ring-buffer/Documentation: Add documentation on buffer_percent file

[linux-2.6-block.git] / Documentation / trace / ftrace.rst

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 9ef362dae9df1d9b1d14843eaf3b307161064d7c..16122a8895ba30b3b31072e61586a470f1d06ee7 100644 (file)
--- 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 watermark 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