[PATCH v2 0/6] Replace CONFIG_DMABUF_SYSFS_STATS with BPF
T.J. Mercier
tjmercier at google.com
Sun May 4 22:41:36 UTC 2025
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
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