bcachefs: Don't call local_clock() twice in trans_begin()

local_clock() is not as cheap as we'd like it to be, alas
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2850,6 +2850,7 @@ static noinline void bch2_trans_reset_srcu_lock(struct btree_trans *trans)
 u32 bch2_trans_begin(struct btree_trans *trans)
 {
 	struct btree_path *path;
+	u64 now;
 
 	bch2_trans_reset_updates(trans);
 
@@ -2878,13 +2879,16 @@ u32 bch2_trans_begin(struct btree_trans *trans)
 			path->preserve = false;
 	}
 
+	now = local_clock();
 	if (!trans->restarted &&
 	    (need_resched() ||
-	     local_clock() - trans->last_begin_time > BTREE_TRANS_MAX_LOCK_HOLD_TIME_NS)) {
+	     now - trans->last_begin_time > BTREE_TRANS_MAX_LOCK_HOLD_TIME_NS)) {
 		bch2_trans_unlock(trans);
 		cond_resched();
 		bch2_trans_relock(trans);
+		now = local_clock();
 	}
+	trans->last_begin_time = now;
 
 	if (unlikely(time_after(jiffies, trans->srcu_lock_time + msecs_to_jiffies(10))))
 		bch2_trans_reset_srcu_lock(trans);
@@ -2895,7 +2899,6 @@ u32 bch2_trans_begin(struct btree_trans *trans)
 		trans->notrace_relock_fail = false;
 	}
 
-	trans->last_begin_time = local_clock();
 	return trans->restart_count;
 }