[PATCH v2] ring-buffer/Documentation: Add documentation on buffer_percent file

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

When the buffer_percent file was added to the kernel, the documentation
should have been updated to document what that file does.

Fixes: 03329f9939781 ("tracing: Add tracefs file buffer_percentage")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
Changes since v1: https://lore.kernel.org/all/20231226123525.71a6d0fb@xxxxxxxxxxxxxxxxxx/

- Fixed some grammar issues.

 Documentation/trace/ftrace.rst | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 933e7efb9f1b..84b810a6cf76 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -180,6 +180,21 @@ of ftrace. Here is a list of some of the key files:
 	Only active when the file contains a number greater than 0.
 	(in microseconds)
 
+  buffer_percent:
+
+	This is the water-mark for how much the ring buffer needs to be filled
+	before a waiter is woken up. That is, if an application calls a
+	blocking read syscall on one of the per_cpu trace_pipe_raw files, it
+	will block until the given amount of data specified by buffer_percent
+	is in the ring buffer before it wakes the reader up. This also
+	controls how the splice system calls are blocked on this file.
+
+	0   - means to wake up as soon as there is any data in the ring buffer.
+	50  - means to wake up when roughly half of the ring buffer sub-buffers
+	      are full.
+	100 - means to block until the ring buffer is totally full and is
+	      about to start overwriting the older data.
+
   buffer_size_kb:
 
 	This sets or displays the number of kilobytes each CPU
-- 
2.42.0