block, bfq: avoid selecting a queue w/o budget

To boost throughput on devices with internal queueing and in scenarios
where device idling is not strictly needed, bfq immediately starts
serving a new bfq_queue if the in-service bfq_queue remains without
pending I/O, even if new I/O may arrive soon for the latter queue. Then,
if such I/O actually arrives soon, bfq preempts the new in-service
bfq_queue so as to give the previous queue a chance to go on being
served (in case the previous queue should actually be the one to be
served, according to its timestamps).

However, the in-service bfq_queue, say Q, may also be without further
budget when it remains also pending I/O. Since bfq changes budgets
dynamically to fit the needs of bfq_queues, this happens more often than
one may expect. If this happens, then there is no point in trying to go
on serving Q when new I/O arrives for it soon: Q would be expired
immediately after being selected for service. This would only cause
useless overhead. This commit avoids such a useless selection.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -1380,7 +1380,15 @@ static bool bfq_bfqq_update_budg_for_activation(struct bfq_data *bfqd,
 {
 	struct bfq_entity *entity = &bfqq->entity;
 
-	if (bfq_bfqq_non_blocking_wait_rq(bfqq) && arrived_in_time) {
+	/*
+	 * In the next compound condition, we check also whether there
+	 * is some budget left, because otherwise there is no point in
+	 * trying to go on serving bfqq with this same budget: bfqq
+	 * would be expired immediately after being selected for
+	 * service. This would only cause useless overhead.
+	 */
+	if (bfq_bfqq_non_blocking_wait_rq(bfqq) && arrived_in_time &&
+	    bfq_bfqq_budget_left(bfqq) > 0) {
 		/*
 		 * We do not clear the flag non_blocking_wait_rq here, as
 		 * the latter is used in bfq_activate_bfqq to signal