Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb

[Steven Rostedt]

On Thu, 14 Nov 2019 23:24:28 -0500
"Frank A. Cancio Bello" <frank@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxx> 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.

-- Steve