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Subject[RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb
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@joelfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
---
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
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