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
