Re: [PATCH v5 3/3] doc: add dispatcher programming guide

[David Marchand <david.marchand@redhat.com>]

On Thu, Sep 28, 2023 at 9:37 AM Mattias Rönnblom
<mattias.ronnblom@ericsson.com> wrote:
>
> Provide programming guide for the dispatcher library.
>
> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>

Checkpatch complains about empty lines, can you double check?

For example.

ERROR:TRAILING_WHITESPACE: trailing whitespace
#63: FILE: doc/guides/prog_guide/dispatcher_lib.rst:33:
+    $

ERROR:TRAILING_WHITESPACE: trailing whitespace
#66: FILE: doc/guides/prog_guide/dispatcher_lib.rst:36:
+    $


>
> --
> 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 43890cad0e..ab35498204 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -1730,6 +1730,7 @@ 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..951db06081
> --- /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 nessarily translate to a system-level performance degradation. See

Typo on necessarily?



> +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::
> +
> +    [e0: qid=1], [e1: qid=1], [e2: qid=0], [e3: qid=1]
> +
> +The dispatcher may deliver the events in the following manner:
> +
> +.. code-block::
> +
> +   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::
> +
> +   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
>


-- 
David Marchand