linux-kernel - [PATCH v3 1/3] docs: ftrace: Clarify the RAM impact of buffer_size

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Message-ID: <6f33be5f3d60e5ffc061d8d2b329d3d3ccf22a8c.1577231751.git.frank@generalsoftwareinc.com>
Date:   Tue, 24 Dec 2019 19:06:05 -0500
From:   "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     joel@...lfernandes.org, saiprakash.ranjan@...eaurora.org,
        nachukannan@...il.com, rdunlap@...radead.org
Subject: [PATCH v3 1/3] 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.

Clarifies that a few extra pages may be allocated to accommodate buffer
management meta-data.

Suggested-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Suggested-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@...eralsoftwareinc.com>
---
 Documentation/trace/ftrace.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index d2b5657ed33e..5a037bedbf6a 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -185,7 +185,8 @@ of ftrace. Here is a list of some of the key files:
 	CPU buffer and not total size of all buffers. 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
+	A few extra pages may be allocated to accommodate buffer management
+	meta-data. 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
-- 
2.17.1