From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dpdk.org (dpdk.org [92.243.14.124]) by inbox.dpdk.org (Postfix) with ESMTP id 3B6A6A052B; Thu, 30 Jul 2020 21:54:05 +0200 (CEST) Received: from [92.243.14.124] (localhost [127.0.0.1]) by dpdk.org (Postfix) with ESMTP id 44F2C1C0B6; Thu, 30 Jul 2020 21:53:16 +0200 (CEST) Received: from mga02.intel.com (mga02.intel.com [134.134.136.20]) by dpdk.org (Postfix) with ESMTP id 296E637B7 for ; Thu, 30 Jul 2020 21:53:09 +0200 (CEST) IronPort-SDR: Ppzsd4rCxZ9bmHnEgYlqZRbJShRrjNV2gACJ10gT+ubTOv2uPWDlziIMI6Tbdymrbaru/hsetd Up2mzTfX1V+A== X-IronPort-AV: E=McAfee;i="6000,8403,9698"; a="139672292" X-IronPort-AV: E=Sophos;i="5.75,415,1589266800"; d="scan'208";a="139672292" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Jul 2020 12:53:07 -0700 IronPort-SDR: bspNrRhXTLiryGLAyXZ0UqBkLLdswrOQQ7GxStH4uU+2JWGUMTwaMHIJc2qsTwPe+ph9hqi955 lPrWmINHAbCQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.75,415,1589266800"; d="scan'208";a="465378082" Received: from txasoft-yocto.an.intel.com ([10.123.72.192]) by orsmga005.jf.intel.com with ESMTP; 30 Jul 2020 12:53:06 -0700 From: "McDaniel, Timothy" To: jerinj@marvell.com Cc: mattias.ronnblom@ericsson.com, dev@dpdk.org, gage.eads@intel.com, harry.van.haaren@intel.com, "McDaniel, Timothy" Date: Thu, 30 Jul 2020 14:49:52 -0500 Message-Id: <1596138614-17409-6-git-send-email-timothy.mcdaniel@intel.com> X-Mailer: git-send-email 1.7.10 In-Reply-To: <1596138614-17409-1-git-send-email-timothy.mcdaniel@intel.com> References: <1593232671-5690-0-git-send-email-timothy.mcdaniel@intel.com> <1596138614-17409-1-git-send-email-timothy.mcdaniel@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject: [dpdk-dev] =?utf-8?q?=5BPATCH_05/27=5D_event/dlb=3A_add_DLB_docum?= =?utf-8?q?entation?= X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" From: "McDaniel, Timothy" Signed-off-by: McDaniel, Timothy --- 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 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 ` 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= + +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= + +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