block: consolidate struct request timestamp fields

Currently, struct request has four timestamp fields:

- A start time, set at get_request time, in jiffies, used for iostats
- An I/O start time, set at start_request time, in ktime nanoseconds,
  used for blk-stats (i.e., wbt, kyber, hybrid polling)
- Another start time and another I/O start time, used for cfq and bfq

These can all be consolidated into one start time and one I/O start
time, both in ktime nanoseconds, shaving off up to 16 bytes from struct
request depending on the kernel config.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 5 insertions, 6 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 782940c65d8a7..5573d0fbec536 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -724,13 +724,12 @@ static struct request *attempt_merge(struct request_queue *q,
 	}
 
 	/*
-	 * At this point we have either done a back merge
-	 * or front merge. We need the smaller start_time of
-	 * the merged requests to be the current request
-	 * for accounting purposes.
+	 * At this point we have either done a back merge or front merge. We
+	 * need the smaller start_time_ns of the merged requests to be the
+	 * current request for accounting purposes.
 	 */
-	if (time_after(req->start_time, next->start_time))
-		req->start_time = next->start_time;
+	if (next->start_time_ns < req->start_time_ns)
+		req->start_time_ns = next->start_time_ns;
 
 	req->biotail->bi_next = next->bio;
 	req->biotail = next->biotail;