[PATCH v7 3/3] doc: add dispatcher programming guide

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

Provide programming guide for the dispatcher library.

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>

--

PATCH v7:
 o Mark pseudo code blocks as being type "none", to avoid Sphinx failures
   on non-Ubuntu systems. (David Marchand)
 o "Necessarily" necessarily needs to be spelled just so. (David Marchand)

PATCH v6:
 o Eliminate unneeded white space in code blocks. (David Marchand)

PATCH v5:
 o Update guide to match API changes related to dispatcher ids.

PATCH v3:
 o Adapt guide to the dispatcher API name changes.

PATCH:
 o Improve grammar and spelling.

RFC v4:
 o Extend event matching section of the programming guide.
 o Improve grammar and spelling.
---
 MAINTAINERS                              |   1 +
 doc/guides/prog_guide/dispatcher_lib.rst | 433 +++++++++++++++++++++++
 doc/guides/prog_guide/index.rst          |   1 +
 3 files changed, 435 insertions(+)
 create mode 100644 doc/guides/prog_guide/dispatcher_lib.rst

diff --git a/MAINTAINERS b/MAINTAINERS
index 262401d43d..748c15cfe9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1737,6 +1737,7 @@ Dispatcher - EXPERIMENTAL
 M: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
 F: lib/dispatcher/
 F: app/test/test_dispatcher.c
+F: doc/guides/prog_guide/dispatcher_lib.rst
 
 
 Test Applications
diff --git a/doc/guides/prog_guide/dispatcher_lib.rst b/doc/guides/prog_guide/dispatcher_lib.rst
new file mode 100644
index 0000000000..6de1ea78b0
--- /dev/null
+++ b/doc/guides/prog_guide/dispatcher_lib.rst
@@ -0,0 +1,433 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2023 Ericsson AB.
+
+Dispatcher
+==========
+
+Overview
+--------
+
+The purpose of the dispatcher is to help reduce coupling in an
+:doc:`Eventdev <eventdev>`-based DPDK application.
+
+In particular, the dispatcher addresses a scenario where an
+application's modules share the same event device and event device
+ports, and performs work on the same lcore threads.
+
+The dispatcher replaces the conditional logic that follows an event
+device dequeue operation, where events are dispatched to different
+parts of the application, typically based on fields in the
+``rte_event``, such as the ``queue_id``, ``sub_event_type``, or
+``sched_type``.
+
+Below is an excerpt from a fictitious application consisting of two
+modules; A and B. In this example, event-to-module routing is based
+purely on queue id, where module A expects all events to a certain
+queue id, and module B two other queue ids. [#Mapping]_
+
+.. code-block:: c
+
+    for (;;) {
+            struct rte_event events[MAX_BURST];
+            unsigned int n;
+
+            n = rte_event_dequeue_burst(dev_id, port_id, events,
+	                                MAX_BURST, 0);
+
+            for (i = 0; i < n; i++) {
+                    const struct rte_event *event = &events[i];
+
+                    switch (event->queue_id) {
+                    case MODULE_A_QUEUE_ID:
+                            module_a_process(event);
+                            break;
+                    case MODULE_B_STAGE_0_QUEUE_ID:
+                            module_b_process_stage_0(event);
+                            break;
+                    case MODULE_B_STAGE_1_QUEUE_ID:
+                            module_b_process_stage_1(event);
+                            break;
+                    }
+            }
+    }
+
+The issue this example attempts to illustrate is that the centralized
+conditional logic has knowledge of things that should be private to
+the modules. In other words, this pattern leads to a violation of
+module encapsulation.
+
+The shared conditional logic contains explicit knowledge about what
+events should go where. In case, for example, the
+``module_a_process()`` is broken into two processing stages — a
+module-internal affair — the shared conditional code must be updated
+to reflect this change.
+
+The centralized event routing code becomes an issue in larger
+applications, where modules are developed by different organizations.
+This pattern also makes module reuse across different application more
+difficult. The part of the conditional logic relevant for a particular
+application may need to be duplicated across many module
+instantiations (e.g., applications and test setups).
+
+The dispatcher separates the mechanism (routing events to their
+receiver) from the policy (which events should go where).
+
+The basic operation of the dispatcher is as follows:
+
+* Dequeue a batch of events from the event device.
+* For each event determine which handler should receive the event, using
+  a set of application-provided, per-handler event matching callback
+  functions.
+* Provide events matching a particular handler, to that handler, using
+  its process callback.
+
+If the above application would have made use of the dispatcher, the
+code relevant for its module A may have looked something like this:
+
+.. code-block:: c
+
+    static bool
+    module_a_match(const struct rte_event *event, void *cb_data)
+    {
+           return event->queue_id == MODULE_A_QUEUE_ID;
+    }
+
+    static void
+    module_a_process_events(uint8_t event_dev_id, uint8_t event_port_id,
+                            const struct rte_event *events,
+			    uint16_t num, void *cb_data)
+    {
+            uint16_t i;
+
+            for (i = 0; i < num; i++)
+                    module_a_process_event(&events[i]);
+    }
+
+    /* In the module's initialization code */
+    rte_dispatcher_register(dispatcher, module_a_match, NULL,
+			    module_a_process_events, module_a_data);
+
+(Error handling is left out of this and future example code in this
+chapter.)
+
+When the shared conditional logic is removed, a new question arise:
+which part of the system actually runs the dispatching mechanism? Or
+phrased differently, what is replacing the function hosting the shared
+conditional logic (typically launched on all lcores using
+``rte_eal_remote_launch()``)? To solve this issue, the dispatcher is a
+run as a DPDK :doc:`Service <service_cores>`.
+
+The dispatcher is a layer between the application and the event device
+in the receive direction. In the transmit (i.e., item of work
+submission) direction, the application directly accesses the Eventdev
+core API (e.g., ``rte_event_enqueue_burst()``) to submit new or
+forwarded event to the event device.
+
+Dispatcher Creation
+-------------------
+
+A dispatcher is created with using
+``rte_dispatcher_create()``.
+
+The event device must be configured before the dispatcher is created.
+
+Usually, only one dispatcher is needed per event device. A dispatcher
+handles exactly one event device.
+
+An dispatcher is freed using the ``rte_dispatcher_free()``
+function. The dispatcher's service functions must not be running on
+any lcore at the point of this call.
+
+Event Port Binding
+------------------
+
+To be able to dequeue events, the dispatcher must know which event
+ports are to be used, on all the lcores it uses. The application
+provides this information using
+``rte_dispatcher_bind_port_to_lcore()``.
+
+This call is typically made from the part of the application that
+deals with deployment issues (e.g., iterating lcores and determining
+which lcore does what), at the time of application initialization.
+
+The ``rte_dispatcher_unbind_port_from_lcore()`` is used to undo
+this operation.
+
+Multiple lcore threads may not safely use the same event
+port. [#Port-MT-Safety]
+
+Event ports cannot safely be bound or unbound while the dispatcher's
+service function is running on any lcore.
+
+Event Handlers
+--------------
+
+The dispatcher handler is an interface between the dispatcher and an
+application module, used to route events to the appropriate part of
+the application.
+
+Handler Registration
+^^^^^^^^^^^^^^^^^^^^
+
+The event handler interface consists of two function pointers:
+
+* The ``rte_dispatcher_match_t`` callback, which job is to
+  decide if this event is to be the property of this handler.
+* The ``rte_dispatcher_process_t``, which is used by the
+  dispatcher to deliver matched events.
+
+An event handler registration is valid on all lcores.
+
+The functions pointed to by the match and process callbacks resides in
+the application's domain logic, with one or more handlers per
+application module.
+
+A module may use more than one event handler, for convenience or to
+further decouple sub-modules. However, the dispatcher may impose an
+upper limit of the number handlers. In addition, installing a large
+number of handlers increase dispatcher overhead, although this does
+not necessarily translate to a system-level performance degradation. See
+the section on :ref:`Event Clustering` for more information.
+
+Handler registration and unregistration cannot safely be done while
+the dispatcher's service function is running on any lcore.
+
+Event Matching
+^^^^^^^^^^^^^^
+
+A handler's match callback function decides if an event should be
+delivered to this handler, or not.
+
+An event is routed to no more than one handler. Thus, if a match
+function returns true, no further match functions will be invoked for
+that event.
+
+Match functions must not depend on being invocated in any particular
+order (e.g., in the handler registration order).
+
+Events failing to match any handler are dropped, and the
+``ev_drop_count`` counter is updated accordingly.
+
+Event Delivery
+^^^^^^^^^^^^^^
+
+The handler callbacks are invocated by the dispatcher's service
+function, upon the arrival of events to the event ports bound to the
+running service lcore.
+
+A particular event is delivery to at most one handler.
+
+The application must not depend on all match callback invocations for
+a particular event batch being made prior to any process calls are
+being made. For example, if the dispatcher dequeues two events from
+the event device, it may choose to find out the destination for the
+first event, and deliver it, and then continue to find out the
+destination for the second, and then deliver that event as well. The
+dispatcher may also choose a strategy where no event is delivered
+until the destination handler for both events have been determined.
+
+The events provided in a single process call always belong to the same
+event port dequeue burst.
+
+.. _Event Clustering:
+
+Event Clustering
+^^^^^^^^^^^^^^^^
+
+The dispatcher maintains the order of events destined for the same
+handler.
+
+*Order* here refers to the order in which the events were delivered
+from the event device to the dispatcher (i.e., in the event array
+populated by ``rte_event_dequeue_burst()``), in relation to the order
+in which the dispatcher deliveres these events to the application.
+
+The dispatcher *does not* guarantee to maintain the order of events
+delivered to *different* handlers.
+
+For example, assume that ``MODULE_A_QUEUE_ID`` expands to the value 0,
+and ``MODULE_B_STAGE_0_QUEUE_ID`` expands to the value 1. Then
+consider a scenario where the following events are dequeued from the
+event device (qid is short for event queue id).
+
+.. code-block:: none
+
+    [e0: qid=1], [e1: qid=1], [e2: qid=0], [e3: qid=1]
+
+The dispatcher may deliver the events in the following manner:
+
+.. code-block:: none
+
+   module_b_stage_0_process([e0: qid=1], [e1: qid=1])
+   module_a_process([e2: qid=0])
+   module_b_stage_0_process([e2: qid=1])
+
+The dispatcher may also choose to cluster (group) all events destined
+for ``module_b_stage_0_process()`` into one array:
+
+.. code-block:: none
+
+   module_b_stage_0_process([e0: qid=1], [e1: qid=1], [e3: qid=1])
+   module_a_process([e2: qid=0])
+
+Here, the event ``e2`` is reordered and placed behind ``e3``, from a
+delivery order point of view. This kind of reshuffling is allowed,
+since the events are destined for different handlers.
+
+The dispatcher may also deliver ``e2`` before the three events
+destined for module B.
+
+An example of what the dispatcher may not do, is to reorder event
+``e1`` so, that it precedes ``e0`` in the array passed to the module
+B's stage 0 process callback.
+
+Although clustering requires some extra work for the dispatcher, it
+leads to fewer process function calls. In addition, and likely more
+importantly, it improves temporal locality of memory accesses to
+handler-specific data structures in the application, which in turn may
+lead to fewer cache misses and improved overall performance.
+
+Finalize
+--------
+
+The dispatcher may be configured to notify one or more parts of the
+application when the matching and processing of a batch of events has
+completed.
+
+The ``rte_dispatcher_finalize_register`` call is used to
+register a finalize callback. The function
+``rte_dispatcher_finalize_unregister`` is used to remove a
+callback.
+
+The finalize hook may be used by a set of event handlers (in the same
+modules, or a set of cooperating modules) sharing an event output
+buffer, since it allows for flushing of the buffers at the last
+possible moment. In particular, it allows for buffering of
+``RTE_EVENT_OP_FORWARD`` events, which must be flushed before the next
+``rte_event_dequeue_burst()`` call is made (assuming implicit release
+is employed).
+
+The following is an example with an application-defined event output
+buffer (the ``event_buffer``):
+
+.. code-block:: c
+
+    static void
+    finalize_batch(uint8_t event_dev_id, uint8_t event_port_id,
+                   void *cb_data)
+    {
+            struct event_buffer *buffer = cb_data;
+            unsigned lcore_id = rte_lcore_id();
+            struct event_buffer_lcore *lcore_buffer =
+                    &buffer->lcore_buffer[lcore_id];
+
+            event_buffer_lcore_flush(lcore_buffer);
+    }
+
+    /* In the module's initialization code */
+    rte_dispatcher_finalize_register(dispatcher, finalize_batch,
+                                     shared_event_buffer);
+
+The dispatcher does not track any relationship between a handler and a
+finalize callback, and all finalize callbacks will be called, if (and
+only if) at least one event was dequeued from the event device.
+
+Finalize callback registration and unregistration cannot safely be
+done while the dispatcher's service function is running on any lcore.
+
+Service
+-------
+
+The dispatcher is a DPDK service, and is managed in a manner similar
+to other DPDK services (e.g., an Event Timer Adapter).
+
+Below is an example of how to configure a particular lcore to serve as
+a service lcore, and to map an already-configured dispatcher
+(identified by ``DISPATCHER_ID``) to that lcore.
+
+.. code-block:: c
+
+    static void
+    launch_dispatcher_core(struct rte_dispatcher *dispatcher,
+                           unsigned lcore_id)
+    {
+            uint32_t service_id;
+
+            rte_service_lcore_add(lcore_id);
+
+            rte_dispatcher_service_id_get(dispatcher, &service_id);
+
+            rte_service_map_lcore_set(service_id, lcore_id, 1);
+
+            rte_service_lcore_start(lcore_id);
+
+            rte_service_runstate_set(service_id, 1);
+    }
+
+As the final step, the dispatcher must be started.
+
+.. code-block:: c
+
+    rte_dispatcher_start(dispatcher);
+
+
+Multi Service Dispatcher Lcores
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In an Eventdev application, most (or all) compute-intensive and
+performance-sensitive processing is done in an event-driven manner,
+where CPU cycles spent on application domain logic is the direct
+result of items of work (i.e., ``rte_event`` events) dequeued from an
+event device.
+
+In the light of this, it makes sense to have the dispatcher service be
+the only DPDK service on all lcores used for packet processing — at
+least in principle.
+
+However, there is nothing in DPDK that prevents colocating other
+services with the dispatcher service on the same lcore.
+
+Tasks that prior to the introduction of the dispatcher into the
+application was performed on the lcore, even though no events were
+received, are prime targets for being converted into such auxiliary
+services, running on the dispatcher core set.
+
+An example of such a task would be the management of a per-lcore timer
+wheel (i.e., calling ``rte_timer_manage()``).
+
+For applications employing :doc:`Read-Copy-Update (RCU) <rcu_lib>` (or
+similar technique), may opt for having quiescent state (e.g., calling
+``rte_rcu_qsbr_quiescent()``) signaling factored out into a separate
+service, to assure resource reclaimination occurs even in though some
+lcores currently do not process any events.
+
+If more services than the dispatcher service is mapped to a service
+lcore, it's important that the other service are well-behaved and
+don't interfere with event processing to the extent the system's
+throughput and/or latency requirements are at risk of not being met.
+
+In particular, to avoid jitter, they should have an small upper bound
+for the maximum amount of time spent in a single service function
+call.
+
+An example of scenario with a more CPU-heavy colocated service is a
+low-lcore count deployment, where the event device lacks the
+``RTE_EVENT_ETH_RX_ADAPTER_CAP_INTERNAL_PORT`` capability (and thus
+require software to feed incoming packets into the event device). In
+this case, the best performance may be achieved if the Event Ethernet
+RX and/or TX Adapters are mapped to lcores also used by for event
+dispatching, since otherwise the adapter lcores would have a lot of
+idle CPU cycles.
+
+.. rubric:: Footnotes
+
+.. [#Mapping]
+   Event routing may reasonably be done based on other ``rte_event``
+   fields (or even event user data). Indeed, that's the very reason to
+   have match callback functions, instead of a simple queue
+   id-to-handler mapping scheme. Queue id-based routing serves well in
+   a simple example.
+
+.. [#Port-MT-Safety]
+   This property (which is a feature, not a bug) is inherited from the
+   core Eventdev APIs.
diff --git a/doc/guides/prog_guide/index.rst b/doc/guides/prog_guide/index.rst
index 52a6d9e7aa..ab05bd6074 100644
--- a/doc/guides/prog_guide/index.rst
+++ b/doc/guides/prog_guide/index.rst
@@ -60,6 +60,7 @@ Programmer's Guide
     event_ethernet_tx_adapter
     event_timer_adapter
     event_crypto_adapter
+    dispatcher_lib
     qos_framework
     power_man
     packet_classif_access_ctrl
-- 
2.34.1