event/slb: introducing software load balancer eventdev PMD

From: Pravin Pathak <[email protected]>

This series introduces the Software Load Balancer (SLB) Eventdev PMD (event_slb) supporting parallel, atomic and ordered queues. It is a distributed software scheduler that scales well.

Motivation
----------
Existing software eventdev PMDs face trade-offs between centralized bottlenecks and feature coverage:
- event_sw relies on a dedicated centralized scheduler consuming an extra CPU core. The single scheduling core also limits maximum throughput and it does not scale.
- event_dsw distributes scheduling across worker cores without a central scheduler core, but does not support ordered queues and incurs per-port control ring messaging overhead during atomic flow migrations. It also does not support service priority.

Design Overview
---------------
event_slb is a distributed (RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED), maintenance-free (RTE_EVENT_DEV_CAP_MAINTENANCE_FREE) software eventdev PMD that supports Atomic, Ordered, Parallel, and Single-Link queues:

1. Queue-Centric Ring Architecture:
- Parallel and Single-Link queues map directly to rte_event_ring instances (MP/MC for parallel, MP/SC for single-link), allowing zero-copy burst enqueue and dequeue without history tracking.
- Ordered queues use the ring slot index at dequeue time as an implicit sequence number into a per-queue reorder buffer, draining contiguous completed events in order without global sequence counters. This makes ordered queue implementaion highly efficient performing close to parallel queues.
- Atomic queues use a main MP/SC queue ring alongside per-port SP/SC rings and a 2048-entry flow-to-port affinity table. All cores served by the atomic queue are used for scheduling.

2. Lock-Free / Zero-Pause Atomic Flow Migration:
- When an atomic queue refills per-port rings and detects persistent congestion on a destination port, SLB selects the lightest flow on that port (outside the migration cooldown window) and waits for queues in-flight events to drain naturally before switching the flow's port affinity and schedule hold-buffer entries.
- Hold buffer is used for buffering events towards migrating ports while allowing serving other ports.
- Implementaion avoids cross-port pause/unpause control rings while preserving strict per-flow ordering.

3. Credit-Based Backpressure & Priority Scheduling:
- A global atomic credit pool with per-port local credit caches bounds total in-flight events (nb_events_limit) and enforces new_event_threshold with minimal atomic contention.
- Ports support two strict service priority levels with round-robin scheduling within each priority level.

4. Observability & Testing:
- Device and per-port/per-queue extended statistics (xstats), detailed rte_event_dev_dump() diagnostics, and a unit self-test suite (eventdev_selftest_slb) covering parallel, atomic, ordered, single-link, priority, backpressure, stop-flush, and multi-core atomic migration scenarios.

5. Future Plan:
- Supporting Event priority
- Optimizing atomic flow migration

Pravin Pathak (8):
event/slb: introduce software load balancer PMD event/slb: add support for ordered queues
event/slb: add support for atomic queues
event/slb: add dynamic flow migration for atomic queues event/slb: add extended statistics and diagnostics event/slb: add self-test and documentation
event/slb: update resets and credit defaults event/slb: improve port linking, config checks and atomic refill

 MAINTAINERS                           |    6 +
 app/test/test_eventdev.c              |    7 +
 doc/guides/eventdevs/features/slb.ini |   25 +
 doc/guides/eventdevs/index.rst        |    1 +
 doc/guides/eventdevs/slb.rst          |   61 ++
 drivers/event/meson.build             |    1 +
 drivers/event/slb/meson.build         |   21 +
 drivers/event/slb/slb_evdev.c         |  904 ++++++++++++++++++
 drivers/event/slb/slb_evdev.h         |  263 ++++++
 drivers/event/slb/slb_event.c         | 1219 +++++++++++++++++++++++++
 drivers/event/slb/slb_selftest.c      |  522 +++++++++++
 drivers/event/slb/slb_xstats.c        |  347 +++++++
 12 files changed, 3377 insertions(+)
 create mode 100644 doc/guides/eventdevs/features/slb.ini
 create mode 100644 doc/guides/eventdevs/slb.rst
 create mode 100644 drivers/event/slb/meson.build
 create mode 100644 drivers/event/slb/slb_evdev.c
 create mode 100644 drivers/event/slb/slb_evdev.h
 create mode 100644 drivers/event/slb/slb_event.c
 create mode 100644 drivers/event/slb/slb_selftest.c
 create mode 100644 drivers/event/slb/slb_xstats.c

2.53.0