[Cake] [PATCH net-next v4 0/5] Multi-queue aware sch_cake

["Toke Høiland-Jørgensen" <toke@redhat.com>]

This series adds a multi-queue aware variant of the sch_cake scheduler,
called 'cake_mq'. Using this makes it possible to scale the rate shaper
of sch_cake across multiple CPUs, while still enforcing a single global
rate on the interface.

The approach taken in this patch series is to implement a separate qdisc
called 'cake_mq', which is based on the existing 'mq' qdisc, but differs
in a couple of aspects:

- It will always install a cake instance on each hardware queue (instead
  of using the default qdisc for each queue like 'mq' does).

- The cake instances on the queues will share their configuration, which
  can only be modified through the parent cake_mq instance.

Doing things this way simplifies user configuration by centralising
all configuration through the cake_mq qdisc (which also serves as an
obvious way of opting into the multi-queue aware behaviour). The cake_mq
qdisc takes all the same configuration parameters as the cake qdisc.

An earlier version of this work was presented at this year's Netdevconf:
https://netdevconf.info/0x19/sessions/talk/mq-cake-scaling-software-rate-limiting-across-cpu-cores.html

The patch series is structured as follows:

- Patch 1 exports the mq qdisc functions for reuse.

- Patch 2 factors out the sch_cake configuration variables into a
  separate struct that can be shared between instances.

- Patch 3 adds the basic cake_mq qdisc, reusing the exported mq code

- Patch 4 adds configuration sharing across the cake instances installed
  under cake_mq

- Patch 5 adds the shared shaper state that enables the multi-core rate
  shaping

A selftest, and a patch to iproute2 to make it aware of the cake_mq
qdisc, will be submitted separately.

---
Changes in v4:
- A bunch of bot nits:
 - Fix null pointer deref in cake_destroy()
 - Unwind qdisc registration on failure
 - Use rcu_dereference() instead of rtnl_dereference() in data path
 - Use WRITE_ONCE() for q->last_active
 - Store num_active_qs to stats value after computing it
- Link to v3: https://lore.kernel.org/r/20251130-mq-cake-sub-qdisc-v3-0-5f66c548ecdc@redhat.com

Changes in v3:
- Export the functions from sch_mq and reuse them instead of copy-pasting
- Dropped Jamal's reviewed-by on the patches that changed due to the above
- Fixed a crash if cake_mq_init is called with a NULL opt parameter
- Link to v2: https://lore.kernel.org/r/20251127-mq-cake-sub-qdisc-v2-0-24d9ead047b9@redhat.com

Changes in v2:
- Rebase on top of net-next, incorporating Eric's changes
- Link to v1: https://lore.kernel.org/r/20251124-mq-cake-sub-qdisc-v1-0-a2ff1dab488f@redhat.com

Changes in v1 (since RFC):
- Drop the sync_time parameter for now and always use the 200 us value.
  We are planning to explore auto-configuration of the sync time, so
  this is to avoid committing to a UAPI. If needed, a parameter can be
  added back later.
- Keep the tc yaml spec in sync with the new stats member
- Rebase on net-next
- Link to RFC: https://lore.kernel.org/r/20250924-mq-cake-sub-qdisc-v1-0-43a060d1112a@redhat.com

---
Jonas Köppeler (1):
      net/sched: sch_cake: share shaper state across sub-instances of cake_mq

Toke Høiland-Jørgensen (4):
      net/sched: Export mq functions for reuse
      net/sched: sch_cake: Factor out config variables into separate struct
      net/sched: sch_cake: Add cake_mq qdisc for using cake on mq devices
      net/sched: sch_cake: Share config across cake_mq sub-qdiscs

 Documentation/netlink/specs/tc.yaml |   3 +
 include/net/sch_generic.h           |  19 ++
 include/uapi/linux/pkt_sched.h      |   1 +
 net/sched/sch_cake.c                | 485 ++++++++++++++++++++++++++----------
 net/sched/sch_mq.c                  |  69 +++--
 5 files changed, 431 insertions(+), 146 deletions(-)
---
base-commit: 0177f0f07886e54e12c6f18fa58f63e63ddd3c58
change-id: 20250902-mq-cake-sub-qdisc-cdf0b59d2fe5