trace doc: add ftrace-uses.rst to doc tree

The ftrace context

==================

.. warning::

WARNING: The ability to add a callback to almost any function within the

  The ability to add a callback to almost any function within the

  kernel comes with risks. A callback can be called from any context

  (normal, softirq, irq, and NMI). Callbacks can also be called just before

  going to idle, during CPU bring up and takedown, or going to user space.

Both .flags and .private are optional. Only .func is required.

To enable tracing call::

To enable tracing call:

.. c:function::  register_ftrace_function(&ops);

To disable tracing call::

To disable tracing call:

.. c:function::  unregister_ftrace_function(&ops);

The above is defined by including the header::

The above is defined by including the header:

.. c:function:: #include <linux/ftrace.h>

See Filter Commands in :file:`Documentation/trace/ftrace.txt`.

To just trace the schedule function::

To just trace the schedule function:

.. code-block:: c

@reset parameter set to zero. To remove the current filter set and replace it

with new functions defined by @buf, have @reset be non-zero.

To remove all the filtered functions and trace all functions::

To remove all the filtered functions and trace all functions:

.. code-block:: c

   :maxdepth: 2

   ftrace-design

   ftrace-uses