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

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Message-ID: <20230329070353.1e1b443b@gandalf.local.home>
Date:   Wed, 29 Mar 2023 07:03:53 -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, 29 Mar 2023 10:19:44 +0100
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> > 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.

Yeah, I was trying to see how a non consuming read would work, and was
having issues figuring that out without the tail page being updated.

> 
> 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.

I don't really want to get rid of it, but perhaps break it up where we
don't have it in the first release, but add it in a second one. That will
also make sure that we can expand the API if necessary (one reason I wanted
the "data_start" in the first place).

Let's drop it for now, but be able to add it later, an have the current
structure be:

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 */
};

BTW, shouldn't the nr_data_pages take into account the reader page? As it
is part of the array we traverse isn't it?

-- Steve