linux-kernel - [PATCH v2] docs: ftrace: Specifies when buffers get clear

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Message-ID: <20191218191553.q4lwyxmquvtjzjfz@frank-laptop>
Date:   Wed, 18 Dec 2019 14:15:53 -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, nachukannan@...il.com,
        saiprakash.ranjan@...look.com
Subject: [PATCH v2] docs: ftrace: Specifies when buffers get clear

Clarify a few places where the ring buffer and the "snapshot" buffer
are cleared as a side effect of an operation.

This will avoid users lost of tracing data because of these so far
undocumented behavior.

Signed-off-by: Frank A. Cancio Bello <frank@...eralsoftwareinc.com>
---
Changes in v2:
  - Per Steven comment correct the fact that the "snapshot" buffer is
    not touched when writing in the "trace" file.
  - Use tab instead of spaces for alignment.

 Documentation/trace/ftrace.rst | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index d2b5657ed33e..46df39300d22 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -95,7 +95,8 @@ of ftrace. Here is a list of some of the key files:
   current_tracer:
 
 	This is used to set or display the current tracer
-	that is configured.
+	that is configured. Changing the current tracer clears
+	the ring buffer content as well as the "snapshot" buffer.
 
   available_tracers:
 
@@ -126,7 +127,8 @@ of ftrace. Here is a list of some of the key files:
 	This file holds the output of the trace in a human
 	readable format (described below). Note, tracing is temporarily
 	disabled when the file is open for reading. Once all readers
-	are closed, tracing is re-enabled.
+	are closed, tracing is re-enabled. Opening this file for
+	writing with the O_TRUNC flag clears the ring buffer content.
 
   trace_pipe:
 
@@ -490,6 +492,9 @@ of ftrace. Here is a list of some of the key files:
 
 	  # echo global > trace_clock
 
+	Setting a clock clears the ring buffer content as well as the
+	"snapshot" buffer.
+
   trace_marker:
 
 	This is a very useful file for synchronizing user space
-- 
2.17.1