net_sched: plug in qdisc ops change_tx_queue_len

Introduce a new qdisc ops ->change_tx_queue_len() so that
each qdisc could decide how to implement this if it wants.
Previously we simply read dev->tx_queue_len, after pfifo_fast
switches to skb array, we need this API to resize the skb array
when we change dev->tx_queue_len.

To avoid handling race conditions with TX BH, we need to
deactivate all TX queues before change the value and bring them
back after we are done, this also makes implementation easier.

Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -200,6 +200,7 @@ struct Qdisc_ops {
 					  struct nlattr *arg,
 					  struct netlink_ext_ack *extack);
 	void			(*attach)(struct Qdisc *sch);
+	int			(*change_tx_queue_len)(struct Qdisc *, unsigned int);
 
 	int			(*dump)(struct Qdisc *, struct sk_buff *);
 	int			(*dump_stats)(struct Qdisc *, struct gnet_dump *);
@@ -489,6 +490,7 @@ void qdisc_class_hash_remove(struct Qdisc_class_hash *,
 void qdisc_class_hash_grow(struct Qdisc *, struct Qdisc_class_hash *);
 void qdisc_class_hash_destroy(struct Qdisc_class_hash *);
 
+int dev_qdisc_change_tx_queue_len(struct net_device *dev);
 void dev_init_scheduler(struct net_device *dev);
 void dev_shutdown(struct net_device *dev);
 void dev_activate(struct net_device *dev);

net/core/dev.c

@@ -7070,6 +7070,7 @@ int dev_change_tx_queue_len(struct net_device *dev, unsigned long new_len)
 			dev->tx_queue_len = orig_len;
 			return res;
 		}
+		return dev_qdisc_change_tx_queue_len(dev);
 	}
 
 	return 0;

net/sched/sch_generic.c

@@ -1178,6 +1178,39 @@ void dev_deactivate(struct net_device *dev)
 }
 EXPORT_SYMBOL(dev_deactivate);
 
+static int qdisc_change_tx_queue_len(struct net_device *dev,
+				     struct netdev_queue *dev_queue)
+{
+	struct Qdisc *qdisc = dev_queue->qdisc_sleeping;
+	const struct Qdisc_ops *ops = qdisc->ops;
+
+	if (ops->change_tx_queue_len)
+		return ops->change_tx_queue_len(qdisc, dev->tx_queue_len);
+	return 0;
+}
+
+int dev_qdisc_change_tx_queue_len(struct net_device *dev)
+{
+	bool up = dev->flags & IFF_UP;
+	unsigned int i;
+	int ret = 0;
+
+	if (up)
+		dev_deactivate(dev);
+
+	for (i = 0; i < dev->num_tx_queues; i++) {
+		ret = qdisc_change_tx_queue_len(dev, &dev->_tx[i]);
+
+		/* TODO: revert changes on a partial failure */
+		if (ret)
+			break;
+	}
+
+	if (up)
+		dev_activate(dev);
+	return ret;
+}
+
 static void dev_init_scheduler_queue(struct net_device *dev,
 				     struct netdev_queue *dev_queue,
 				     void *_qdisc)