[RFC v2 0/3] block: Introduce a BPF-based I/O scheduler

[Kaitao cheng <kaitao.cheng@linux.dev>]

I have been working on adding a new BPF-based I/O scheduler. It has both
kernel and user-space parts. In kernel space, using per-ctx, I implemented
a simple elevator that exposes a set of BPF hooks. The goal is to move the
policy side of I/O scheduling out of the kernel and into user space, which
should greatly increase flexibility and applicability. To verify that the
whole stack works end to end, I wrote a simple BPF example program. I am
calling this feature the UFQ (User-programmable Flexible Queueing) I/O
scheduler.

This patch depends on new BPF functionality that I have already posted to
the BPF community but that is not yet in mainline. Details are in the
thread:

https://lore.kernel.org/bpf/20260427165906.84420-1-kaitao.cheng@linux.dev/

To try it, you need to apply the patches from that series first.

I am looking for community feedback on whether this direction and the
implementation approach make sense, and what else we should
consider.

todo:
1. More thorough testing
2. Split the code into multiple sub-patches
3. Identify concrete use cases

Changes in v2:
- Remove bpf_request_put (Alexei Starovoitov)
- Update the UFQ README (Miguel Ojeda)
- Add bio merge support
- Fix synchronization issues during UFQ scheduler transitions

Link to v1:
https://lore.kernel.org/bpf/20260327114741.91500-1-pilgrimtao@gmail.com/

Kaitao Cheng (3):
  bpf: Add KF_SPIN_LOCK flag for kfuncs under bpf_spin_lock
  block: Introduce the UFQ I/O scheduler
  tools/ufq_iosched: add BPF example scheduler and build scaffolding

 block/Kconfig.iosched                         |   8 +
 block/Makefile                                |   1 +
 block/blk-merge.c                             |  28 +-
 block/blk-mq.c                                |   8 +-
 block/blk-mq.h                                |   2 +-
 block/blk.h                                   |   5 +
 block/ufq-bpfops.c                            | 241 +++++++
 block/ufq-iosched.c                           | 640 ++++++++++++++++++
 block/ufq-iosched.h                           |  64 ++
 block/ufq-kfunc.c                             | 131 ++++
 include/linux/btf.h                           |   1 +
 kernel/bpf/verifier.c                         |  20 +-
 tools/ufq_iosched/.gitignore                  |   2 +
 tools/ufq_iosched/Makefile                    | 262 +++++++
 tools/ufq_iosched/README.md                   | 145 ++++
 .../include/bpf-compat/gnu/stubs.h            |  12 +
 tools/ufq_iosched/include/ufq/common.bpf.h    |  75 ++
 tools/ufq_iosched/include/ufq/common.h        |  90 +++
 tools/ufq_iosched/include/ufq/simple_stat.h   |  23 +
 tools/ufq_iosched/ufq_simple.bpf.c            | 604 +++++++++++++++++
 tools/ufq_iosched/ufq_simple.c                | 120 ++++
 21 files changed, 2464 insertions(+), 18 deletions(-)
 create mode 100644 block/ufq-bpfops.c
 create mode 100644 block/ufq-iosched.c
 create mode 100644 block/ufq-iosched.h
 create mode 100644 block/ufq-kfunc.c
 create mode 100644 tools/ufq_iosched/.gitignore
 create mode 100644 tools/ufq_iosched/Makefile
 create mode 100644 tools/ufq_iosched/README.md
 create mode 100644 tools/ufq_iosched/include/bpf-compat/gnu/stubs.h
 create mode 100644 tools/ufq_iosched/include/ufq/common.bpf.h
 create mode 100644 tools/ufq_iosched/include/ufq/common.h
 create mode 100644 tools/ufq_iosched/include/ufq/simple_stat.h
 create mode 100644 tools/ufq_iosched/ufq_simple.bpf.c
 create mode 100644 tools/ufq_iosched/ufq_simple.c

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