linux-kernel - Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping functions

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Message-ID: <20230328224411.0d69e272@gandalf.local.home>
Date:   Tue, 28 Mar 2023 22:44:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vincent Donnefort <vdonnefort@...gle.com>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping
 functions

On Wed, 22 Mar 2023 10:22:43 +0000
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> +#include <linux/types.h>
> +
> +struct ring_buffer_meta_page_header {
> +#if __BITS_PER_LONG == 64
> +	__u64	entries;
> +	__u64	overrun;
> +#else
> +	__u32	entries;
> +	__u32	overrun;
> +#endif
> +	__u32	pages_touched;
> +	__u32	meta_page_size;
> +	__u32	reader_page;	/* page ID for the reader page */
> +	__u32	nr_data_pages;	/* doesn't take into account the reader_page */
> +	__u32	data_page_head;	/* ring-buffer head as an offset from data_start */
> +	__u32	data_start;	/* offset within the meta page */
> +};
> +

I've been playing with this a bit, and I'm thinking, do we need the
data_pages[] array on the meta page?

I noticed that I'm not even using it.

Currently, we need to do a ioctl every time we finish with the reader page,
and that updates the reader_page in the meta data to point to the next page
to read. When do we need to look at the data_start section?

-- Steve