doc/kernel_v2/other/event_logger.rst

.. _event_logger_v2:

Kernel Event Logger [TBD]
#########################

Definition
**********

The kernel event logger is a standardized mechanism to record events within the
Kernel while providing a single interface for the user to collect the data.
This mechanism is currently used to log the following events:

* Sleep events (entering and exiting low power conditions).
* Context switch events.
* Interrupt events.

Kernel Event Logger Configuration
*********************************

Kconfig provides the ability to enable and disable the collection of events and
to configure the size of the buffer used by the event logger.

These options can be found in the following path :file:`kernel/Kconfig`.

General kernel event logger configuration:

* :option:`CONFIG_KERNEL_EVENT_LOGGER_BUFFER_SIZE`

  Default size: 128 words, 32-bit length.

Profiling points configuration:

* :option:`CONFIG_KERNEL_EVENT_LOGGER_DYNAMIC`

  Allows modifying at runtime the events to record. At boot no event is
  recorded if enabled This flag adds functions allowing to enable/disable
  recording of kernel event logger.

* :option:`CONFIG_KERNEL_EVENT_LOGGER_CUSTOM_TIMESTAMP`

  Enables the possibility to set the timer function to be used to populate
  kernel event logger timestamp. This has to be done at runtime by calling
  sys_k_event_logger_set_timer and providing the function callback.

Adding a Kernel Event Logging Point
***********************************

Custom trace points can be added with the following API:

* :c:func:`sys_k_event_logger_put()`

  Adds the profile of a new event with custom data.

* :cpp:func:`sys_k_event_logger_put_timed()`

  Adds timestamped profile of a new event.

.. important::

   The data must be in 32-bit sized blocks.

Retrieving Kernel Event Data
****************************

Applications are required to implement a cooperative thread for accessing the
recorded event messages.  Developers can use the provided API to retrieve the
data, or may write their own routines using the ring buffer provided by the
event logger.

The API functions provided are:

* :c:func:`sys_k_event_logger_get()`
* :c:func:`sys_k_event_logger_get_wait()`
* :c:func:`sys_k_event_logger_get_wait_timeout()`

The above functions specify various ways to retrieve a event message and to
copy it to the provided buffer. When the buffer size is smaller than the
message, the function will return an error. All three functions retrieve
messages via a FIFO method. The :literal:`wait` and :literal:`wait_timeout`
functions allow the caller to pend until a new message is logged, or until the
timeout expires.

Enabling/disabling event recording
**********************************

If KERNEL_EVENT_LOGGER_DYNAMIC is enabled, following functions must be checked
for dynamically enabling/disabling event recording at runtime:

* :cpp:func:`sys_k_event_logger_set_mask()`
* :cpp:func:`sys_k_event_logger_get_mask()`

Each mask bit corresponds to the corresponding event ID (mask is starting at
bit 1 not bit 0).

More details are provided in function description.

Timestamp
*********

The timestamp used by the kernel event logger is 32-bit LSB of platform HW
timer (for example Lakemont APIC timer for Quark SE). This timer period is very
small and leads to timestamp wraparound happening quite often (e.g. every 134s
for Quark SE).

see :option:`CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC`

This wraparound must be considered when analyzing kernel event logger data and
care must be taken when tickless idle is enabled and sleep duration can exceed
maximum HW timer value.

Timestamp used by the kernel event logger can be customized by enabling
following option: :option:`CONFIG_KERNEL_EVENT_LOGGER_CUSTOM_TIMESTAMP`

In case this option is enabled, a callback function returning a 32-bit
timestamp must be provided to the kernel event logger by calling the following
function at runtime: :cpp:func:`sys_k_event_logger_set_timer()`

Message Formats
***************

Interrupt-driven Event Messaging
--------------------------------

The data of the interrupt-driven event message comes in two block of 32 bits:

* The first block contains the timestamp occurrence of the interrupt event.
* The second block contains the Id of the interrupt.

Example:

.. code-block:: c

   uint32_t data[2];
   data[0] = timestamp_event;
   data[1] = interrupt_id;

Context-switch Event Messaging
------------------------------

The data of the context-switch event message comes in two block of 32 bits:

* The first block contains the timestamp occurrence of the context-switch event.
* The second block contains the thread id of the context involved.

Example:

.. code-block:: c

   uint32_t data[2];
   data[0] = timestamp_event;
   data[1] = context_id;

Sleep Event Messaging
---------------------

The data of the sleep event message comes in three block of 32 bits:

* The first block contains the timestamp when the CPU went to sleep mode.
* The second block contains the timestamp when the CPU woke up.
* The third block contains the interrupt Id that woke the CPU up.

Example:

.. code-block:: c

   uint32_t data[3];
   data[0] = timestamp_went_sleep;
   data[1] = timestamp woke_up.
   data[2] = interrupt_id.


Example: Retrieving Profiling Messages
======================================

.. code-block:: c

   uint32_t data[3];
   uint8_t data_length = SIZE32_OF(data);
   uint8_t dropped_count;

   while(1) {
      /* collect the data */
      res = sys_k_event_logger_get_wait(&event_id, &dropped_count, data,
         &data_length);

      if (dropped_count > 0) {
         /* process the message dropped count */
      }

      if (res > 0) {
         /* process the data */
         switch (event_id) {
         case KERNEL_EVENT_CONTEXT_SWITCH_EVENT_ID:
            /* ... Process the context switch event data ... */
            break;
         case KERNEL_EVENT_INTERRUPT_EVENT_ID:
            /* ... Process the interrupt event data ... */
            break;
         case KERNEL_EVENT_SLEEP_EVENT_ID:
            /* ... Process the data for a sleep event ... */
            break;
         default:
            printf("unrecognized event id %d\n", event_id);
         }
      } else {
         if (res == -EMSGSIZE) {
            /* ERROR - The buffer provided to collect the
             * profiling events is too small.
             */
         } else if (ret == -EAGAIN) {
            /* There is no message available in the buffer */
         }
      }
   }

.. note::

   To see an example that shows how to collect the kernel event data, check the
   project :file:`samples/kernel_event_logger`.

Example: Adding a Kernel Event Logging Point
============================================

.. code-block:: c

   uint32_t data[2];

   if (sys_k_must_log_event(KERNEL_EVENT_LOGGER_CUSTOM_ID)) {
      data[0] = custom_data_1;
      data[1] = custom_data_2;

      sys_k_event_logger_put(KERNEL_EVENT_LOGGER_CUSTOM_ID, data,
			     ARRAY_SIZE(data));
   }

Use the following function to register only the time of an event.

.. code-block:: c

   if (sys_k_must_log_event(KERNEL_EVENT_LOGGER_CUSTOM_ID)) {
      sys_k_event_logger_put_timed(KERNEL_EVENT_LOGGER_CUSTOM_ID);
   }

APIs
****

The following APIs are provided by the :file:`k_event_logger.h` file:

:cpp:func:`sys_k_event_logger_register_as_collector()`
   Register the current cooperative thread as the collector thread.

:c:func:`sys_k_event_logger_put()`
   Enqueue a kernel event logger message with custom data.

:cpp:func:`sys_k_event_logger_put_timed()`
   Enqueue a kernel event logger message with the current time.

:c:func:`sys_k_event_logger_get()`
   De-queue a kernel event logger message.

:c:func:`sys_k_event_logger_get_wait()`
   De-queue a kernel event logger message. Wait if the buffer is empty.

:c:func:`sys_k_event_logger_get_wait_timeout()`
   De-queue a kernel event logger message. Wait if the buffer is empty until
   the timeout expires.

:cpp:func:`sys_k_must_log_event()`
   Check if an event type has to be logged or not

In case KERNEL_EVENT_LOGGER_DYNAMIC is enabled:

:cpp:func:`sys_k_event_logger_set_mask()`
   Set kernel event logger event mask

:cpp:func:`sys_k_event_logger_get_mask()`
   Get kernel event logger event mask

In case KERNEL_EVENT_LOGGER_CUSTOM_TIMESTAMP is enabled:

:cpp:func:`sys_k_event_logger_set_timer()`
   Set kernel event logger timestamp function