bcachefs: Unlock in bch2_trans_begin() if we've held locks more than 10us

We try to ensure we never hold btree locks for too long - bcachefs tries
to be soft realtime. This adds a check when restarting a transaction,
where a transaction restart is cheap - if we've been holding locks for
too long, drop and retake them.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_iter.c

@@ -3224,12 +3224,19 @@ void bch2_trans_begin(struct btree_trans *trans)
 			path->preserve = false;
 	}
 
-	bch2_trans_cond_resched(trans);
+	if (!trans->restarted &&
+	    (need_resched() ||
+	     ktime_get_ns() - trans->last_begin_time > BTREE_TRANS_MAX_LOCK_HOLD_TIME_NS)) {
+		bch2_trans_unlock(trans);
+		cond_resched();
+		bch2_trans_relock(trans);
+	}
 
 	if (trans->restarted)
 		bch2_btree_path_traverse_all(trans);
 
 	trans->restarted = false;
+	trans->last_begin_time = ktime_get_ns();
 }
 
 static void bch2_trans_alloc_paths(struct btree_trans *trans, struct bch_fs *c)
@@ -3259,6 +3266,7 @@ void __bch2_trans_init(struct btree_trans *trans, struct bch_fs *c,
 	memset(trans, 0, sizeof(*trans));
 	trans->c		= c;
 	trans->fn		= fn;
+	trans->last_begin_time	= ktime_get_ns();
 	trans->task		= current;
 	trans->journal_replay_not_finished =
 		!test_bit(JOURNAL_REPLAY_DONE, &c->journal.flags);

fs/bcachefs/btree_types.h

@@ -382,10 +382,13 @@ struct btree_trans_commit_hook {
 
 #define BTREE_TRANS_MEM_MAX	(1U << 16)
 
+#define BTREE_TRANS_MAX_LOCK_HOLD_TIME_NS	10000
+
 struct btree_trans {
 	struct bch_fs		*c;
 	const char		*fn;
 	struct list_head	list;
+	u64			last_begin_time;
 	struct btree		*locking;
 	unsigned		locking_path_idx;
 	struct bpos		locking_pos;