[PATCH v8 02/10] block: record I/O request start time for passthru request

While starting an I/O request, blk_mq_start_request() records the
request start timestamp only for non-passthrough requests when
QUEUE_FLAG_STATS is enabled.

However, the latency based multipath policy uses request completion
latency to evaluate path performance, and I/O is issued as passthrough
requests. Since passthru requests never initialize rq->io_start_time_ns,
their latency cannot be computed.

Record io_start_time_ns for all requests whenever QUEUE_FLAG_STATS is
enabled.

Reviewed-by: Guixin Liu <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Nilay Shroff <[email protected]>
---
 block/blk-mq.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2c850330a32b..38922209a24f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1340,12 +1340,14 @@ void blk_mq_start_request(struct request *rq)
 
 	trace_block_rq_issue(rq);
 
-	if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags) &&
-	    !blk_rq_is_passthrough(rq)) {
+	if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
 		rq->io_start_time_ns = blk_time_get_ns();
-		rq->stats_sectors = blk_rq_sectors(rq);
-		rq->rq_flags |= RQF_STATS;
-		rq_qos_issue(q, rq);
+
+		if (!blk_rq_is_passthrough(rq)) {
+			rq->stats_sectors = blk_rq_sectors(rq);
+			rq->rq_flags |= RQF_STATS;
+			rq_qos_issue(q, rq);
+		}
 	}
 
 	WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_IDLE);
-- 
2.53.0