The 'trace_logger' component can be used to easily gather, process and export
different types of tracing data. Which subjects to select is configurable via
session label policies and thread names. Which data to collect from the
selected subjects can be configured for each subject individually, for groups
of subjects, or for all subjects. The gathered data can be exported as log
output.


Configuration
~~~~~~~~~~~~~

This is an example configuration of the 'trace_logger' component which shows
the default value for each attribute except the policy.thread and
policy.label:

! <config verbose="no"
!         session_ram="10M"
!         session_arg_buffer="4K"
!         session_parent_levels="0"
!         period_sec="5"
!         default_policy="null"
!         default_buffer="4K">
!
!    <policy label="init -> timer" />
!    <policy label_suffix=" -> ram_fs" />
!    <policy label_prefix="init -> encryption -> "
!            thread="worker"
!            buffer="4K"
!            policy="null" />
! </config>


This is a short description of the tags and attributes:

:config.verbose:
  Optional. Toggles whether the trace_logger shall log debugging information.
  If enabled, even inactive trace subjects appear in the log.

:config.sc_time:
  Optionally log scheduling-context execution time in addition to the
  thread-context. This may be relevant on kernel platforms that do not
  schedule threads on their own CPU share only but implement some kind of
  donation scheme, for example, NOVA.

:config.session_ram:
  Optional. Amount of RAM donated to the trace session.

:config.session_arg_buffer:
  Optional. Size of the trace sessions argument buffer.

:config.session_parent_levels:
  Optional. Number of parent levels to trace.

:config.period_sec:
  Optional. Length of processing/export interval in seconds.

:config.default_policy:
  Optional. Name of tracing policy for subjects without individual config.

:config.default_buffer:
  Optional. Size of tracing buffer for subjects without individual config.

:config.policy:
  Subject selector. For matching subjects, tracing is enabled and the defined
  individual configuration is applied.

:config.policy.label:
:config.policy.label_prefix:
:config.policy.label_suffix:
  Mutually exclusive. Filters subjects according to their session label.

:config.policy.thread:
  Optional. Filters subjects according to their exact thread name.

:config.policy.buffer:
  Optional. Size of tracing buffer used for matching subjects.

:config.policy.policy:
  Optional. Name of tracing policy used for matching subjects.


Sessions
~~~~~~~~

This is an overview of the sessions required and provided by the
'trace_logger' component apart from the environment sessions:

* Requires ROM sessions to all configured tracing policies.
* Requires one TRACE session that provides the desired subjects.
* Requires one Timer session.


Examples
~~~~~~~~

An Example of how to use the trace_logger component can be found in the test
package at 'os/recipes/pkg/test-trace_logger/'.
