Fix for "Reduce latency effects of non-interactive I/O"

It was found that setting min_active tunables for non-interactive I/Os
makes them stuck.  It is caused by zfs_vdev_nia_delay, that can never
be reached if we never issue any I/Os due to min_active set to zero.

Fix this by issuing at least one non-interactive I/O at a time when
there are no interactive I/Os.  When there are interactive I/Os, zero
min_active allows to completely block any non-interactive I/O.  It may
min_active starvation in some scenarios, but who we are to deny foot
shooting?

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes openzfs#11261

module/zfs/vdev_queue.c

@@ -380,21 +380,21 @@ vdev_queue_class_max_active(spa_t *spa, vdev_queue_t *vq, zio_priority_t p)
 			return (MIN(vq->vq_nia_credit,
 			    zfs_vdev_scrub_min_active));
 		} else if (vq->vq_nia_credit < zfs_vdev_nia_delay)
-			return (zfs_vdev_scrub_min_active);
+			return (MAX(1, zfs_vdev_scrub_min_active));
 		return (zfs_vdev_scrub_max_active);
 	case ZIO_PRIORITY_REMOVAL:
 		if (vq->vq_ia_active > 0) {
 			return (MIN(vq->vq_nia_credit,
 			    zfs_vdev_removal_min_active));
 		} else if (vq->vq_nia_credit < zfs_vdev_nia_delay)
-			return (zfs_vdev_removal_min_active);
+			return (MAX(1, zfs_vdev_removal_min_active));
 		return (zfs_vdev_removal_max_active);
 	case ZIO_PRIORITY_INITIALIZING:
 		if (vq->vq_ia_active > 0) {
 			return (MIN(vq->vq_nia_credit,
 			    zfs_vdev_initializing_min_active));
 		} else if (vq->vq_nia_credit < zfs_vdev_nia_delay)
-			return (zfs_vdev_initializing_min_active);
+			return (MAX(1, zfs_vdev_initializing_min_active));
 		return (zfs_vdev_initializing_max_active);
 	case ZIO_PRIORITY_TRIM:
 		return (zfs_vdev_trim_max_active);
@@ -403,7 +403,7 @@ vdev_queue_class_max_active(spa_t *spa, vdev_queue_t *vq, zio_priority_t p)
 			return (MIN(vq->vq_nia_credit,
 			    zfs_vdev_rebuild_min_active));
 		} else if (vq->vq_nia_credit < zfs_vdev_nia_delay)
-			return (zfs_vdev_rebuild_min_active);
+			return (MAX(1, zfs_vdev_rebuild_min_active));
 		return (zfs_vdev_rebuild_max_active);
 	default:
 		panic("invalid priority %u", p);