vm_pageout_scan_inactive: take a lock break

In vm_pageout_scan_inactive, release the object lock when we go to
refill the scan batch queue so that someone else has a chance to acquire
it.  This improves access latency to the object when the pagedaemon is
processing many consecutive pages from a single object, and also in any
case avoids a hiccup during refill for the last touched object.

Reviewed by:	alc, markj (previous version)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D45288

sys/vm/vm_pageout.c

@@ -1451,7 +1451,21 @@ vm_pageout_scan_inactive(struct vm_domain *vmd, int page_shortage)
 	pq = &vmd->vmd_pagequeues[PQ_INACTIVE];
 	vm_pagequeue_lock(pq);
 	vm_pageout_init_scan(&ss, pq, marker, NULL, pq->pq_cnt);
-	while (page_shortage > 0 && (m = vm_pageout_next(&ss, true)) != NULL) {
+	while (page_shortage > 0) {
+		/*
+		 * If we need to refill the scan batch queue, release any
+		 * optimistically held object lock.  This gives someone else a
+		 * chance to grab the lock, and also avoids holding it while we
+		 * do unrelated work.
+		 */
+		if (object != NULL && vm_batchqueue_empty(&ss.bq)) {
+			VM_OBJECT_WUNLOCK(object);
+			object = NULL;
+		}
+
+		m = vm_pageout_next(&ss, true);
+		if (m == NULL)
+			break;
 		KASSERT((m->flags & PG_MARKER) == 0,
 		    ("marker page %p was dequeued", m));
 

sys/vm/vm_pagequeue.h

@@ -354,6 +354,12 @@ vm_batchqueue_init(struct vm_batchqueue *bq)
 	bq->bq_cnt = 0;
 }
 
+static inline bool
+vm_batchqueue_empty(const struct vm_batchqueue *bq)
+{
+	return (bq->bq_cnt == 0);
+}
+
 static inline int
 vm_batchqueue_insert(struct vm_batchqueue *bq, vm_page_t m)
 {