linux-kernel - [PATCH 0/4] Replace CONFIG_DMABUF_SYSFS

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Message-ID: <20250414225227.3642618-1-tjmercier@google.com>
Date: Mon, 14 Apr 2025 22:52:23 +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
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, song@...nel.org, 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 0/4] 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. While the kernel must have CONFIG_DEBUG_FS for
the dmabuf_iter to be available, debugfs does not need to be mounted.
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 (even if it were mounted).

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/

T.J. Mercier (4):
  dma-buf: Rename and expose debugfs symbols
  bpf: Add dmabuf iterator
  selftests/bpf: Add test for 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                     |  40 +---
 include/linux/btf_ids.h                       |   1 +
 include/linux/dma-buf.h                       |   6 +
 kernel/bpf/Makefile                           |   3 +
 kernel/bpf/dmabuf_iter.c                      | 130 +++++++++++
 tools/testing/selftests/bpf/config            |   1 +
 .../selftests/bpf/prog_tests/dmabuf_iter.c    | 116 ++++++++++
 .../testing/selftests/bpf/progs/dmabuf_iter.c |  31 +++
 14 files changed, 299 insertions(+), 311 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

-- 
2.49.0.604.gff1f9ca942-goog