blk-throttle: Avoid tracking latency if low limit is invalid

The IO latency tracking is only for LOW limit, so we should add a
validation to avoid redundant latency tracking if the LOW limit
is not valid.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -2100,7 +2100,7 @@ static void throtl_update_latency_buckets(struct throtl_data *td)
 	unsigned long last_latency[2] = { 0 };
 	unsigned long latency[2];
 
-	if (!blk_queue_nonrot(td->queue))
+	if (!blk_queue_nonrot(td->queue) || !td->limit_valid[LIMIT_LOW])
 		return;
 	if (time_before(jiffies, td->last_calculate_time + HZ))
 		return;
@@ -2338,6 +2338,8 @@ void blk_throtl_bio_endio(struct bio *bio)
 	if (!blkg)
 		return;
 	tg = blkg_to_tg(blkg);
+	if (!tg->td->limit_valid[LIMIT_LOW])
+		return;
 
 	finish_time_ns = ktime_get_ns();
 	tg->last_finish_time = finish_time_ns >> 10;