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

Masami Hiramatsu
2024-01-09T13:04:50Z

Hi Vincent,

On Fri, 5 Jan 2024 09:47:27 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> 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.

I think this is very nice feature. But this series seems just a feature,
no document and no example code. Can you add 2 patches to add those?
I know libtracefs already provide a support code, but I think it is
better to have a test code under tools/testing/selftests/ring_buffer.

I also wonder what happen if other operation (e.g. taking snapshot) happens
while mmaping the ring buffer.

Thank you,

>
> 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
>
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>