| From d651aa1d68a2f0a7ee65697b04c6a92f8c0a12f2 Mon Sep 17 00:00:00 2001 |
| From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org> |
| 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@goodmis.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@goodmis.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| kernel/trace/ring_buffer.c | 7 +++++++ |
| 1 file changed, 7 insertions(+) |
| |
| --- a/kernel/trace/ring_buffer.c |
| +++ b/kernel/trace/ring_buffer.c |
| @@ -2396,6 +2396,13 @@ __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, |
