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

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Message-ID: <20191115155955.4khvnlnzjhnp5bxa@ubuntu1804-desktop>
Date:   Fri, 15 Nov 2019 10:59:55 -0500
From:   "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        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 Fri, Nov 15, 2019 at 08:30:00AM -0500, Steven Rostedt wrote:
> On Thu, 14 Nov 2019 23:24:28 -0500
> "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com> wrote:
> 
> > On Thu, Nov 14, 2019 at 04:36:39PM -0500, Steven Rostedt wrote:
> > > 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.
> > > >   
> > 
> > Not exactly in my opinion ;) The deleted text was not the problem. I
> > just deleted it because with the added text it turns to be redundant.
> > 
> > The issue that I found with the documentation (maybe just to my
> > newbie's eyes) is in this part:
> > 
> > "The trace buffers are allocated in pages (blocks of memory that the
> > kernel uses for allocation, usually 4 KB in size). If the last page
> > allocated has room for more bytes than requested, the rest of the
> > page will be used, making the actual allocation bigger than requested
> > or shown."
> > 
> > For me that "suggests" the interpretation that the number of pages
> > allocated in the current implementation correspond with the round
> > integer division of buffer_size_kb / PAGE_SIZE, which is inaccurate
> > (for 5 pages in the example that I mentioned).
> 
> If you would like, you could reword that to something more accurate,
> but still not detailing the implementation.
> 
> > Understood and agreed. It is funny that what I spotted as "a problem"
> > was precisely an incomplete description of the implementation (the
> > sentences that I quoted above). What do you think about removing
> > those two sentences?
> 
> I wouldn't remove them, just reword them to something you find more
> accurate.
> 

I feel that adding:

"A few extra pages may be allocated to accommodate buffer management
meta-data."

between the two sentences that I quoted will address the issue. If
that is OK with you I will proceed to package this change in a new
patchset along with a few fixes of typos that I spotted in other
parts of the doc.

thanks one more time for your quick response.
frank a.