linux-kernel - Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size

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Date:   Thu, 14 Nov 2019 16:36:39 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, saiprakash.ranjan@...eaurora.org
Subject: Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of
 buffer_size_kb

On Thu, 14 Nov 2019 15:20:59 -0500
Joel Fernandes <joel@...lfernandes.org> wrote:

> On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote:
> > On Wed, 13 Nov 2019 11:32:36 -0500
> > "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com> wrote:  
> [snip]
> > > +
> > > +        The number of pages allocated for each CPU buffer may not
> > > +        be the same than the round up of the division:
> > > +        buffer_size_kb / PAGE_SIZE. This is because part of each page is
> > > +        used to store a page header with metadata. E.g. with
> > > +        buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
> > > +        BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
> > > +        page header with metadata) the number of pages allocated for each
> > > +        CPU buffer is 1029, not 1024. The formula for calculating the
> > > +        number of pages allocated for each CPU buffer is the round up of:
> > > +        buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).  
> > 
> > I have no problem with this patch, but the concern of documenting the
> > implementation here, which will most likely not be updated if the
> > implementation is ever changed, which is why I was vague to begin with.
> > 
> > But it may never be changed as that code has been like that for a
> > decade now.  
> 
> Agreed. To give some context, Frank is an outreachy intern I am working with and
> one of his starter tasks was to understand the ring buffer's basics.  I asked
> him to send a patch since I thought he mentioned there was an error in the
> documnentation. It looks like all that was missing is some explanation which
> the deleted text in brackets above should already cover.
> 
> Steve, your call if you want this patch. Looks like Frank understands the
> page header taking up some space, so one of the goals of the exercise is
> accomplished ;-)

Yes agreed, what was written was not wrong (thus understood). But the
more I think about this, the less I like the implementation details in
the documentation directory. Now I am looking forward for some other
patches from Frank, and perhaps he could add some comments in
ring_buffer.c about this. ;-)

-- Steve