LTTng » LTTng-tools

It is possible to explicitly create a kernel channel named metadata in the kernel domain.

$ lttng enable-channel -k metadata --num-subbuf 256
Kernel channel metadata enabled for session youssef_robert

This channel can be listed:

- metadata: [enabled]

    Attributes:
      Event-loss mode:  discard
      Sub-buffer size:  1048576 bytes
      Sub-buffer count: 256
      Switch timer:     inactive
      Read timer:       200000 µs
      Monitor timer:    1000000 µs
      Blocking timeout: 0 µs
      Trace file count: 1 per stream
      Trace file size:  unlimited
      Output mode:      splice

    Statistics:
      Discarded events: 0

    Event rules:
      None

It is surprisingly possible to add event rules to such a channel.

lttng enable-event -k -a -c metadata
All Kernel events are enabled in channel metadata

Predictably, since this channel is not identified as a special metadata channel, the configuration is not used during the creation of the metadata stream.

Instead, a dedicated ltt_kernel_metadata field is used. This field is initialized during the creation of the kernel session with a set of default attributes.

It is unclear to me if this ever worked since I couldn't find a trace of the code that should handle this.