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Message-ID: <b1e53d0b-04ba-4ad7-abdf-2406283a9cfb@amd.com>
Date: Fri, 9 May 2025 08:03:50 +0200
From: Christian König <christian.koenig@....com>
To: "T.J. Mercier" <tjmercier@...gle.com>, sumit.semwal@...aro.org,
ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
martin.lau@...ux.dev, skhan@...uxfoundation.org, alexei.starovoitov@...il.com
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, android-mm@...gle.com,
simona@...ll.ch, eddyz87@...il.com, yonghong.song@...ux.dev,
john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
jolsa@...nel.org, mykolal@...com, shuah@...nel.org, song@...nel.org
Subject: Re: [PATCH bpf-next v4 0/5] Replace CONFIG_DMABUF_SYSFS_STATS with
BPF
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
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