1 .. SPDX-License-Identifier: BSD-3-Clause
2 Copyright(c) 2020 Intel Corporation.
4 Driver for the Intel® Dynamic Load Balancer (DLB2)
5 ==================================================
7 The DPDK dlb poll mode driver supports the Intel® Dynamic Load Balancer.
12 Follow the DPDK :ref:`Getting Started Guide for Linux <linux_gsg>` to setup
13 the basic DPDK environment.
18 The DLB2 PF PMD is a user-space PMD that uses VFIO to gain direct
19 device access. To use this operation mode, the PCIe PF device must be bound
20 to a DPDK-compatible VFIO driver, such as vfio-pci.
25 The DLB2 provides the functions of a DPDK event device; specifically, it
26 supports atomic, ordered, and parallel scheduling events from queues to ports.
27 However, the DLB2 hardware is not a perfect match to the eventdev API. Some DLB2
28 features are abstracted by the PMD such as directed ports.
30 In general the dlb PMD is designed for ease-of-use and does not require a
31 detailed understanding of the hardware, but these details are important when
32 writing high-performance code. This section describes the places where the
33 eventdev API and DLB2 misalign.
35 Scheduling Domain Configuration
36 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
38 There are 32 scheduling domainis the DLB2.
39 When one is configured, it allocates load-balanced and
40 directed queues, ports, credits, and other hardware resources. Some
41 resource allocations are user-controlled -- the number of queues, for example
42 -- and others, like credit pools (one directed and one load-balanced pool per
43 scheduling domain), are not.
45 The DLB2 is a closed system eventdev, and as such the ``nb_events_limit`` device
46 setup argument and the per-port ``new_event_threshold`` argument apply as
47 defined in the eventdev header file. The limit is applied to all enqueues,
48 regardless of whether it will consume a directed or load-balanced credit.
53 A load-balanced queue can support atomic and ordered scheduling, or atomic and
54 unordered scheduling, but not atomic and unordered and ordered scheduling. A
55 queue's scheduling types are controlled by the event queue configuration.
57 If the user sets the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag, the
58 ``nb_atomic_order_sequences`` determines the supported scheduling types.
59 With non-zero ``nb_atomic_order_sequences``, the queue is configured for atomic
60 and ordered scheduling. In this case, ``RTE_SCHED_TYPE_PARALLEL`` scheduling is
61 supported by scheduling those events as ordered events. Note that when the
62 event is dequeued, its sched_type will be ``RTE_SCHED_TYPE_ORDERED``. Else if
63 ``nb_atomic_order_sequences`` is zero, the queue is configured for atomic and
64 unordered scheduling. In this case, ``RTE_SCHED_TYPE_ORDERED`` is unsupported.
66 If the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag is not set, schedule_type
67 dictates the queue's scheduling type.
69 The ``nb_atomic_order_sequences`` queue configuration field sets the ordered
70 queue's reorder buffer size. DLB2 has 4 groups of ordered queues, where each
71 group is configured to contain either 1 queue with 1024 reorder entries, 2
72 queues with 512 reorder entries, and so on down to 32 queues with 32 entries.
74 When a load-balanced queue is created, the PMD will configure a new sequence
75 number group on-demand if num_sequence_numbers does not match a pre-existing
76 group with available reorder buffer entries. If all sequence number groups are
77 in use, no new group will be created and queue configuration will fail. (Note
78 that when the PMD is used with a virtual DLB2 device, it cannot change the
79 sequence number configuration.)
81 The queue's ``nb_atomic_flows`` parameter is ignored by the DLB2 PMD, because
82 the DLB2 does not limit the number of flows a queue can track. In the DLB2, all
83 load-balanced queues can use the full 16-bit flow ID range.
88 A load-balanced queue can support atomic and ordered scheduling, or atomic and
89 unordered scheduling, but not atomic and unordered and ordered scheduling. A
90 queue's scheduling types are controlled by the event queue configuration.
92 If the user sets the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag, the
93 ``nb_atomic_order_sequences`` determines the supported scheduling types.
94 With non-zero ``nb_atomic_order_sequences``, the queue is configured for atomic
95 and ordered scheduling. In this case, ``RTE_SCHED_TYPE_PARALLEL`` scheduling is
96 supported by scheduling those events as ordered events. Note that when the
97 event is dequeued, its sched_type will be ``RTE_SCHED_TYPE_ORDERED``. Else if
98 ``nb_atomic_order_sequences`` is zero, the queue is configured for atomic and
99 unordered scheduling. In this case, ``RTE_SCHED_TYPE_ORDERED`` is unsupported.
101 If the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag is not set, schedule_type
102 dictates the queue's scheduling type.
104 The ``nb_atomic_order_sequences`` queue configuration field sets the ordered
105 queue's reorder buffer size. DLB2 has 4 groups of ordered queues, where each
106 group is configured to contain either 1 queue with 1024 reorder entries, 2
107 queues with 512 reorder entries, and so on down to 32 queues with 32 entries.
109 When a load-balanced queue is created, the PMD will configure a new sequence
110 number group on-demand if num_sequence_numbers does not match a pre-existing
111 group with available reorder buffer entries. If all sequence number groups are
112 in use, no new group will be created and queue configuration will fail. (Note
113 that when the PMD is used with a virtual DLB2 device, it cannot change the
114 sequence number configuration.)
116 The queue's ``nb_atomic_flows`` parameter is ignored by the DLB2 PMD, because
117 the DLB2 does not limit the number of flows a queue can track. In the DLB2, all
118 load-balanced queues can use the full 16-bit flow ID range.
120 Load-balanced and Directed Ports
121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
123 DLB2 ports come in two flavors: load-balanced and directed. The eventdev API
124 does not have the same concept, but it has a similar one: ports and queues that
125 are singly-linked (i.e. linked to a single queue or port, respectively).
127 The ``rte_event_dev_info_get()`` function reports the number of available
128 event ports and queues (among other things). For the DLB2 PMD, max_event_ports
129 and max_event_queues report the number of available load-balanced ports and
130 queues, and max_single_link_event_port_queue_pairs reports the number of
131 available directed ports and queues.
133 When a scheduling domain is created in ``rte_event_dev_configure()``, the user
134 specifies ``nb_event_ports`` and ``nb_single_link_event_port_queues``, which
135 control the total number of ports (load-balanced and directed) and the number
136 of directed ports. Hence, the number of requested load-balanced ports is
137 ``nb_event_ports - nb_single_link_event_ports``. The ``nb_event_queues`` field
138 specifies the total number of queues (load-balanced and directed). The number
139 of directed queues comes from ``nb_single_link_event_port_queues``, since
140 directed ports and queues come in pairs.
142 When a port is setup, the ``RTE_EVENT_PORT_CFG_SINGLE_LINK`` flag determines
143 whether it should be configured as a directed (the flag is set) or a
144 load-balanced (the flag is unset) port. Similarly, the
145 ``RTE_EVENT_QUEUE_CFG_SINGLE_LINK`` queue configuration flag controls
146 whether it is a directed or load-balanced queue.
148 Load-balanced ports can only be linked to load-balanced queues, and directed
149 ports can only be linked to directed queues. Furthermore, directed ports can
150 only be linked to a single directed queue (and vice versa), and that link
151 cannot change after the eventdev is started.
153 The eventdev API does not have a directed scheduling type. To support directed
154 traffic, the dlb PMD detects when an event is being sent to a directed queue
155 and overrides its scheduling type. Note that the originally selected scheduling
156 type (atomic, ordered, or parallel) is not preserved, and an event's sched_type
157 will be set to ``RTE_SCHED_TYPE_ATOMIC`` when it is dequeued from a directed
163 The flow ID field is preserved in the event when it is scheduled in the
169 DLB2 uses a hardware credit scheme to prevent software from overflowing hardware
170 event storage, with each unit of storage represented by a credit. A port spends
171 a credit to enqueue an event, and hardware refills the ports with credits as the
172 events are scheduled to ports. Refills come from credit pools, and each port is
173 a member of a load-balanced credit pool and a directed credit pool. The
174 load-balanced credits are used to enqueue to load-balanced queues, and directed
175 credits are used for directed queues.
177 A DLB2 eventdev contains one load-balanced and one directed credit pool. These
178 pools' sizes are controlled by the nb_events_limit field in struct
179 rte_event_dev_config. The load-balanced pool is sized to contain
180 nb_events_limit credits, and the directed pool is sized to contain
181 nb_events_limit/4 credits. The directed pool size can be overridden with the
182 num_dir_credits vdev argument, like so:
184 .. code-block:: console
186 --vdev=dlb1_event,num_dir_credits=<value>
188 This can be used if the default allocation is too low or too high for the
189 specific application needs. The PMD also supports a vdev arg that limits the
190 max_num_events reported by rte_event_dev_info_get():
192 .. code-block:: console
194 --vdev=dlb1_event,max_num_events=<value>
196 By default, max_num_events is reported as the total available load-balanced
197 credits. If multiple DLB2-based applications are being used, it may be desirable
198 to control how many load-balanced credits each application uses, particularly
199 when application(s) are written to configure nb_events_limit equal to the
200 reported max_num_events.
202 Each port is a member of both credit pools. A port's credit allocation is
203 defined by its low watermark, high watermark, and refill quanta. These three
204 parameters are calculated by the dlb PMD like so:
206 - The load-balanced high watermark is set to the port's enqueue_depth.
207 The directed high watermark is set to the minimum of the enqueue_depth and
208 the directed pool size divided by the total number of ports.
209 - The refill quanta is set to half the high watermark.
210 - The low watermark is set to the minimum of 16 and the refill quanta.
212 When the eventdev is started, each port is pre-allocated a high watermark's
213 worth of credits. For example, if an eventdev contains four ports with enqueue
214 depths of 32 and a load-balanced credit pool size of 4096, each port will start
215 with 32 load-balanced credits, and there will be 3968 credits available to
216 replenish the ports. Thus, a single port is not capable of enqueueing up to the
217 nb_events_limit (without any events being dequeued), since the other ports are
218 retaining their initial credit allocation; in short, all ports must enqueue in
219 order to reach the limit.
221 If a port attempts to enqueue and has no credits available, the enqueue
222 operation will fail and the application must retry the enqueue. Credits are
223 replenished asynchronously by the DLB2 hardware.
228 The DLB2 is a "closed system" event dev, and the DLB2 PMD layers a software
229 credit scheme on top of the hardware credit scheme in order to comply with
230 the per-port backpressure described in the eventdev API.
232 The DLB2's hardware scheme is local to a queue/pipeline stage: a port spends a
233 credit when it enqueues to a queue, and credits are later replenished after the
234 events are dequeued and released.
236 In the software credit scheme, a credit is consumed when a new (.op =
237 RTE_EVENT_OP_NEW) event is injected into the system, and the credit is
238 replenished when the event is released from the system (either explicitly with
239 RTE_EVENT_OP_RELEASE or implicitly in dequeue_burst()).
241 In this model, an event is "in the system" from its first enqueue into eventdev
242 until it is last dequeued. If the event goes through multiple event queues, it
243 is still considered "in the system" while a worker thread is processing it.
245 A port will fail to enqueue if the number of events in the system exceeds its
246 ``new_event_threshold`` (specified at port setup time). A port will also fail
247 to enqueue if it lacks enough hardware credits to enqueue; load-balanced
248 credits are used to enqueue to a load-balanced queue, and directed credits are
249 used to enqueue to a directed queue.
251 The out-of-credit situations are typically transient, and an eventdev
252 application using the DLB2 ought to retry its enqueues if they fail.
253 If enqueue fails, DLB2 PMD sets rte_errno as follows:
255 - -ENOSPC: Credit exhaustion (either hardware or software)
256 - -EINVAL: Invalid argument, such as port ID, queue ID, or sched_type.
258 Depending on the pipeline the application has constructed, it's possible to
259 enter a credit deadlock scenario wherein the worker thread lacks the credit
260 to enqueue an event, and it must dequeue an event before it can recover the
261 credit. If the worker thread retries its enqueue indefinitely, it will not
262 make forward progress. Such deadlock is possible if the application has event
263 "loops", in which an event in dequeued from queue A and later enqueued back to
266 Due to this, workers should stop retrying after a time, release the events it
267 is attempting to enqueue, and dequeue more events. It is important that the
268 worker release the events and don't simply set them aside to retry the enqueue
269 again later, because the port has limited history list size (by default, twice
270 the port's dequeue_depth).
275 The DLB2 supports event priority and per-port queue service priority, as
276 described in the eventdev header file. The DLB2 does not support 'global' event
277 queue priority established at queue creation time.
279 DLB2 supports 8 event and queue service priority levels. For both priority
280 types, the PMD uses the upper three bits of the priority field to determine the
281 DLB2 priority, discarding the 5 least significant bits. The 5 least significant
282 event priority bits are not preserved when an event is enqueued.
287 The Eventdev API allows one to reconfigure a device, its ports, and its queues
288 by first stopping the device, calling the configuration function(s), then
289 restarting the device. The DLB2 does not support configuring an individual queue
290 or port without first reconfiguring the entire device, however, so there are
291 certain reconfiguration sequences that are valid in the eventdev API but not
292 supported by the PMD.
294 Specifically, the PMD supports the following configuration sequence:
295 1. Configure and start the device
297 3. (Optional) Reconfigure the device
298 4. (Optional) If step 3 is run:
300 a. Setup queue(s). The reconfigured queue(s) lose their previous port links.
301 b. The reconfigured port(s) lose their previous queue links.
303 5. (Optional, only if steps 4a and 4b are run) Link port(s) to queue(s)
304 6. Restart the device. If the device is reconfigured in step 3 but one or more
305 of its ports or queues are not, the PMD will apply their previous
306 configuration (including port->queue links) at this time.
308 The PMD does not support the following configuration sequences:
309 1. Configure and start the device
311 3. Setup queue or setup port
314 This sequence is not supported because the event device must be reconfigured
315 before its ports or queues can be.
317 Atomic Inflights Allocation
318 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
320 In the last stage prior to scheduling an atomic event to a CQ, DLB2 holds the
321 inflight event in a temporary buffer that is divided among load-balanced
322 queues. If a queue's atomic buffer storage fills up, this can result in
323 head-of-line-blocking. For example:
325 - An LDB queue allocated N atomic buffer entries
326 - All N entries are filled with events from flow X, which is pinned to CQ 0.
328 Until CQ 0 releases 1+ events, no other atomic flows for that LDB queue can be
329 scheduled. The likelihood of this case depends on the eventdev configuration,
330 traffic behavior, event processing latency, potential for a worker to be
331 interrupted or otherwise delayed, etc.
333 By default, the PMD allocates 16 buffer entries for each load-balanced queue,
334 which provides an even division across all 128 queues but potentially wastes
335 buffer space (e.g. if not all queues are used, or aren't used for atomic
338 The PMD provides a dev arg to override the default per-queue allocation. To
339 increase a vdev's per-queue atomic-inflight allocation to (for example) 64:
341 .. code-block:: console
343 --vdev=dlb1_event,atm_inflights=64
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