block, bfq: balance I/O injection among underutilized actuators

Upon the invocation of its dispatch function, BFQ returns the next I/O
request of the in-service bfq_queue, unless some exception holds. One
such exception is that there is some underutilized actuator, different
from the actuator for which the in-service queue contains I/O, and
that some other bfq_queue happens to contain I/O for such an
actuator. In this case, the next I/O request of the latter bfq_queue,
and not of the in-service bfq_queue, is returned (I/O is injected from
that bfq_queue). To find such an actuator, a linear scan, in
increasing index order, is performed among actuators.

Performing a linear scan entails a prioritization among actuators: an
underutilized actuator may be considered for injection only if all
actuators with a lower index are currently fully utilized, or if there
is no pending I/O for any lower-index actuator that happens to be
underutilized.

This commits breaks this prioritization and tends to distribute
injection uniformly across actuators. This is obtained by adding the
following condition to the linear scan: even if an actuator A is
underutilized, A is however skipped if its load is higher than that of
the next actuator.
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Davide Zini <davidezini2@gmail.com>
Link: https://lore.kernel.org/r/20230103145503.71712-9-paolo.valente@linaro.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -4767,10 +4767,16 @@ bfq_find_active_bfqq_for_actuator(struct bfq_data *bfqd, int idx)
 
 /*
  * Perform a linear scan of each actuator, until an actuator is found
- * for which the following two conditions hold: the load of the
- * actuator is below the threshold (see comments on actuator_load_threshold
- * for details), and there is a queue that contains I/O for that
- * actuator. On success, return that queue.
+ * for which the following three conditions hold: the load of the
+ * actuator is below the threshold (see comments on
+ * actuator_load_threshold for details) and lower than that of the
+ * next actuator (comments on this extra condition below), and there
+ * is a queue that contains I/O for that actuator. On success, return
+ * that queue.
+ *
+ * Performing a plain linear scan entails a prioritization among
+ * actuators. The extra condition above breaks this prioritization and
+ * tends to distribute injection uniformly across actuators.
  */
 static struct bfq_queue *
 bfq_find_bfqq_for_underused_actuator(struct bfq_data *bfqd)
@@ -4778,7 +4784,9 @@ bfq_find_bfqq_for_underused_actuator(struct bfq_data *bfqd)
 	int i;
 
 	for (i = 0 ; i < bfqd->num_actuators; i++) {
-		if (bfqd->rq_in_driver[i] < bfqd->actuator_load_threshold) {
+		if (bfqd->rq_in_driver[i] < bfqd->actuator_load_threshold &&
+		    (i == bfqd->num_actuators - 1 ||
+		     bfqd->rq_in_driver[i] < bfqd->rq_in_driver[i+1])) {
 			struct bfq_queue *bfqq =
 				bfq_find_active_bfqq_for_actuator(bfqd, i);