block, bfq: remove superfluous check in queue-merging setup

When two or more processes do I/O in a way that the their requests are
sequential in respect to one another, BFQ merges the bfq_queues associated
with the processes. This way the overall I/O pattern becomes sequential,
and thus there is a boost in througput.
These cooperating processes usually start or restart to do I/O shortly
after each other. So, in order to avoid merging non-cooperating processes,
BFQ ensures that none of these queues has been in weight raising for too
long.

In this respect, from commit "block, bfq-sq, bfq-mq: let a queue be merged
only shortly after being created", BFQ checks whether any queue (and not
only weight-raised ones) is doing I/O continuously from too long to be
merged.

This new additional check makes the first one useless: a queue doing
I/O from long enough, if being weight-raised, is also a queue in
weight raising for too long to be merged. Accordingly, this commit
removes the first check.

Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -1990,20 +1990,6 @@ static bool bfq_may_be_close_cooperator(struct bfq_queue *bfqq,
 	return true;
 }
 
-/*
- * If this function returns true, then bfqq cannot be merged. The idea
- * is that true cooperation happens very early after processes start
- * to do I/O. Usually, late cooperations are just accidental false
- * positives. In case bfqq is weight-raised, such false positives
- * would evidently degrade latency guarantees for bfqq.
- */
-static bool wr_from_too_long(struct bfq_queue *bfqq)
-{
-	return bfqq->wr_coeff > 1 &&
-		time_is_before_jiffies(bfqq->last_wr_start_finish +
-				       msecs_to_jiffies(100));
-}
-
 /*
  * Attempt to schedule a merge of bfqq with the currently in-service
  * queue or with a close queue among the scheduled queues.  Return
@@ -2017,11 +2003,6 @@ static bool wr_from_too_long(struct bfq_queue *bfqq)
  * to maintain. Besides, in such a critical condition as an out of memory,
  * the benefits of queue merging may be little relevant, or even negligible.
  *
- * Weight-raised queues can be merged only if their weight-raising
- * period has just started. In fact cooperating processes are usually
- * started together. Thus, with this filter we avoid false positives
- * that would jeopardize low-latency guarantees.
- *
  * WARNING: queue merging may impair fairness among non-weight raised
  * queues, for at least two reasons: 1) the original weight of a
  * merged queue may change during the merged state, 2) even being the
@@ -2052,9 +2033,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 	if (bfqq->new_bfqq)
 		return bfqq->new_bfqq;
 
-	if (!io_struct ||
-	    wr_from_too_long(bfqq) ||
-	    unlikely(bfqq == &bfqd->oom_bfqq))
+	if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
 		return NULL;
 
 	/* If there is only one backlogged queue, don't search. */
@@ -2063,12 +2042,9 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 
 	in_service_bfqq = bfqd->in_service_queue;
 
-	if (!in_service_bfqq || in_service_bfqq == bfqq
-	    || wr_from_too_long(in_service_bfqq) ||
-	    unlikely(in_service_bfqq == &bfqd->oom_bfqq))
-		goto check_scheduled;
-
-	if (bfq_rq_close_to_sector(io_struct, request, bfqd->last_position) &&
+	if (in_service_bfqq && in_service_bfqq != bfqq &&
+	    likely(in_service_bfqq != &bfqd->oom_bfqq) &&
+	    bfq_rq_close_to_sector(io_struct, request, bfqd->last_position) &&
 	    bfqq->entity.parent == in_service_bfqq->entity.parent &&
 	    bfq_may_be_close_cooperator(bfqq, in_service_bfqq)) {
 		new_bfqq = bfq_setup_merge(bfqq, in_service_bfqq);
@@ -2080,12 +2056,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 	 * queues. The only thing we need is that the bio/request is not
 	 * NULL, as we need it to establish whether a cooperator exists.
 	 */
-check_scheduled:
 	new_bfqq = bfq_find_close_cooperator(bfqd, bfqq,
 			bfq_io_struct_pos(io_struct, request));
 
-	if (new_bfqq && !wr_from_too_long(new_bfqq) &&
-	    likely(new_bfqq != &bfqd->oom_bfqq) &&
+	if (new_bfqq && likely(new_bfqq != &bfqd->oom_bfqq) &&
 	    bfq_may_be_close_cooperator(bfqq, new_bfqq))
 		return bfq_setup_merge(bfqq, new_bfqq);