This is a note to let you know that I've just added the patch titled
ring-buffer: Fix first commit on sub-buffer having non-zero delta
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ring-buffer-fix-first-commit-on-sub-buffer-having-non-zero-delta.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From d651aa1d68a2f0a7ee65697b04c6a92f8c0a12f2 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
Date: Tue, 11 Feb 2014 13:38:54 -0500
Subject: ring-buffer: Fix first commit on sub-buffer having non-zero delta
From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
commit d651aa1d68a2f0a7ee65697b04c6a92f8c0a12f2 upstream.
Each sub-buffer (buffer page) has a full 64 bit timestamp. The events on
that page use a 27 bit delta against that timestamp in order to save on
bits written to the ring buffer. If the time between events is larger than
what the 27 bits can hold, a "time extend" event is added to hold the
entire 64 bit timestamp again and the events after that hold a delta from
that timestamp.
As a "time extend" is always paired with an event, it is logical to just
allocate the event with the time extend, to make things a bit more efficient.
Unfortunately, when the pairing code was written, it removed the "delta = 0"
from the first commit on a page, causing the events on the page to be
slightly skewed.
Fixes: 69d1b839f7ee "ring-buffer: Bind time extend and data events together"
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/ring_buffer.c | 7 +++++++
1 file changed, 7 insertions(+)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2008,6 +2008,13 @@ __rb_reserve_next(struct ring_buffer_per
write &= RB_WRITE_MASK;
tail = write - length;
+ /*
+ * If this is the first commit on the page, then it has the same
+ * timestamp as the page itself.
+ */
+ if (!tail)
+ delta = 0;
+
/* See if we shot pass the end of this buffer page */
if (unlikely(write > BUF_PAGE_SIZE))
return rb_move_tail(cpu_buffer, length, tail,
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-3.4/ring-buffer-fix-first-commit-on-sub-buffer-having-non-zero-delta.patch
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