Re: [PATCH bpf-next v4 0/5] Replace CONFIG_DMABUF_SYSFS_STATS with BPF

["Christian König" <christian.koenig@amd.com>]

Feel free to add my Acked-by to the patches which don't have my rb yet.

And ping me when I should upstream this through drm-misc-next, but if you want to upstream this through some other branch then that is fine with me as well.

Regards,
Christian.

On 5/8/25 20:20, T.J. Mercier wrote:
> 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
> 
> 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                      | 149 ++++++++++
>  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    | 258 ++++++++++++++++++
>  .../testing/selftests/bpf/progs/dmabuf_iter.c |  91 ++++++
>  9 files changed, 594 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