linux-kernel - Re: [PATCH v19 RESEND 4/5] Documentation: tracing: Add ring-buffer mapping

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Date: Wed, 27 Mar 2024 17:32:49 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Vincent Donnefort <vdonnefort@...gle.com>, rostedt@...dmis.org,
 mhiramat@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Cc: mathieu.desnoyers@...icios.com, kernel-team@...roid.com
Subject: Re: [PATCH v19 RESEND 4/5] Documentation: tracing: Add ring-buffer
 mapping

Hi,

On 3/26/24 03:08, Vincent Donnefort wrote:
> It is now possible to mmap() a ring-buffer to stream its content. Add
> some documentation and a code example.
> 
> Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> 

> diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/ring-buffer-map.rst
> new file mode 100644
> index 000000000000..0426ab4bcf3d
> --- /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@...gle.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 as the meta-page. One of the most important field of

                      referred to as                                       fields

> +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 ascendant ID. It is

                                                                ascending
might be better IMO.

> +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.


[snip]

Thanks.
-- 
#Randy