[PATCH 03/17] block, bfq: only preempt plugged in_service_queue if bfq_better_to_idle say no

[Kemeng Shi]

Why we preempt in_service_queue when bfq_better_to_idle say no is (from
comment above the preemption expiration):
As for throughput, we ask bfq_better_to_idle() whether we still need to
plug I/O dispatching. If bfq_better_to_idle() says no, then plugging is
not needed any longer, either to boost throughput or to perserve service
guarantees. Then the best option is to stop plugging I/O, as not doing so
would certainly lower throughput.
This preemption only makes sense that IO of in_service_queue is currently
plugged and then there is a need to stop plugging.
Make sure bfqq is plugged before bfq_better_to_idle check to improve this.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 block/bfq-iosched.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 4868538c9745..7b416f9471b3 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -1978,8 +1978,9 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
 	if (bfqd->in_service_queue &&
 	    ((bfqq_wants_to_preempt &&
 	      bfqq->wr_coeff >= bfqd->in_service_queue->wr_coeff) ||
-	     bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue) ||
-	     !bfq_better_to_idle(bfqd->in_service_queue)) &&
+	      bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue) ||
+	      (bfq_bfqq_wait_request(bfqq) &&
+	      !bfq_better_to_idle(bfqd->in_service_queue))) &&
 	    next_queue_may_preempt(bfqd))
 		bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
 				false, BFQQE_PREEMPTED);
-- 
2.30.0