Btrfs: Reduce contention on the root node

This calls unlock_up sooner in btrfs_search_slot in order to decrease the
amount of work done with the higher level tree locks held.

Also, it changes btrfs_tree_lock to spin for a big against the page lock
before scheduling.  This makes a big difference in context switch rate under
highly contended workloads.

Longer term, a better locking structure is needed than the page lock.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -1313,16 +1313,13 @@ again:
 				slot = p->slots[level];
 				BUG_ON(btrfs_header_nritems(b) == 1);
 			}
+			unlock_up(p, level, lowest_unlock);
+
 			/* this is only true while dropping a snapshot */
 			if (level == lowest_level) {
-				unlock_up(p, level, lowest_unlock);
 				break;
 			}
 
-			if (should_reada)
-				reada_for_search(root, p, level, slot,
-						 key->objectid);
-
 			blocknr = btrfs_node_blockptr(b, slot);
 			gen = btrfs_node_ptr_generation(b, slot);
 			blocksize = btrfs_level_size(root, level - 1);
@@ -1340,6 +1337,11 @@ again:
 					btrfs_release_path(NULL, p);
 					if (tmp)
 						free_extent_buffer(tmp);
+					if (should_reada)
+						reada_for_search(root, p,
+								 level, slot,
+								 key->objectid);
+
 					tmp = read_tree_block(root, blocknr,
 							 blocksize, gen);
 					if (tmp)
@@ -1348,12 +1350,15 @@ again:
 				} else {
 					if (tmp)
 						free_extent_buffer(tmp);
+					if (should_reada)
+						reada_for_search(root, p,
+								 level, slot,
+								 key->objectid);
 					b = read_node_slot(root, b, slot);
 				}
 			}
 			if (!p->skip_locking)
 				btrfs_tree_lock(b);
-			unlock_up(p, level, lowest_unlock);
 		} else {
 			p->slots[level] = slot;
 			if (ins_len > 0 && btrfs_leaf_free_space(root, b) <

fs/btrfs/locking.c

@@ -27,6 +27,16 @@
 
 int btrfs_tree_lock(struct extent_buffer *eb)
 {
+	int i;
+
+	if (!TestSetPageLocked(eb->first_page))
+		return 0;
+	for (i = 0; i < 512; i++) {
+		cpu_relax();
+		if (!TestSetPageLocked(eb->first_page))
+			return 0;
+	}
+	cpu_relax();
 	lock_page(eb->first_page);
 	return 0;
 }