it's possible to check if some probe is currently enabled or not (to avoid argument preparation for disabled probes).
> But I'm unable to find similar feature in lttng-ust (at least it's not mentioned in lttng-ust man page).
>
> Am I missed it or it's not implemented on purpose?

   Use the list command.  A command such as 'lttng list <session_name>' will list the known events and their enabled/disabled status.  You can filter the command's output by specifying a domain (--userspace) and/or a channel (--channel <channel_name>).  For example:

(begin example)
$ lttng create some_session
Session some_session created.
Traces will be written in /home/username/lttng-traces/ some_session-20140625-102709
$ lttng enable-event -u some_event
UST event some_event created in channel channel0
$ lttng list some_session -u --channel channel0
Tracing session some_session: [inactive]
    Trace path: /home/username/lttng-traces/ some_session-20140625-102709

=== Domain: UST global ===

Buffer type: per UID

- channel0: [enabled]

    Attributes:
      overwrite mode: 0
      subbuffers size: 131072
      number of subbufers: 4
      switch time interval: 0
      read time interval: 0
      output: mmap()

    Events:
      some_event (type: tracepoint) [enabled]
(end example)

   You can take a look at /usr/src/lttng-tools/src/bin/lttng/commands/list.c to see how the information is obtained.

Daniel U. Thibault
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