Merge branch 'bpf-qdisc'

Amery Hung says:

====================
bpf qdisc

Hi all,

This patchset aims to support implementing qdisc using bpf struct_ops.
This version takes a step back and only implements the minimum support
for bpf qdisc. 1) support of adding skb to bpf_list and bpf_rbtree
directly and 2) classful qdisc are deferred to future patchsets. In
addition, we only allow attaching bpf qdisc to root or mq for now.
This is to prevent accidentally breaking exisiting classful qdiscs
that rely on data in a child qdisc. This limit may be lifted in the
future after careful inspection.

* Overview *

This series supports implementing qdisc using bpf struct_ops. bpf qdisc
aims to be a flexible and easy-to-use infrastructure that allows users to
quickly experiment with different scheduling algorithms/policies. It only
requires users to implement core qdisc logic using bpf and implements the
mundane part for them. In addition, the ability to easily communicate
between qdisc and other components will also bring new opportunities for
new applications and optimizations.

* Performance of bpf qdisc *

This patchset includes two qdisc examples, bpf_fifo and bpf_fq, for
__testing__ purposes. For performance test, we compare selftests and their
kernel counterparts to give you a sense of the performance of qdisc
implemented in bpf.

The implementation of bpf_fq is fairly complex and slightly different from
fq so later we only compare the two fifo qdiscs. bpf_fq implements a
scheduling algorithm similar to fq before commit 29f834aa326e ("net_sched:
sch_fq: add 3 bands and WRR scheduling") was introduced. bpf_fifo uses a
single bpf_list as a queue instead of three queues for different
priorities in pfifo_fast. The time complexity of fifo however should be
similar since the queue selection time is negligible.

Test setup:

    client -> qdisc ------------->  server
    ~~~~~~~~~~~~~~~                 ~~~~~~
    nested VM1 @ DC1               VM2 @ DC2

Throghput: iperf3 -t 600, 5 times

      Qdisc        Average (GBits/sec)
    ----------     -------------------
    pfifo_fast       12.52 ± 0.26
    bpf_fifo         11.72 ± 0.32
    fq               10.24 ± 0.13
    bpf_fq           11.92 ± 0.64

Latency: sockperf pp --tcp -t 600, 5 times

      Qdisc        Average (usec)
    ----------     --------------
    pfifo_fast      244.58 ± 7.93
    bpf_fifo        244.92 ± 15.22
    fq              234.30 ± 19.25
    bpf_fq          221.34 ± 10.76

Looking at the two fifo qdiscs, the 6.4% drop in throughput in the bpf
implementatioin is consistent with previous observation (v8 throughput
test on a loopback device). This should be able to be mitigated by
supporting adding skb to bpf_list or bpf_rbtree directly in the future.

* Clean up skb in bpf qdisc during reset *

The current implementation relies on bpf qdisc implementors to correctly
release skbs in queues (bpf graphs or maps) in .reset, which might not be
a safe thing to do. The solution as Martin has suggested would be
supporting private data in struct_ops. This can also help simplifying
implementation of qdisc that works with mq. For examples, qdiscs in the
selftest mostly use global data. Therefore, even if user add multiple
qdisc instances under mq, they would still share the same queue.
====================

Link: https://patch.msgid.link/20250409214606.2000194-1-ameryhung@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>