linux-kernel - [PATCH BUGFIX 4/4] block, bfq: fix service being wrongly set to zero in case of preemption

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Date:   Mon, 25 Jun 2018 21:48:06 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, broonie@...nel.org,
        linus.walleij@...aro.org, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX 4/4] block, bfq: fix service being wrongly set to zero in case of preemption

If
- a bfq_queue Q preempts another queue, because one request of Q
arrives in time,
- but, after this preemption, Q is not the queue that is set in service,
then Q->entity.service is set to 0 when Q is eventually set in
service. But Q should have continued receiving service with its old
budget (which is why preemption has occurred) and its old service.

This commit addresses this issue by resetting service on queue real
expiration.

Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-iosched.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index d579cc8e0db6..41d9036b1822 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -634,7 +634,7 @@ static bool bfq_differentiated_weights(struct bfq_data *bfqd)
  * The following function returns true if every queue must receive the
  * same share of the throughput (this condition is used when deciding
  * whether idling may be disabled, see the comments in the function
- * bfq_bfqq_may_idle()).
+ * bfq_better_to_idle()).
  *
  * Such a scenario occurs when:
  * 1) all active queues have the same weight,
@@ -3355,7 +3355,7 @@ static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq)
  * issues taken into account are not trivial. We discuss these issues
  * individually while introducing the variables.
  */
-static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
+static bool bfq_better_to_idle(struct bfq_queue *bfqq)
 {
 	struct bfq_data *bfqd = bfqq->bfqd;
 	bool rot_without_queueing =
@@ -3588,19 +3588,19 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 }
 
 /*
- * If the in-service queue is empty but the function bfq_bfqq_may_idle
+ * If the in-service queue is empty but the function bfq_better_to_idle
  * returns true, then:
  * 1) the queue must remain in service and cannot be expired, and
  * 2) the device must be idled to wait for the possible arrival of a new
  *    request for the queue.
- * See the comments on the function bfq_bfqq_may_idle for the reasons
+ * See the comments on the function bfq_better_to_idle for the reasons
  * why performing device idling is the best choice to boost the throughput
- * and preserve service guarantees when bfq_bfqq_may_idle itself
+ * and preserve service guarantees when bfq_better_to_idle itself
  * returns true.
  */
 static bool bfq_bfqq_must_idle(struct bfq_queue *bfqq)
 {
-	return RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_may_idle(bfqq);
+	return RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_better_to_idle(bfqq);
 }
 
 /*
@@ -3686,7 +3686,7 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
 	 * may idle after their completion, then keep it anyway.
 	 */
 	if (bfq_bfqq_wait_request(bfqq) ||
-	    (bfqq->dispatched != 0 && bfq_bfqq_may_idle(bfqq))) {
+	    (bfqq->dispatched != 0 && bfq_better_to_idle(bfqq))) {
 		bfqq = NULL;
 		goto keep_queue;
 	}
@@ -4734,7 +4734,7 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
 					BFQQE_BUDGET_TIMEOUT);
 		else if (RB_EMPTY_ROOT(&bfqq->sort_list) &&
 			 (bfqq->dispatched == 0 ||
-			  !bfq_bfqq_may_idle(bfqq)))
+			  !bfq_better_to_idle(bfqq)))
 			bfq_bfqq_expire(bfqd, bfqq, false,
 					BFQQE_NO_MORE_REQUESTS);
 	}
-- 
2.16.1