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>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index c7a4a15c7c19e56d1e9662a863b06a33b1123f4f..9ea2c4f42501f0267bb27779e8d10410d44821d3 100644 (file)
--- 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