[PATCH v10 0/2] Introducing trace buffer mapping by user-space

Vincent Donnefort
2024-01-05T09:47:56Z

The tracing ring-buffers can be stored on disk or sent to network
without any copy via splice. However the later doesn't allow real time
processing of the traces. A solution is to give userspace direct access
to the ring-buffer pages via a mapping. An application can now become a
consumer of the ring-buffer, in a similar fashion to what trace_pipe
offers.

Support for this new feature in libtracefs can be found here:

https://lore.kernel.org/all/20231228201100.78aae259@rorschach.local.home

Vincent

v9 -> v10:
* Refactor rb_update_meta_page()
* In-loop declaration for foreach_subbuf_page()
* Check for cpu_buffer->mapped overflow

v8 -> v9:
* Fix the unlock path in ring_buffer_map()
* Fix cpu_buffer cast with rb_work_rq->is_cpu_buffer
* Rebase on linux-trace/for-next (3cb3091138ca0921c4569bcf7ffa062519639b6a)

v7 -> v8:
* Drop the subbufs renaming into bpages
* Use subbuf as a name when relevant

v6 -> v7:
* Rebase onto lore.kernel.org/lkml/20231215175502.106587604@goodmis.org/
* Support for subbufs
* Rename subbufs into bpages

v5 -> v6:
* Rebase on next-20230802.
* (unsigned long) -> (void *) cast for virt_to_page().
* Add a wait for the GET_READER_PAGE ioctl.
* Move writer fields update (overrun/pages_lost/entries/pages_touched)
in the irq_work.
* Rearrange id in struct buffer_page.
* Rearrange the meta-page.
* ring_buffer_meta_page -> trace_buffer_meta_page.
* Add meta_struct_len into the meta-page.

v4 -> v5:
* Trivial rebase onto 6.5-rc3 (previously 6.4-rc3)

v3 -> v4:
* Add to the meta-page:
- pages_lost / pages_read (allow to compute how full is the
ring-buffer)
- read (allow to compute how many entries can be read)
- A reader_page struct.
* Rename ring_buffer_meta_header -> ring_buffer_meta
* Rename ring_buffer_get_reader_page -> ring_buffer_map_get_reader_page
* Properly consume events on ring_buffer_map_get_reader_page() with
rb_advance_reader().

v2 -> v3:
* Remove data page list (for non-consuming read)
** Implies removing order > 0 meta-page
* Add a new meta page field ->read
* Rename ring_buffer_meta_page_header into ring_buffer_meta_header

v1 -> v2:
* Hide data_pages from the userspace struct
* Fix META_PAGE_MAX_PAGES
* Support for order > 0 meta-page
* Add missing page->mapping.

---

Vincent Donnefort (2):
ring-buffer: Introducing ring-buffer mapping functions
tracing: Allow user-space mapping of the ring-buffer

include/linux/ring_buffer.h | 7 +
include/uapi/linux/trace_mmap.h | 31 +++
kernel/trace/ring_buffer.c | 384 +++++++++++++++++++++++++++++++-
kernel/trace/trace.c | 79 ++++++-
4 files changed, 497 insertions(+), 4 deletions(-)
create mode 100644 include/uapi/linux/trace_mmap.h

base-commit: 3cb3091138ca0921c4569bcf7ffa062519639b6a
--
2.43.0.472.g3155946c3a-goog