block: inherit request start time from bio for BLK_CGROUP

Doing high IOPS testing with blk-cgroups enabled spends ~15-20% of the
time just doing ktime_get_ns() -> readtsc. We essentially read and
set the start time twice, one for the bio and then again when that bio
is mapped to a request.

Given that the time between the two is very short, inherit the bio
start time instead of reading it again. This cuts 1/3rd of the overhead
of the time keeping.
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -725,7 +725,14 @@ void blk_mq_start_request(struct request *rq)
 	trace_block_rq_issue(rq);
 
 	if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
-		rq->io_start_time_ns = ktime_get_ns();
+		u64 start_time;
+#ifdef CONFIG_BLK_CGROUP
+		if (rq->bio)
+			start_time = bio_issue_time(&rq->bio->bi_issue);
+		else
+#endif
+			start_time = ktime_get_ns();
+		rq->io_start_time_ns = start_time;
 		rq->stats_sectors = blk_rq_sectors(rq);
 		rq->rq_flags |= RQF_STATS;
 		rq_qos_issue(q, rq);