block: move issue side time stamping to blk_account_io_start()

It's known needed at that point, and it's cleaner to just assign it
there rather than rely on it being reliably set before hitting the
IO accounting. Hence, move it out of blk_mq_rq_time_init(), which is
now only doing the allocation side timing.

While at it, get rid of the '0' time passing to blk_mq_rq_time_init(),
just pass in blk_time_get_ns() for the two cases where 0 is being
explicitly passed in. The rest pass in the previously cached allocation
time.

Signed-off-by: Jens Axboe <[e-mail]>

block/blk-mq.c

···
 /* Set start and alloc time when the allocated request is actually used */
 static inline void blk_mq_rq_time_init(struct request *rq, u64 alloc_time_ns)
 {
-	if (blk_queue_io_stat(rq->q))
-		rq->start_time_ns = blk_time_get_ns();
-	else
-		rq->start_time_ns = 0;
-
 #ifdef CONFIG_BLK_RQ_ALLOC_TIME
 	if (blk_queue_rq_alloc_time(rq->q))
-		rq->alloc_time_ns = alloc_time_ns ?: rq->start_time_ns;
+		rq->alloc_time_ns = alloc_time_ns;
 	else
 		rq->alloc_time_ns = 0;
 #endif
···
 			return NULL;
 
 		plug->cached_rq = rq_list_next(rq);
-		blk_mq_rq_time_init(rq, 0);
+		blk_mq_rq_time_init(rq, blk_time_get_ns());
 	}
 
 	rq->cmd_flags = opf;
···
 		return;
 
 	req->rq_flags |= RQF_IO_STAT;
+	req->start_time_ns = blk_time_get_ns();
 
 	/*
 	 * All non-passthrough requests are created from a bio with one
···
 	plug->cached_rq = rq_list_next(rq);
 	rq_qos_throttle(rq->q, bio);
 
-	blk_mq_rq_time_init(rq, 0);
+	blk_mq_rq_time_init(rq, blk_time_get_ns());
 	rq->cmd_flags = bio->bi_opf;
 	INIT_LIST_HEAD(&rq->queuelist);
 }