linux-kernel - [PATCH v2 0/6] Replace CONFIG_DMABUF_SYSFS

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Message-ID: <20250504224149.1033867-1-tjmercier@google.com>
Date: Sun,  4 May 2025 22:41:36 +0000
From: "T.J. Mercier" <tjmercier@...gle.com>
To: sumit.semwal@...aro.org, christian.koenig@....com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	skhan@...uxfoundation.org, song@...nel.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, 
	linux-doc@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, android-mm@...gle.com, simona@...ll.ch, 
	corbet@....net, eddyz87@...il.com, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	jolsa@...nel.org, mykolal@...com, "T.J. Mercier" <tjmercier@...gle.com>
Subject: [PATCH v2 0/6] Replace CONFIG_DMABUF_SYSFS_STATS with BPF

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.

As this is a replacement for our use of CONFIG_DMABUF_SYSFS_STATS, the
last patch is a RFC for removing it from the kernel. Please see my
suggestion there regarding the timeline for that.

[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

T.J. Mercier (6):
  dma-buf: Rename and expose 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
  RFC: dma-buf: Remove DMA-BUF statistics

 .../ABI/testing/sysfs-kernel-dmabuf-buffers   |  24 --
 Documentation/driver-api/dma-buf.rst          |   5 -
 drivers/dma-buf/Kconfig                       |  15 -
 drivers/dma-buf/Makefile                      |   1 -
 drivers/dma-buf/dma-buf-sysfs-stats.c         | 202 --------------
 drivers/dma-buf/dma-buf-sysfs-stats.h         |  35 ---
 drivers/dma-buf/dma-buf.c                     |  58 +---
 include/linux/dma-buf.h                       |   6 +-
 kernel/bpf/Makefile                           |   3 +
 kernel/bpf/dmabuf_iter.c                      | 177 ++++++++++++
 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 ++++++
 15 files changed, 561 insertions(+), 327 deletions(-)
 delete mode 100644 Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
 delete mode 100644 drivers/dma-buf/dma-buf-sysfs-stats.c
 delete mode 100644 drivers/dma-buf/dma-buf-sysfs-stats.h
 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: 0af2f6be1b4281385b618cb86ad946eded089ac8
-- 
2.49.0.906.g1f30a19c02-goog