1 files changed, 68 insertions, 5 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index f59efee7a601..b957e9db87d8 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -595,7 +595,16 @@ static bool bfq_too_late_for_merging(struct bfq_queue *bfqq)
 				       bfq_merge_time_limit);
 }
 
-void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
+/*
+ * The following function is not marked as __cold because it is
+ * actually cold, but for the same performance goal described in the
+ * comments on the likely() at the beginning of
+ * bfq_setup_cooperator(). Unexpectedly, to reach an even lower
+ * execution time for the case where this function is not invoked, we
+ * had to add an unlikely() in each involved if().
+ */
+void __cold
+bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 {
 	struct rb_node **p, *parent;
 	struct bfq_queue *__bfqq;
@@ -1849,8 +1858,9 @@ static void bfq_add_request(struct request *rq)
 
 	/*
 	 * Adjust priority tree position, if next_rq changes.
+	 * See comments on bfq_pos_tree_add_move() for the unlikely().
 	 */
-	if (prev != bfqq->next_rq)
+	if (unlikely(!bfqd->nonrot_with_queueing && prev != bfqq->next_rq))
 		bfq_pos_tree_add_move(bfqd, bfqq);
 
 	if (!bfq_bfqq_busy(bfqq)) /* switching to busy ... */
@@ -1990,7 +2000,9 @@ static void bfq_remove_request(struct request_queue *q,
 			bfqq->pos_root = NULL;
 		}
 	} else {
-		bfq_pos_tree_add_move(bfqd, bfqq);
+		/* see comments on bfq_pos_tree_add_move() for the unlikely() */
+		if (unlikely(!bfqd->nonrot_with_queueing))
+			bfq_pos_tree_add_move(bfqd, bfqq);
 	}
 
 	if (rq->cmd_flags & REQ_META)
@@ -2075,7 +2087,12 @@ static void bfq_request_merged(struct request_queue *q, struct request *req,
 		 */
 		if (prev != bfqq->next_rq) {
 			bfq_updated_next_req(bfqd, bfqq);
-			bfq_pos_tree_add_move(bfqd, bfqq);
+			/*
+			 * See comments on bfq_pos_tree_add_move() for
+			 * the unlikely().
+			 */
+			if (unlikely(!bfqd->nonrot_with_queueing))
+				bfq_pos_tree_add_move(bfqd, bfqq);
 		}
 	}
 }
@@ -2358,6 +2375,46 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 	struct bfq_queue *in_service_bfqq, *new_bfqq;
 
 	/*
+	 * Do not perform queue merging if the device is non
+	 * rotational and performs internal queueing. In fact, such a
+	 * device reaches a high speed through internal parallelism
+	 * and pipelining. This means that, to reach a high
+	 * throughput, it must have many requests enqueued at the same
+	 * time. But, in this configuration, the internal scheduling
+	 * algorithm of the device does exactly the job of queue
+	 * merging: it reorders requests so as to obtain as much as
+	 * possible a sequential I/O pattern. As a consequence, with
+	 * the workload generated by processes doing interleaved I/O,
+	 * the throughput reached by the device is likely to be the
+	 * same, with and without queue merging.
+	 *
+	 * Disabling merging also provides a remarkable benefit in
+	 * terms of throughput. Merging tends to make many workloads
+	 * artificially more uneven, because of shared queues
+	 * remaining non empty for incomparably more time than
+	 * non-merged queues. This may accentuate workload
+	 * asymmetries. For example, if one of the queues in a set of
+	 * merged queues has a higher weight than a normal queue, then
+	 * the shared queue may inherit such a high weight and, by
+	 * staying almost always active, may force BFQ to perform I/O
+	 * plugging most of the time. This evidently makes it harder
+	 * for BFQ to let the device reach a high throughput.
+	 *
+	 * Finally, the likely() macro below is not used because one
+	 * of the two branches is more likely than the other, but to
+	 * have the code path after the following if() executed as
+	 * fast as possible for the case of a non rotational device
+	 * with queueing. We want it because this is the fastest kind
+	 * of device. On the opposite end, the likely() may lengthen
+	 * the execution time of BFQ for the case of slower devices
+	 * (rotational or at least without queueing). But in this case
+	 * the execution time of BFQ matters very little, if not at
+	 * all.
+	 */
+	if (likely(bfqd->nonrot_with_queueing))
+		return NULL;
+
+	/*
 	 * Prevent bfqq from being merged if it has been created too
 	 * long ago. The idea is that true cooperating processes, and
 	 * thus their associated bfq_queues, are supposed to be
@@ -2986,8 +3043,10 @@ static void __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 		bfq_requeue_bfqq(bfqd, bfqq, true);
 		/*
 		 * Resort priority tree of potential close cooperators.
+		 * See comments on bfq_pos_tree_add_move() for the unlikely().
 		 */
-		bfq_pos_tree_add_move(bfqd, bfqq);
+		if (unlikely(!bfqd->nonrot_with_queueing))
+			bfq_pos_tree_add_move(bfqd, bfqq);
 	}
 
 	/*
@@ -5051,6 +5110,9 @@ static void bfq_update_hw_tag(struct bfq_data *bfqd)
 	bfqd->hw_tag = bfqd->max_rq_in_driver > BFQ_HW_QUEUE_THRESHOLD;
 	bfqd->max_rq_in_driver = 0;
 	bfqd->hw_tag_samples = 0;
+
+	bfqd->nonrot_with_queueing =
+		blk_queue_nonrot(bfqd->queue) && bfqd->hw_tag;
 }
 
 static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
@@ -5882,6 +5944,7 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
 	INIT_HLIST_HEAD(&bfqd->burst_list);
 
 	bfqd->hw_tag = -1;
+	bfqd->nonrot_with_queueing = blk_queue_nonrot(bfqd->queue);
 
 	bfqd->bfq_max_budget = bfq_default_max_budget;