bcachefs: Add btree node prefetching to bch2_btree_and_journal_walk()

bch2_btree_and_journal_walk() walks the btree overlaying keys from the
journal; it was introduced so that we could read in the alloc btree
prior to journal replay being done, when journalling of updates to
interior btree nodes was introduced.

But it didn't have btree node prefetching, which introduced a severe
regression with mount times, particularly on spinning rust. This patch
implements btree node prefetching for the btree + journal walk,
hopefully fixing that.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 31 insertions, 6 deletions

diff --git a/fs/bcachefs/recovery.c b/fs/bcachefs/recovery.c
index 5a43682c26ef..c700b12b2ac0 100644
--- a/fs/bcachefs/recovery.c
+++ b/fs/bcachefs/recovery.c
@@ -206,6 +206,31 @@ void bch2_btree_and_journal_iter_init_node_iter(struct btree_and_journal_iter *i
 
 /* Walk btree, overlaying keys from the journal: */
 
+static void btree_and_journal_iter_prefetch(struct bch_fs *c, struct btree *b,
+					   struct btree_and_journal_iter iter)
+{
+	unsigned i = 0, nr = b->c.level > 1 ? 2 : 16;
+	struct bkey_s_c k;
+	struct bkey_buf tmp;
+
+	BUG_ON(!b->c.level);
+
+	bch2_bkey_buf_init(&tmp);
+
+	while (i < nr &&
+	       (k = bch2_btree_and_journal_iter_peek(&iter)).k) {
+		bch2_bkey_buf_reassemble(&tmp, c, k);
+
+		bch2_btree_node_prefetch(c, NULL, tmp.k,
+					b->c.btree_id, b->c.level - 1);
+
+		bch2_btree_and_journal_iter_advance(&iter);
+		i++;
+	}
+
+	bch2_bkey_buf_exit(&tmp, c);
+}
+
 static int bch2_btree_and_journal_walk_recurse(struct bch_fs *c, struct btree *b,
 				struct journal_keys *journal_keys,
 				enum btree_id btree_id,
@@ -214,8 +239,11 @@ static int bch2_btree_and_journal_walk_recurse(struct bch_fs *c, struct btree *b
 {
 	struct btree_and_journal_iter iter;
 	struct bkey_s_c k;
+	struct bkey_buf tmp;
+	struct btree *child;
 	int ret = 0;
 
+	bch2_bkey_buf_init(&tmp);
 	bch2_btree_and_journal_iter_init_node_iter(&iter, journal_keys, b);
 
 	while ((k = bch2_btree_and_journal_iter_peek(&iter)).k) {
@@ -224,23 +252,19 @@ static int bch2_btree_and_journal_walk_recurse(struct bch_fs *c, struct btree *b
 			break;
 
 		if (b->c.level) {
-			struct btree *child;
-			struct bkey_buf tmp;
-
-			bch2_bkey_buf_init(&tmp);
 			bch2_bkey_buf_reassemble(&tmp, c, k);
-			k = bkey_i_to_s_c(tmp.k);
 
 			bch2_btree_and_journal_iter_advance(&iter);
 
 			child = bch2_btree_node_get_noiter(c, tmp.k,
 						b->c.btree_id, b->c.level - 1);
-			bch2_bkey_buf_exit(&tmp, c);
 
 			ret = PTR_ERR_OR_ZERO(child);
 			if (ret)
 				break;
 
+			btree_and_journal_iter_prefetch(c, b, iter);
+
 			ret   = (node_fn ? node_fn(c, b) : 0) ?:
 				bch2_btree_and_journal_walk_recurse(c, child,
 					journal_keys, btree_id, node_fn, key_fn);
@@ -253,6 +277,7 @@ static int bch2_btree_and_journal_walk_recurse(struct bch_fs *c, struct btree *b
 		}
 	}
 
+	bch2_bkey_buf_exit(&tmp, c);
 	return ret;
 }