bcachefs: Add hysteresis to waiting on btree key cache flush

This helps ensure key cache reclaim isn't contending with threads
waiting for the key cache to be helped, and fixes a severe performance
bug.

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

Author: Kent Overstreet

fs/bcachefs/btree_key_cache.h

@@ -20,6 +20,15 @@ static inline bool bch2_btree_key_cache_must_wait(struct bch_fs *c)
 	return nr_dirty > max_dirty;
 }
 
+static inline bool bch2_btree_key_cache_wait_done(struct bch_fs *c)
+{
+	size_t nr_dirty = atomic_long_read(&c->btree_key_cache.nr_dirty);
+	size_t nr_keys = atomic_long_read(&c->btree_key_cache.nr_keys);
+	size_t max_dirty = 2048 + (nr_keys * 5) / 8;
+
+	return nr_dirty <= max_dirty;
+}
+
 int bch2_btree_key_cache_journal_flush(struct journal *,
 				struct journal_entry_pin *, u64);
 

fs/bcachefs/btree_trans_commit.c

@@ -927,7 +927,7 @@ static inline int do_bch2_trans_commit(struct btree_trans *trans, unsigned flags
 static int journal_reclaim_wait_done(struct bch_fs *c)
 {
 	int ret = bch2_journal_error(&c->journal) ?:
-		!bch2_btree_key_cache_must_wait(c);
+		bch2_btree_key_cache_wait_done(c);
 
 	if (!ret)
 		journal_reclaim_kick(&c->journal);