doc: debugging: Add section about CPU load

Add documentation for CPU load module.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

doc/services/debugging/cpu_load.rst

@@ -0,0 +1,26 @@
+.. _cpu_load:
+
+CPU load
+########
+
+Module can be used to track how much time is spent in idle. It is using tracing hooks
+which are called before and after CPU goes to idle. Compared to :ref:`thread_analyzer`
+it is more accurate since it takes into account time spent in the interrupt context as well.
+
+Function :c:func:`cpu_load_get` is used to get the latest value. It is also used to reset
+the measurement. By default, module is using :c:func:`k_cycle_get_32` but in cases when higher
+precision is needed a :ref:`counter_api` device can be used.
+
+Load can also be reported periodically using a logging message. Period is configured using :kconfig:option:`CONFIG_CPU_LOAD_LOG_PERIODICALLY`.
+
+Using counter device
+********************
+
+In order to use :ref:`counter_api` device :kconfig:option:`CONFIG_CPU_LOAD_USE_COUNTER` must be
+enabled and chosen in devicetree must be set.
+
+.. code-block:: devicetree
+
+   chosen {
+     zephyr,cpu-load-counter = &counter_device;
+   };

doc/services/debugging/index.rst

@@ -7,6 +7,7 @@ Debugging
    :maxdepth: 1
 
    thread-analyzer.rst
+   cpu_load.rst
    coredump.rst
    gdbstub.rst
    debugmon.rst