On Fri, Jul 31, 2020 at 1:24 AM McDaniel, Timothy <timothy.mcdan...@intel.com> wrote: > > From: "McDaniel, Timothy" <timothy.mcdan...@intel.com> > > Signed-off-by: McDaniel, Timothy <timothy.mcdan...@intel.com> > --- > doc/guides/eventdevs/dlb.rst | 343 > ++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 343 insertions(+) > create mode 100644 doc/guides/eventdevs/dlb.rst > > diff --git a/doc/guides/eventdevs/dlb.rst b/doc/guides/eventdevs/dlb.rst
Please don't have a separate patch for documentation. Have documentation base in the first patch and update the doc as and when code is added. > new file mode 100644 > index 0000000..5c42969 > --- /dev/null > +++ b/doc/guides/eventdevs/dlb.rst > @@ -0,0 +1,343 @@ > +.. 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 >
