[PATCH 6.6 105/283] Documentation: tracing: Add ring-buffer mapping

Sasha Levin
2026-02-28T18:08:36Z

From: Vincent Donnefort <vdonnefort@google.com>

[ Upstream commit a1e0dd7ce38af3fb1a3bc54a222a7c5e4eaa4202 ]

It is now possible to mmap() a ring-buffer to stream its content. Add
some documentation and a code example.

Link: https://lore.kernel.org/linux-trace-kernel/20240510140435.3550353-5-vdonnefort@google.com

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Stable-dep-of: 8236fc613d44 ("Documentation: tracing: Add PCI tracepoint documentation")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Documentation/trace/index.rst | 1 +
Documentation/trace/ring-buffer-map.rst | 106 ++++++++++++++++++++++++
2 files changed, 107 insertions(+)
create mode 100644 Documentation/trace/ring-buffer-map.rst

diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index 5092d6c13af5e..0b300901fd750 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -29,6 +29,7 @@ Linux Tracing Technologies
timerlat-tracer
intel_th
ring-buffer-design
+ ring-buffer-map
stm
sys-t
coresight/index
diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/ring-buffer-map.rst
new file mode 100644
index 0000000000000..8e296bcc0d7f3
--- /dev/null
+++ b/Documentation/trace/ring-buffer-map.rst
@@ -0,0 +1,106 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Tracefs ring-buffer memory mapping
+==================================
+
+:Author: Vincent Donnefort <vdonnefort@google.com>
+
+Overview
+========
+Tracefs ring-buffer memory map provides an efficient method to stream data
+as no memory copy is necessary. The application mapping the ring-buffer becomes
+then a consumer for that ring-buffer, in a similar fashion to trace_pipe.
+
+Memory mapping setup
+====================
+The mapping works with a mmap() of the trace_pipe_raw interface.
+
+The first system page of the mapping contains ring-buffer statistics and
+description. It is referred to as the meta-page. One of the most important
+fields of the meta-page is the reader. It contains the sub-buffer ID which can
+be safely read by the mapper (see ring-buffer-design.rst).
+
+The meta-page is followed by all the sub-buffers, ordered by ascending ID. It is
+therefore effortless to know where the reader starts in the mapping:
+
+.. code-block:: c
+
+ reader_id = meta->reader->id;
+ reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
+
+When the application is done with the current reader, it can get a new one using
+the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also updates
+the meta-page fields.
+
+Limitations
+===========
+When a mapping is in place on a Tracefs ring-buffer, it is not possible to
+either resize it (either by increasing the entire size of the ring-buffer or
+each subbuf). It is also not possible to use snapshot and causes splice to copy
+the ring buffer data instead of using the copyless swap from the ring buffer.
+
+Concurrent readers (either another application mapping that ring-buffer or the
+kernel with trace_pipe) are allowed but not recommended. They will compete for
+the ring-buffer and the output is unpredictable, just like concurrent readers on
+trace_pipe would be.
+
+Example
+=======
+
+.. code-block:: c
+
+ #include <fcntl.h>
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <unistd.h>
+
+ #include <linux/trace_mmap.h>
+
+ #include <sys/mman.h>
+ #include <sys/ioctl.h>
+
+ #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw"
+
+ int main(void)
+ {
+ int page_size = getpagesize(), fd, reader_id;
+ unsigned long meta_len, data_len;
+ struct trace_buffer_meta *meta;
+ void *map, *reader, *data;
+
+ fd = open(TRACE_PIPE_RAW, O_RDONLY | O_NONBLOCK);
+ if (fd < 0)
+ exit(EXIT_FAILURE);
+
+ map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
+ if (map == MAP_FAILED)
+ exit(EXIT_FAILURE);
+
+ meta = (struct trace_buffer_meta *)map;
+ meta_len = meta->meta_page_size;
+
+ printf("entries: %llu\n", meta->entries);
+ printf("overrun: %llu\n", meta->overrun);
+ printf("read: %llu\n", meta->read);
+ printf("nr_subbufs: %u\n", meta->nr_subbufs);
+
+ data_len = meta->subbuf_size * meta->nr_subbufs;
+ data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len);
+ if (data == MAP_FAILED)
+ exit(EXIT_FAILURE);
+
+ if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0)
+ exit(EXIT_FAILURE);
+
+ reader_id = meta->reader.id;
+ reader = data + meta->subbuf_size * reader_id;
+
+ printf("Current reader address: %p\n", reader);
+
+ munmap(data, data_len);
+ munmap(meta, meta_len);
+ close (fd);
+
+ return 0;
+ }
--
2.51.0