On Wed, 13 Nov 2019 11:32:36 -0500 "Frank A. Cancio Bello" <frank@xxxxxxxxxxxxxxxxxxxxxx> wrote: > The current text could mislead the user into believing that the number > of pages allocated by each CPU ring buffer is calculated by the round > up of the division: buffer_size_kb / PAGE_SIZE. > > Clarify that the number of pages allocated is the round up of the > division: buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE). Add an > example that shows how the number of pages allocated could be off by > 5 pages more compared with how the current text suggests it should be. > > Suggested-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Signed-off-by: Frank A. Cancio Bello <frank@xxxxxxxxxxxxxxxxxxxxxx> > --- > Documentation/trace/ftrace.rst | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst > index e3060eedb22d..ec2c4eff95a6 100644 > --- a/Documentation/trace/ftrace.rst > +++ b/Documentation/trace/ftrace.rst > @@ -188,8 +188,17 @@ of ftrace. Here is a list of some of the key files: > 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. > - ( Note, the size may not be a multiple of the page size > - due to buffer management meta-data. ) The above is not untrue ;-) > + > + 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. -- Steve > > Buffer sizes for individual CPUs may vary > (see "per_cpu/cpu0/buffer_size_kb" below), and if they do
