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. > > 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
