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Date:   Wed, 29 Mar 2023 08:47:35 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Vincent Donnefort <vdonnefort@google.com>
Cc:     mhiramat@kernel.org, linux-kernel@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping
 functions
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On Wed, 29 Mar 2023 13:23:01 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> On Wed, Mar 29, 2023 at 07:03:53AM -0400, Steven Rostedt wrote:
> > On Wed, 29 Mar 2023 10:19:44 +0100
> > Vincent Donnefort <vdonnefort@google.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.  
> 
> Would the userspace really need to know where is the tail page? It can just stop
> whenever it finds out a page doesn't have any events, and make sure it does not
> loop once back to the head?

I'm trying to come up with a possible algorithm that doesn't need
ioctls. It would need to know if the writer moved or not. Probably need
a counter that gets incremented every time the writer goes to a new page.

Having the tail page was just a convenient way to know where the end is.

> 
> >   
> > > 
> > > 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:  
> 
> Ok, I will prepare a V3 accordingly.
> 
> > 
> > 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?  
> 
> It depends if the reader page has ever been swapped out. If yes, the reader
> would have to start from reader_page and then switch to the data_pages.
> Which sounds like a fiddly interface for the userspace.
> 
> So yeah, consuming-read only feels like a better start.
> 

I agree. I'd like to get something in that can be extended, but simple
enough that it's not too much of a barrier wrt getting the API correct.

-- Steve