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Bug #1262

Metadata channel properties are not configurable (kernel domain)

Added by Jérémie Galarneau 7 months ago. Updated 7 months ago.

Status:
New
Priority:
Low
Assignee:
-
Target version:
-
Start date:
05/05/2020
Due date:
% Done:

0%

Estimated time:

Description

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.

#1

Updated by Mathieu Desnoyers 7 months ago

Is it possible to create a "special" metadata channel for user-space ?

In any case, we should decide whether we want to simply disallow specifying metadata channel options (possibly printing a warning if users try to), or wire up that configuration.

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