[PATCH] blk-mq: don't call ktime_get_ns() if we don't need it

[Jens Axboe <axboe@kernel.dk>]

We only need the request fields and the end_io time if we have stats
enabled, or if we have a scheduler attached as those may use it for
completion time stats.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-mq.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7dcef565dc0f..2b458e9da946 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -316,7 +316,8 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	RB_CLEAR_NODE(&rq->rb_node);
 	rq->rq_disk = NULL;
 	rq->part = NULL;
-	rq->start_time_ns = ktime_get_ns();
+	if (data->q->elevator)
+		rq->start_time_ns = ktime_get_ns();
 	rq->io_start_time_ns = 0;
 	rq->nr_phys_segments = 0;
 #if defined(CONFIG_BLK_DEV_INTEGRITY)
@@ -522,12 +523,18 @@ EXPORT_SYMBOL_GPL(blk_mq_free_request);
 
 inline void __blk_mq_end_request(struct request *rq, blk_status_t error)
 {
-	u64 now = ktime_get_ns();
+	u64 now = 0;
 
+	/*
+	 * Only read ktime if we need it, which is if stats are enabled
+	 * or we have a scheduler attached that may use it.
+	 */
 	if (rq->rq_flags & RQF_STATS) {
 		blk_mq_poll_stats_start(rq->q);
+		now = ktime_get_ns();
 		blk_stat_add(rq, now);
-	}
+	} else if (rq->q->elevator)
+		now = ktime_get_ns();
 
 	if (rq->internal_tag != -1)
 		blk_mq_sched_completed_request(rq, now);
-- 
2.17.1

-- 
Jens Axboe