From f0bf1b240e83a54c7e0df7c0da697b05945edbcd Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (Red Hat)" Date: Tue, 11 Feb 2014 13:38:54 -0500 Subject: [PATCH] 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 Signed-off-by: Greg Kroah-Hartman --- 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 e444ff88f0a4..fd12cc56371f 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -2396,6 +2396,13 @@ __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, -- 2.34.1