bfq: Use only idle IO periods for think time calculations

Currently whenever bfq queue has a request queued we add now -
last_completion_time to the think time statistics. This is however
misleading in case the process is able to submit several requests in
parallel because e.g. if the queue has request completed at time T0 and
then queues new requests at times T1, T2, then we will add T1-T0 and
T2-T0 to think time statistics which just doesn't make any sence (the
queue's think time is penalized by the queue being able to submit more
IO). So add to think time statistics only time intervals when the queue
had no IO pending.

Signed-off-by: Jan Kara <[email protected]>
Acked-by: Paolo Valente <[email protected]>
[axboe: fix whitespace on empty line]
Signed-off-by: Jens Axboe <[email protected]>

block/bfq-iosched.c

@@ -5278,8 +5278,16 @@ static void bfq_update_io_thinktime(struct bfq_data *bfqd,
 				    struct bfq_queue *bfqq)
 {
 	struct bfq_ttime *ttime = &bfqq->ttime;
-	u64 elapsed = ktime_get_ns() - bfqq->ttime.last_end_request;
+	u64 elapsed;
 
+	/*
+	 * We are really interested in how long it takes for the queue to
+	 * become busy when there is no outstanding IO for this queue. So
+	 * ignore cases when the bfq queue has already IO queued.
+	 */
+	if (bfqq->dispatched || bfq_bfqq_busy(bfqq))
+		return;
+	elapsed = ktime_get_ns() - bfqq->ttime.last_end_request;
 	elapsed = min_t(u64, elapsed, 2ULL * bfqd->bfq_slice_idle);
 
 	ttime->ttime_samples = (7*ttime->ttime_samples + 256) / 8;