block: readd plug trace event

This was removed with the queue plug state. But we can easily readd
by checking if this is the first request going to this queue. It's
good information to have when tracing to see how effective the
plugging is.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 76850fc9cf23..52e756c526be 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1311,7 +1311,15 @@ get_rq:
 
         plug = current->plug;
         if (plug) {
-                if (!plug->should_sort && !list_empty(&plug->list)) {
+                /*
+                 * If this is the first request added after a plug, fire
+                 * of a plug trace. If others have been added before, check
+                 * if we have multiple devices in this plug. If so, make a
+                 * note to sort the list before dispatch.
+                 */
+                if (list_empty(&plug->list))
+                        trace_block_plug(q);
+                else if (!plug->should_sort) {
                         struct request *__rq;
 
                         __rq = list_entry_rq(plug->list.prev);