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

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

block/blk-mq.c

@@ -281,6 +281,15 @@ bool blk_mq_can_queue(struct blk_mq_hw_ctx *hctx)
 }
 EXPORT_SYMBOL(blk_mq_can_queue);
 
+/*
+ * Only need start/end time stamping if we have stats enabled, or using
+ * an IO scheduler.
+ */
+static inline bool blk_mq_need_time_stamp(struct request *rq)
+{
+	return (rq->rq_flags & RQF_IO_STAT) || rq->q->elevator;
+}
+
 static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 		unsigned int tag, unsigned int op)
 {
@@ -316,7 +325,10 @@ 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 (blk_mq_need_time_stamp(rq))
+		rq->start_time_ns = ktime_get_ns();
+	else
+		rq->start_time_ns = 0;
 	rq->io_start_time_ns = 0;
 	rq->nr_phys_segments = 0;
 #if defined(CONFIG_BLK_DEV_INTEGRITY)
@@ -522,7 +534,10 @@ 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;
+
+	if (blk_mq_need_time_stamp(rq))
+		now = ktime_get_ns();
 
 	if (rq->rq_flags & RQF_STATS) {
 		blk_mq_poll_stats_start(rq->q);