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

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Date:   Wed, 29 Mar 2023 10:19:44 +0100
From:   Vincent Donnefort <vdonnefort@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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 Tue, Mar 28, 2023 at 10:44:11PM -0400, Steven Rostedt wrote:
> 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?

This is for non-consuming read, to get all the pages in order.

If we remove this section we would lose this ability ... but we'd also simplify
the code by a good order of magnitude (don't need the update ioctl anymore, no
need to keep those pages in order and everything can fit a 0-order meta-page).
And the non-consuming read doesn't bring much to the user over the pipe version.

This will although impact our hypervisor tracing which will only be able to
expose trace_pipe interfaces. But I don't think it is a problem, all userspace
tools only relying on consuming read anyway.

So if you're happy dropping this support, let's get rid of it.

-- 
Vincent