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authorSteven Rostedt (Red Hat) <rostedt@goodmis.org>2014-02-11 13:38:54 -0500
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-02-22 13:34:59 -0800
commit050d4e1049b17f853ddb1fe1913b4f453faa0100 (patch)
treea3537b834ba42637588493f9fbbc527d09b58ba8
parent02c7c7de9644d17016f292e77110ea8f8b3f405a (diff)
ring-buffer: Fix first commit on sub-buffer having non-zero delta
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@goodmis.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--kernel/trace/ring_buffer.c7
1 files changed, 7 insertions, 0 deletions
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index cc2f66f68dc5..0e337eedb909 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2397,6 +2397,13 @@ __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,