[PATCH linux-2.6-block.git] Fix blktrace trace ordering for plug branch

["Alan D. Brunelle" <Alan.Brunelle@pobox.com>]

The attached patch will correct the ordering of trace output between 
request queue insertions (I) and unplug calls (U). Right now the insert 
precedes the unplug, which just isn't right:

65,128  0        1    67.699868965  7882  Q   R 0 + 1 [aiod]
65,128  0        2    67.699876462  7882  G   R 0 + 1 [aiod]
65,128  0        3    67.699878286  7882  P   W [aiod]
65,128  0        4    67.699880491  7882  I   R 0 + 1 [aiod]
65,128  0        5    67.699887589  7882  U   R [aiod] 1
65,128  0        6    67.699898317    54  D   R 0 + 1 [kblockd/0]
65,128  2      153    67.700126590     0  C   R 0 + 1 [0]

With the patch provided the unplug comes first:

65,128  3        1     0.000000000  7045  Q   R 0 + 1 [aiod]
65,128  3        2     0.000002295  7045  G   R 0 + 1 [aiod]
65,128  3        3     0.000002617  7045  P   W [aiod]
65,128  3        4     0.000003685  7045  U   R [aiod] 1
65,128  3        5     0.000004107  7045  I   R 0 + 1 [aiod]
65,128  3        6     0.000009491    57  D   R 0 + 1 [kblockd/3]
65,128  2        1     0.000232447     0  C   R 0 + 1 [0]

Jens: If you agree, the patch can be applied to your plug branch for 
git://git.kernel.dk/data/git/linux-2.6-block.git

Thanks,
Alan

From: Alan D. Brunelle <Alan.Brunelle@hp.com>

Fix unplug/insert trace inversion problem.

Signed-off-by: Alan D. Brunelle <Alan.Brunelle@hp.com>
---
 block/ll_rw_blk.c      |    8 ++++----
 include/linux/blkdev.h |    1 +
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 46d29f7..3bec97f 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -2981,6 +2981,7 @@ out_unlock:
 		if (bio_data_dir(bio) == WRITE && ioc->qrcu_idx == -1)
 			ioc->qrcu_idx = qrcu_read_lock(&q->qrcu);
 		list_add_tail(&req->queuelist, &ioc->plugged_list);
+		ioc->plugged_list_len++;
 	}
 
 out:
@@ -3720,7 +3721,6 @@ void blk_unplug_current(void)
 	struct io_context *ioc = current->io_context;
 	struct request *req;
 	request_queue_t *q;
-	int nr_unplug;
 
 	if (!ioc)
 		return;
@@ -3735,19 +3735,19 @@ void blk_unplug_current(void)
 	if (list_empty(&ioc->plugged_list))
 		goto out;
 
-	nr_unplug = 0;
+	blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL, ioc->plugged_list_len);
+
 	spin_lock_irq(q->queue_lock);
 	do {
 		req = list_entry_rq(ioc->plugged_list.next);
 		list_del_init(&req->queuelist);
 		add_request(q, req);
-		nr_unplug++;
 	} while (!list_empty(&ioc->plugged_list));
+	ioc->plugged_list_len = 0;
 
 	spin_unlock_irq(q->queue_lock);
 
 	queue_delayed_work(kblockd_workqueue, &q->delay_work, 0);
-	blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL, nr_unplug);
 
 out:
 	/*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index f8cdd44..848564c 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -113,6 +113,7 @@ struct io_context {
 	 */
 	int plugged;
 	int qrcu_idx;
+	int plugged_list_len;
 	struct list_head plugged_list;
 	struct request_queue *plugged_queue;