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

[Frank A. Cancio Bello]

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