[dpdk-dev] [PATCH 05/27] event/dlb: add DLB documentation

["McDaniel, Timothy" <timothy.mcdaniel@intel.com>]

From: "McDaniel, Timothy" <timothy.mcdaniel@intel.com>

Signed-off-by: McDaniel, Timothy <timothy.mcdaniel@intel.com>
---
 doc/guides/eventdevs/dlb.rst |  343 ++++++++++++++++++++++++++++++++++++++++++
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 create mode 100644 doc/guides/eventdevs/dlb.rst

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+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2020 Intel Corporation.
+
+Driver for the Intel® Dynamic Load Balancer (DLB)
+==================================================
+
+The DPDK dlb poll mode driver supports the Intel® Dynamic Load Balancer.
+
+Prerequisites
+-------------
+
+- Follow the DPDK :ref:`Getting Started Guide for Linux <linux_gsg>` to setup
+  the basic DPDK environment.
+
+Configuration
+-------------
+
+* The DLB PF PMD is a user-space PMD that uses VFIO to gain direct
+  device access. To use this operation mode, the PCIe PF device must be bound
+  to a DPDK-compatible VFIO driver, such as vfio-pci. The PF PMD does not work
+  with PCIe VFs, but is portable to all environments (Linux, FreeBSD, etc.)
+  that DPDK supports. (Note: PF PMD testing has been limited to Linux at this
+  time.)
+
+Eventdev API Notes
+------------------
+
+The DLB provides the functions of a DPDK event device; specifically, it
+supports atomic, ordered, and parallel scheduling events from queues to ports.
+However, the DLB hardware is not a perfect match to the eventdev API. Some DLB
+features are abstracted by the PMD (e.g. directed ports), some are only
+accessible as vdev command-line parameters, and certain eventdev features are
+not supported (e.g. the event flow ID is not maintained during scheduling).
+
+In general the dlb PMD is designed for ease-of-use and doesn't require a
+detailed understanding of the hardware, but these details are important when
+writing high-performance code. This section describes the places where the
+eventdev API and DLB misalign.
+
+VAS Configuration
+~~~~~~~~~~~~~~~~~
+
+A VAS is a scheduling domain, of which there are 32 in the DLB.
+When a VAS is configured, it allocates load-balanced and
+directed queues, ports, credits, and other hardware resources. Some VAS
+resource allocations are user-controlled -- the number of queues, for example
+-- and others, like credit pools (one directed and one load-balanced pool per
+VAS), are not.
+
+The DLB is a closed system eventdev, and as such the ``nb_events_limit`` device
+setup argument and the per-port ``new_event_threshold`` argument apply as
+defined in the eventdev header file. The limit is applied to all enqueues,
+regardless of whether it will consume a directed or load-balanced credit.
+
+Load-balanced and Directed Ports
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+DLB ports come in two flavors: load-balanced and directed. The eventdev API
+does not have the same concept, but it has a similar one: ports and queues that
+are singly-linked (i.e. linked to a single queue or port, respectively).
+
+The ``rte_event_dev_info_get()`` function reports the number of available
+event ports and queues (among other things). For the DLB PMD, max_event_ports
+and max_event_queues report the number of available load-balanced ports and
+queues, and max_single_link_event_port_queue_pairs reports the number of
+available directed ports and queues.
+
+When a VAS is created in ``rte_event_dev_configure()``, the user specifies
+``nb_event_ports`` and ``nb_single_link_event_port_queues``, which control the
+total number of ports (load-balanced and directed) and the number of directed
+ports. Hence, the number of requested load-balanced ports is ``nb_event_ports
+- nb_single_link_event_ports``. The ``nb_event_queues`` field specifies the
+total number of queues (load-balanced and directed). The number of directed
+queues comes from ``nb_single_link_event_port_queues``, since directed ports
+and queues come in pairs.
+
+When a port is setup, the ``RTE_EVENT_PORT_CFG_SINGLE_LINK`` flag determines
+whether it should be configured as a directed (the flag is set) or a
+load-balanced (the flag is unset) port. Similarly, the
+``RTE_EVENT_QUEUE_CFG_SINGLE_LINK`` queue configuration flag controls
+whether it is a directed or load-balanced queue.
+
+Load-balanced ports can only be linked to load-balanced queues, and directed
+ports can only be linked to directed queues. Furthermore, directed ports can
+only be linked to a single directed queue (and vice versa), and that link
+cannot change after the eventdev is started.
+
+The eventdev API doesn't have a directed scheduling type. To support directed
+traffic, the dlb PMD detects when an event is being sent to a directed queue
+and overrides its scheduling type. Note that the originally selected scheduling
+type (atomic, ordered, or parallel) is not preserved, and an event's sched_type
+will be set to ``RTE_SCHED_TYPE_ATOMIC`` when it is dequeued from a directed
+port.
+
+Flow ID
+~~~~~~~
+
+The flow ID field is not preserved in the event when it is scheduled in the
+DLB, because the DLB hardware control word format does not have sufficient
+space to preserve every event field. As a result, the flow ID specified with
+the enqueued event will not be in the dequeued event. If this field is
+required, the application should pass it through an out-of-band path (for
+example in the mbuf's udata64 field, if the event points to an mbuf) or
+reconstruct the flow ID after receiving the event.
+
+Also, the DLB hardware control word supports a 16-bit flow ID. Since struct
+rte_event's flow_id field is 20 bits, the DLB PMD drops the most significant
+four bits from the event's flow ID.
+
+Hardware Credits
+~~~~~~~~~~~~~~~~
+
+DLB uses a hardware credit scheme to prevent software from overflowing hardware
+event storage, with each unit of storage represented by a credit. A port spends
+a credit to enqueue an event, and hardware refills the ports with credits as the
+events are scheduled to ports. Refills come from credit pools, and each port is
+a member of a load-balanced credit pool and a directed credit pool. The
+load-balanced credits are used to enqueue to load-balanced queues, and directed
+credits are used for directed queues.
+
+A DLB eventdev contains one load-balanced and one directed credit pool. These
+pools' sizes are controlled by the nb_events_limit field in struct
+rte_event_dev_config. The load-balanced pool is sized to contain
+nb_events_limit credits, and the directed pool is sized to contain
+nb_events_limit/4 credits. The directed pool size can be overridden with the
+num_dir_credits vdev argument, like so:
+
+    .. code-block:: console
+
+       --vdev=dlb1_event,num_dir_credits=<value>
+
+This can be used if the default allocation is too low or too high for the
+specific application needs. The PMD also supports a vdev arg that limits the
+max_num_events reported by rte_event_dev_info_get():
+
+    .. code-block:: console
+
+       --vdev=dlb1_event,max_num_events=<value>
+
+By default, max_num_events is reported as the total available load-balanced
+credits. If multiple DLB-based applications are being used, it may be desirable
+to control how many load-balanced credits each application uses, particularly
+when application(s) are written to configure nb_events_limit equal to the
+reported max_num_events.
+
+Each port is a member of both credit pools. A port's credit allocation is
+defined by its low watermark, high watermark, and refill quanta. These three
+parameters are calculated by the dlb PMD like so:
+
+- The load-balanced high watermark is set to the port's enqueue_depth.
+  The directed high watermark is set to the minimum of the enqueue_depth and
+  the directed pool size divided by the total number of ports.
+- The refill quanta is set to half the high watermark.
+- The low watermark is set to the minimum of 16 and the refill quanta.
+
+When the eventdev is started, each port is pre-allocated a high watermark's
+worth of credits. For example, if an eventdev contains four ports with enqueue
+depths of 32 and a load-balanced credit pool size of 4096, each port will start
+with 32 load-balanced credits, and there will be 3968 credits available to
+replenish the ports. Thus, a single port is not capable of enqueueing up to the
+nb_events_limit (without any events being dequeued), since the other ports are
+retaining their initial credit allocation; in short, all ports must enqueue in
+order to reach the limit.
+
+If a port attempts to enqueue and has no credits available, the enqueue
+operation will fail and the application must retry the enqueue. Credits are
+replenished asynchronously by the DLB hardware.
+
+Software Credits
+~~~~~~~~~~~~~~~~
+
+The DLB is a "closed system" event dev, and the DLB PMD layers a software
+credit scheme on top of the hardware credit scheme in order to comply with
+the per-port backpressure described in the eventdev API.
+
+The DLB's hardware scheme is local to a queue/pipeline stage: a port spends a
+credit when it enqueues to a queue, and credits are later replenished after the
+events are dequeued and released.
+
+In the software credit scheme, a credit is consumed when a new (.op =
+RTE_EVENT_OP_NEW) event is injected into the system, and the credit is
+replenished when the event is released from the system (either explicitly with
+RTE_EVENT_OP_RELEASE or implicitly in dequeue_burst()).
+
+In this model, an event is "in the system" from its first enqueue into eventdev
+until it is last dequeued. If the event goes through multiple event queues, it
+is still considered "in the system" while a worker thread is processing it.
+
+A port will fail to enqueue if the number of events in the system exceeds its
+``new_event_threshold`` (specified at port setup time). A port will also fail
+to enqueue if it lacks enough hardware credits to enqueue; load-balanced
+credits are used to enqueue to a load-balanced queue, and directed credits are
+used to enqueue to a directed queue.
+
+The out-of-credit situations are typically transient, and an eventdev
+application using the DLB ought to retry its enqueues if they fail.
+If enqueue fails, DLB PMD sets rte_errno as follows:
+
+- -ENOSPC: Credit exhaustion (either hardware or software)
+- -EINVAL: Invalid argument, such as port ID, queue ID, or sched_type.
+
+Depending on the pipeline the application has constructed, it's possible to
+enter a credit deadlock scenario wherein the worker thread lacks the credit
+to enqueue an event, and it must dequeue an event before it can recover the
+credit. If the worker thread retries its enqueue indefinitely, it will not
+make forward progress. Such deadlock is possible if the application has event
+"loops", in which an event in dequeued from queue A and later enqueued back to
+queue A.
+
+Due to this, workers should stop retrying after a time, release the events it
+is attempting to enqueue, and dequeue more events. It is important that the
+worker release the events and don't simply set them aside to retry the enqueue
+again later, because the port has limited history list size (by default, twice
+the port's dequeue_depth).
+
+Priority
+~~~~~~~~
+
+The DLB supports event priority and per-port queue service priority, as
+described in the eventdev header file. The DLB does not support 'global' event
+queue priority established at queue creation time.
+
+DLB supports 8 event and queue service priority levels. For both priority
+types, the PMD uses the upper three bits of the priority field to determine the
+DLB priority, discarding the 5 least significant bits. The 5 least significant
+event priority bits are not preserved when an event is enqueued.
+
+Load-Balanced Queues
+~~~~~~~~~~~~~~~~~~~~
+
+A load-balanced queue can support atomic and ordered scheduling, or atomic and
+unordered scheduling, but not atomic and unordered and ordered scheduling. A
+queue's scheduling types are controlled by the event queue configuration.
+
+If the user sets the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag, the
+``nb_atomic_order_sequences`` determines the supported scheduling types.
+With non-zero ``nb_atomic_order_sequences``, the queue is configured for atomic
+and ordered scheduling. In this case, ``RTE_SCHED_TYPE_PARALLEL`` scheduling is
+supported by scheduling those events as ordered events.  Note that when the
+event is dequeued, its sched_type will be ``RTE_SCHED_TYPE_ORDERED``. Else if
+``nb_atomic_order_sequences`` is zero, the queue is configured for atomic and
+unordered scheduling. In this case, ``RTE_SCHED_TYPE_ORDERED`` is unsupported.
+
+If the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag is not set, schedule_type
+dictates the queue's scheduling type.
+
+The ``nb_atomic_order_sequences`` queue configuration field sets the ordered
+queue's reorder buffer size.  DLB has 4 groups of ordered queues, where each
+group is configured to contain either 1 queue with 1024 reorder entries, 2
+queues with 512 reorder entries, and so on down to 32 queues with 32 entries.
+
+When a load-balanced queue is created, the PMD will configure a new sequence
+number group on-demand if num_sequence_numbers does not match a pre-existing
+group with available reorder buffer entries. If all sequence number groups are
+in use, no new group will be created and queue configuration will fail. (Note
+that when the PMD is used with a virtual DLB device, it cannot change the
+sequence number configuration.)
+
+The queue's ``nb_atomic_flows`` parameter is ignored by the DLB PMD, because
+the DLB doesn't limit the number of flows a queue can track. In the DLB, all
+load-balanced queues can use the full 16-bit flow ID range.
+
+Reconfiguration
+~~~~~~~~~~~~~~~
+
+The Eventdev API allows one to reconfigure a device, its ports, and its queues
+by first stopping the device, calling the configuration function(s), then
+restarting the device. The DLB doesn't support configuring an individual queue
+or port without first reconfiguring the entire device, however, so there are
+certain reconfiguration sequences that are valid in the eventdev API but not
+supported by the PMD.
+
+Specifically, the PMD supports the following configuration sequence:
+1. Configure and start the device
+2. Stop the device
+3. (Optional) Reconfigure the device
+4. (Optional) If step 3 is run:
+
+   a. Setup queue(s). The reconfigured queue(s) lose their previous port links.
+   b. The reconfigured port(s) lose their previous queue links.
+
+5. (Optional, only if steps 4a and 4b are run) Link port(s) to queue(s)
+6. Restart the device. If the device is reconfigured in step 3 but one or more
+   of its ports or queues are not, the PMD will apply their previous
+   configuration (including port->queue links) at this time.
+
+The PMD does not support the following configuration sequences:
+1. Configure and start the device
+2. Stop the device
+3. Setup queue or setup port
+4. Start the device
+
+This sequence is not supported because the event device must be reconfigured
+before its ports or queues can be.
+
+Deferred Scheduling
+~~~~~~~~~~~~~~~~~~~
+
+The DLB PMD's default behavior for managing a CQ is to "pop" the CQ once per
+dequeued event before returning from rte_event_dequeue_burst(). This frees the
+corresponding entries in the CQ, which enables the DLB to schedule more events
+to it.
+
+To support applications seeking finer-grained scheduling control -- for example
+deferring scheduling to get the best possible priority scheduling and
+load-balancing -- the PMD supports a deferred scheduling mode. In this mode,
+the CQ entry is not popped until the *subsequent* rte_event_dequeue_burst()
+call. This mode only applies to load-balanced event ports with dequeue depth of
+1.
+
+To enable deferred scheduling, use the defer_sched vdev argument like so:
+
+    .. code-block:: console
+
+       --vdev=dlb1_event,defer_sched=on
+
+Atomic Inflights Allocation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the last stage prior to scheduling an atomic event to a CQ, DLB holds the
+inflight event in a temporary buffer that is divided among load-balanced
+queues. If a queue's atomic buffer storage fills up, this can result in
+head-of-line-blocking. For example:
+- An LDB queue allocated N atomic buffer entries
+- All N entries are filled with events from flow X, which is pinned to CQ 0.
+
+Until CQ 0 releases 1+ events, no other atomic flows for that LDB queue can be
+scheduled. The likelihood of this case depends on the eventdev configuration,
+traffic behavior, event processing latency, potential for a worker to be
+interrupted or otherwise delayed, etc.
+
+By default, the PMD allocates 16 buffer entries for each load-balanced queue,
+which provides an even division across all 128 queues but potentially wastes
+buffer space (e.g. if not all queues are used, or aren't used for atomic
+scheduling).
+
+The PMD provides a dev arg to override the default per-queue allocation. To
+increase a vdev's per-queue atomic-inflight allocation to (for example) 64:
+
+    .. code-block:: console
+
+       --vdev=dlb1_event,atm_inflights=64
+
-- 
1.7.10