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 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). Buffer sizes for individual CPUs may vary (see "per_cpu/cpu0/buffer_size_kb" below), and if they do -- 2.17.1
