tracing: Update subbuffer with kilobytes not page order

Using page order for deciding what the size of the ring buffer sub buffers
are is exposing a bit too much of the implementation. Although the sub
buffers are only allocated in orders of pages, allow the user to specify
the minimum size of each sub-buffer via kilobytes like they can with the
buffer size itself.

If the user specifies 3 via:

  echo 3 > buffer_subbuf_size_kb

Then the sub-buffer size will round up to 4kb (on a 4kb page size system).

If they specify:

  echo 6 > buffer_subbuf_size_kb

The sub-buffer size will become 8kb.

and so on.

Link: https://lore.kernel.org/linux-trace-kernel/20231219185631.809766769@goodmis.org

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Tzvetomir Stoyanov <tz.stoyanov@gmail.com>
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 20 insertions, 26 deletions

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 231d26ceedb0..933e7efb9f1b 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -203,32 +203,26 @@ of ftrace. Here is a list of some of the key files:
 
 	This displays the total combined size of all the trace buffers.
 
-  buffer_subbuf_order:
-
-	This sets or displays the sub buffer page size order. The ring buffer
-	is broken up into several same size "sub buffers". An event can not be
-	bigger than the size of the sub buffer. Normally, the sub buffer is
-	the size of the architecture's page (4K on x86). The sub buffer also
-	contains meta data at the start which also limits the size of an event.
-	That means when the sub buffer is a page size, no event can be larger
-	than the page size minus the sub buffer meta data.
-
-	The buffer_subbuf_order allows the user to change the size of the sub
-	buffer. As the sub buffer is a set of pages by the power of 2, thus
-	the sub buffer total size is defined by the order:
-
-	order		size
-	----		----
-	0		PAGE_SIZE
-	1		PAGE_SIZE * 2
-	2		PAGE_SIZE * 4
-	3		PAGE_SIZE * 8
-
-	Changing the order will change the sub buffer size allowing for events
-	to be larger than the page size.
-
-	Note: When changing the order, tracing is stopped and any data in the
-	ring buffer and the snapshot buffer will be discarded.
+  buffer_subbuf_size_kb:
+
+	This sets or displays the sub buffer size. The ring buffer is broken up
+	into several same size "sub buffers". An event can not be bigger than
+	the size of the sub buffer. Normally, the sub buffer is the size of the
+	architecture's page (4K on x86). The sub buffer also contains meta data
+	at the start which also limits the size of an event.  That means when
+	the sub buffer is a page size, no event can be larger than the page
+	size minus the sub buffer meta data.
+
+	Note, the buffer_subbuf_size_kb is a way for the user to specify the
+	minimum size of the subbuffer. The kernel may make it bigger due to the
+	implementation details, or simply fail the operation if the kernel can
+	not handle the request.
+
+	Changing the sub buffer size allows for events to be larger than the
+	page size.
+
+	Note: When changing the sub-buffer size, tracing is stopped and any
+	data in the ring buffer and the snapshot buffer will be discarded.
 
   free_buffer: