linux-kernel - [PATCH 3.5 52/60] ring-buffer: Fix first commit on sub-buffer having non-zero delta

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Message-Id: <1392987687-15367-53-git-send-email-luis.henriques@canonical.com>
Date:	Fri, 21 Feb 2014 13:01:19 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.5 52/60] ring-buffer: Fix first commit on sub-buffer having non-zero delta

3.5.7.31 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>

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@...dmis.org>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 kernel/trace/ring_buffer.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 35bf8f7..177fa4a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2236,6 +2236,13 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 	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,
-- 
1.9.0

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