block, bfq: let also stably merged queues enjoy weight raising

Merged bfq_queues are kept out of weight-raising (low-latency)
mechanisms. The reason is that these queues are usually created for
non-interactive and non-soft-real-time tasks. Yet this is not the case
for stably-merged queues. These queues are merged just because they
are created shortly after each other. So they may easily serve the I/O
of an interactive or soft-real time application, if the application
happens to spawn multiple processes.

To address this issue, this commits lets also stably-merged queued
enjoy weight raising.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20210619140948.98712-2-paolo.valente@linaro.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -1729,10 +1729,23 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
 		bfqq->entity.new_weight == 40;
 	*interactive = !in_burst && idle_for_long_time &&
 		bfqq->entity.new_weight == 40;
+	/*
+	 * Merged bfq_queues are kept out of weight-raising
+	 * (low-latency) mechanisms. The reason is that these queues
+	 * are usually created for non-interactive and
+	 * non-soft-real-time tasks. Yet this is not the case for
+	 * stably-merged queues. These queues are merged just because
+	 * they are created shortly after each other. So they may
+	 * easily serve the I/O of an interactive or soft-real time
+	 * application, if the application happens to spawn multiple
+	 * processes. So let also stably-merged queued enjoy weight
+	 * raising.
+	 */
 	wr_or_deserves_wr = bfqd->low_latency &&
 		(bfqq->wr_coeff > 1 ||
 		 (bfq_bfqq_sync(bfqq) &&
-		  bfqq->bic && (*interactive || soft_rt)));
+		  (bfqq->bic || RQ_BIC(rq)->stably_merged) &&
+		   (*interactive || soft_rt)));
 
 	/*
 	 * Using the last flag, update budget and check whether bfqq