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: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 6fdc629..648f25a 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2040,6 +2040,13 @@ __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,