blk-iolatency: only call ktime_get() if needed

to #29139300

commit 6e2fa4dd683a22a7697e7ff51dad499406094d28 upstream

ktime_to_ns(ktime_get()), which is expensive, does not need to be called
if blk_iolatency_enabled() return false in blkcg_iolatency_done_bio().
Postponing ktime_to_ns(ktime_get()) execution reduces the CPU usage when
blk_iolatency is disabled.
Signed-off-by: NHongnan Li <hongnan.li@linux.alibaba.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Acked-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

block/blk-iolatency.c

@@ -559,7 +559,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
 	struct rq_wait *rqw;
 	struct iolatency_grp *iolat;
 	u64 window_start;
-	u64 now = ktime_to_ns(ktime_get());
+	u64 now;
 	bool issue_as_root = bio_issue_as_root_blkg(bio);
 	bool enabled = false;
 	int inflight = 0;
@@ -576,6 +576,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
 	if (!enabled)
 		return;
 
+	now = ktime_to_ns(ktime_get());
 	while (blkg && blkg->parent) {
 		iolat = blkg_to_lat(blkg);
 		if (!iolat) {