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[bpf-next,v6,0/5] Replace CONFIG_DMABUF_SYSFS_STATS with BPF

Message ID 20250513163601.812317-1-tjmercier@google.com
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Series Replace CONFIG_DMABUF_SYSFS_STATS with BPF | expand

Message

T.J. Mercier May 13, 2025, 4:35 p.m. UTC 
Until CONFIG_DMABUF_SYSFS_STATS was added [1] it was only possible to
perform per-buffer accounting with debugfs which is not suitable for
production environments. Eventually we discovered the overhead with
per-buffer sysfs file creation/removal was significantly impacting
allocation and free times, and exacerbated kernfs lock contention. [2]
dma_buf_stats_setup() is responsible for 39% of single-page buffer
creation duration, or 74% of single-page dma_buf_export() duration when
stressing dmabuf allocations and frees.

I prototyped a change from per-buffer to per-exporter statistics with a
RCU protected list of exporter allocations that accommodates most (but
not all) of our use-cases and avoids almost all of the sysfs overhead.
While that adds less overhead than per-buffer sysfs, and less even than
the maintenance of the dmabuf debugfs_list, it's still *additional*
overhead on top of the debugfs_list and doesn't give us per-buffer info.

This series uses the existing dmabuf debugfs_list to implement a BPF
dmabuf iterator, which adds no overhead to buffer allocation/free and
provides per-buffer info. The list has been moved outside of
CONFIG_DEBUG_FS scope so that it is always populated. The BPF program
loaded by userspace that extracts per-buffer information gets to define
its own interface which avoids the lack of ABI stability with debugfs.

This will allow us to replace our use of CONFIG_DMABUF_SYSFS_STATS, and
the plan is to remove it from the kernel after the next longterm stable
release.

[1] https://lore.kernel.org/linux-media/20201210044400.1080308-1-hridya@google.com
[2] https://lore.kernel.org/all/20220516171315.2400578-1-tjmercier@google.com

v1: https://lore.kernel.org/all/20250414225227.3642618-1-tjmercier@google.com
v1 -> v2:
Make the DMA buffer list independent of CONFIG_DEBUG_FS per Christian
  König
Add CONFIG_DMA_SHARED_BUFFER check to kernel/bpf/Makefile per kernel
  test robot
Use BTF_ID_LIST_SINGLE instead of BTF_ID_LIST_GLOBAL_SINGLE per Song Liu
Fixup comment style, mixing code/declarations, and use ASSERT_OK_FD in
  selftest per Song Liu
Add BPF_ITER_RESCHED feature to bpf_dmabuf_reg_info per Alexei
  Starovoitov
Add open-coded iterator and selftest per Alexei Starovoitov
Add a second test buffer from the system dmabuf heap to selftests
Use the BPF program we'll use in production for selftest per Alexei
  Starovoitov
  https://r.android.com/c/platform/system/bpfprogs/+/3616123/2/dmabufIter.c
  https://r.android.com/c/platform/system/memory/libmeminfo/+/3614259/1/libdmabufinfo/dmabuf_bpf_stats.cpp
v2: https://lore.kernel.org/all/20250504224149.1033867-1-tjmercier@google.com
v2 -> v3:
Rebase onto bpf-next/master
Move get_next_dmabuf() into drivers/dma-buf/dma-buf.c, along with the
  new get_first_dmabuf(). This avoids having to expose the dmabuf list
  and mutex to the rest of the kernel, and keeps the dmabuf mutex
  operations near each other in the same file. (Christian König)
Add Christian's RB to dma-buf: Rename debugfs symbols
Drop RFC: dma-buf: Remove DMA-BUF statistics
v3: https://lore.kernel.org/all/20250507001036.2278781-1-tjmercier@google.com
v3 -> v4:
Fix selftest BPF program comment style (not kdoc) per Alexei Starovoitov
Fix dma-buf.c kdoc comment style per Alexei Starovoitov
Rename get_first_dmabuf / get_next_dmabuf to dma_buf_iter_begin /
  dma_buf_iter_next per Christian König
Add Christian's RB to bpf: Add dmabuf iterator
v4: https://lore.kernel.org/all/20250508182025.2961555-1-tjmercier@google.com
v4 -> v5:
Add Christian's Acks to all patches
Add Song Liu's Acks
Move BTF_ID_LIST_SINGLE and DEFINE_BPF_ITER_FUNC closer to usage per
  Song Liu
Fix open-coded iterator comment style per Song Liu
Move iterator termination check to its own subtest per Song Liu
Rework selftest buffer creation per Song Liu
Fix spacing in sanitize_string per BPF CI
v5: https://lore.kernel.org/all/20250512174036.266796-1-tjmercier@google.com
v5 -> v6:
Song Liu:
  Init test buffer FDs to -1
  Zero-init udmabuf_create for future proofing
  Bail early for iterator fd/FILE creation failure
  Dereference char ptr to check for NUL in sanitize_string()
  Move map insertion from create_test_buffers() to test_dmabuf_iter()
  Add ACK to selftests/bpf: Add test for open coded dmabuf_iter

T.J. Mercier (5):
  dma-buf: Rename debugfs symbols
  bpf: Add dmabuf iterator
  bpf: Add open coded dmabuf iterator
  selftests/bpf: Add test for dmabuf_iter
  selftests/bpf: Add test for open coded dmabuf_iter

 drivers/dma-buf/dma-buf.c                     |  98 ++++--
 include/linux/dma-buf.h                       |   4 +-
 kernel/bpf/Makefile                           |   3 +
 kernel/bpf/dmabuf_iter.c                      | 150 +++++++++
 kernel/bpf/helpers.c                          |   5 +
 .../testing/selftests/bpf/bpf_experimental.h  |   5 +
 tools/testing/selftests/bpf/config            |   3 +
 .../selftests/bpf/prog_tests/dmabuf_iter.c    | 285 ++++++++++++++++++
 .../testing/selftests/bpf/progs/dmabuf_iter.c |  91 ++++++
 9 files changed, 622 insertions(+), 22 deletions(-)
 create mode 100644 kernel/bpf/dmabuf_iter.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/dmabuf_iter.c
 create mode 100644 tools/testing/selftests/bpf/progs/dmabuf_iter.c


base-commit: 43745d11bfd9683abdf08ad7a5cc403d6a9ffd15