[PATCH BUGFIX IMPROVEMENT 02/14] block, bfq: avoid selecting a queue w/o budget

[Paolo Valente]

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@xxxxxxxxxx>
---
 block/bfq-iosched.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index c7a4a15c7c19..9ea2c4f42501 100644
--- a/block/bfq-iosched.c
+++ b/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
-- 
2.20.1