[PATCH 0/2] block: expose blkcg io.stat to BPF

[Ziyang Men <ziyang.meme@gmail.com>]

This series exposes the block I/O controller's per-device statistics
(io.stat) to BPF via read-only kfuncs, mirroring the kfuncs for the memory
controller in mm/bpf_memcontrol.c.

Motivation:
Collecting cgroup statistics is expensive: the existing method is to open
and parse a cgroup file for every cgroup of interest. memcg already has an
efficient alternative through BPF; this series extends that idea to block.

The main difference from the memory controller is that blkcg keeps one
blkg (one io.stat line) per block device, so the readers operate on a
blkg and the iterator yields them under RCU. Reads take the same
u64_stats seqlock as the io.stat file, so the kfuncs add no fast-path
cost.

The root cgroup needs its own path. It is not accounted through rstat at
all, so bpf_blkcg_flush_stats() branches the way blkcg_print_stat() does
and refills the root's per-device aggregates from the disks' own
statistics. That is the only reason the series touches blk-cgroup.c and
blk-cgroup.h: blkcg_fill_root_iostats() is no longer static. Nothing on
the I/O path changes.

Patch 1 adds the kfuncs (block/bpf_blkcg.c); patch 2 adds a test_progs
selftest that drives direct I/O to a loop device and checks 1) that the
measured values are non-zero (as cgroup_iter_memcg does), 2) that each
kfunc value exactly matches the cgroup's io.stat file for that device,
and 3) that the same device read through the root block cgroup is at or
above what the test cgroup was charged.

Tested on v7.2-rc5 with the series applied: all five cgroup_iter_io
subtests pass (write/read/dev/match/root).

Ziyang Men (2):
  block: add BPF kfuncs to read blkcg io.stat
  selftests/bpf: add test for blkcg io.stat BPF kfuncs

 MAINTAINERS                                   |   1 +
 block/Makefile                                |   3 +
 block/blk-cgroup.c                            |   2 +-
 block/blk-cgroup.h                            |   1 +
 block/bpf_blkcg.c                             | 315 ++++++++++++++++++
 tools/testing/selftests/bpf/cgroup_iter_io.h  |  17 +
 tools/testing/selftests/bpf/config            |   1 +
 .../selftests/bpf/prog_tests/cgroup_iter_io.c | 310 +++++++++++++++++
 .../selftests/bpf/progs/cgroup_iter_io.c      | 107 ++++++
 9 files changed, 756 insertions(+), 1 deletion(-)
 create mode 100644 block/bpf_blkcg.c
 create mode 100644 tools/testing/selftests/bpf/cgroup_iter_io.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_iter_io.c
 create mode 100644 tools/testing/selftests/bpf/progs/cgroup_iter_io.c

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2.53.0-Meta