[v19,0/5] Introducing trace buffer mapping by user-space

Message

Vincent Donnefort March 26, 2024, 10:03 a.m. UTC

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 can already be found in libtracefs from
version 1.8, when built with EXTRA_CFLAGS=-DFORCE_MMAP_ENABLE.

Vincent

v18 -> v19:
  * Use VM_PFNMAP and vm_insert_pages
  * Allocate ring-buffer subbufs with __GFP_COMP
  * Pad the meta-page with the zero-page to align on the subbuf_order
  * Extend the ring-buffer test with mmap() dedicated suite

v17 -> v18:
  * Fix lockdep_assert_held
  * Fix spin_lock_init typo
  * Fix CONFIG_TRACER_MAX_TRACE typo

v16 -> v17:
  * Documentation and comments improvements.
  * Create get/put_snapshot_map() for clearer code.
  * Replace kzalloc with kcalloc.
  * Fix -ENOMEM handling in rb_alloc_meta_page().
  * Move flush(cpu_buffer->reader_page) behind the reader lock.
  * Move all inc/dec of cpu_buffer->mapped behind reader lock and buffer
    mutex. (removes READ_ONCE/WRITE_ONCE accesses).

v15 -> v16:
  * Add comment for the dcache flush.
  * Remove now unnecessary WRITE_ONCE for the meta-page.

v14 -> v15:
  * Add meta-page and reader-page flush. Intends to fix the mapping
    for VIVT and aliasing-VIPT data caches.
  * -EPERM on VM_EXEC.
  * Fix build warning !CONFIG_TRACER_MAX_TRACE.

v13 -> v14:
  * All cpu_buffer->mapped readers use READ_ONCE (except for swap_cpu)
  * on unmap, sync meta-page teardown with the reader_lock instead of
    the synchronize_rcu.
  * Add a dedicated spinlock for trace_array ->snapshot and ->mapped.
    (intends to fix a lockdep issue)
  * Add kerneldoc for flags and Reserved fields.
  * Add kselftest for snapshot/map mutual exclusion.

v12 -> v13:
  * Swap subbufs_{touched,lost} for Reserved fields.
  * Add a flag field in the meta-page.
  * Fix CONFIG_TRACER_MAX_TRACE.
  * Rebase on top of trace/urgent.
  * Add a comment for try_unregister_trigger()

v11 -> v12:
  * Fix code sample mmap bug.
  * Add logging in sample code.
  * Reset tracer in selftest.
  * Add a refcount for the snapshot users.
  * Prevent mapping when there are snapshot users and vice versa.
  * Refine the meta-page.
  * Fix types in the meta-page.
  * Collect Reviewed-by.

v10 -> v11:
  * Add Documentation and code sample.
  * Add a selftest.
  * Move all the update to the meta-page into a single
    rb_update_meta_page().
  * rb_update_meta_page() is now called from
    ring_buffer_map_get_reader() to fix NOBLOCK callers.
  * kerneldoc for struct trace_meta_page.
  * Add a patch to zero all the ring-buffer allocations.

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/[e-mail]/
  * 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 (5):
  ring-buffer: allocate sub-buffers with __GFP_COMP
  ring-buffer: Introducing ring-buffer mapping functions
  tracing: Allow user-space mapping of the ring-buffer
  Documentation: tracing: Add ring-buffer mapping
  ring-buffer/selftest: Add ring-buffer mapping test

 Documentation/trace/index.rst                 |   1 +
 Documentation/trace/ring-buffer-map.rst       | 106 +++++
 include/linux/ring_buffer.h                   |   6 +
 include/uapi/linux/trace_mmap.h               |  48 +++
 kernel/trace/ring_buffer.c                    | 402 +++++++++++++++++-
 kernel/trace/trace.c                          | 122 +++++-
 kernel/trace/trace.h                          |   1 +
 tools/testing/selftests/ring-buffer/Makefile  |   8 +
 tools/testing/selftests/ring-buffer/config    |   2 +
 .../testing/selftests/ring-buffer/map_test.c  | 302 +++++++++++++
 10 files changed, 987 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/trace/ring-buffer-map.rst
 create mode 100644 include/uapi/linux/trace_mmap.h
 create mode 100644 tools/testing/selftests/ring-buffer/Makefile
 create mode 100644 tools/testing/selftests/ring-buffer/config
 create mode 100644 tools/testing/selftests/ring-buffer/map_test.c


base-commit: 7604256cecef34a82333d9f78262d3180f4eb525