net: sched: convert qdisc linked list to hashtable

Convert the per-device linked list into a hashtable. The primary
motivation for this change is that currently, we're not tracking all the
qdiscs in hierarchy (e.g. excluding default qdiscs), as the lookup
performed over the linked list by qdisc_match_from_root() is rather
expensive.

The ultimate goal is to get rid of hidden qdiscs completely, which will
bring much more determinism in user experience.

Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -423,7 +423,6 @@ struct Qdisc noop_qdisc = {
 	.dequeue	=	noop_dequeue,
 	.flags		=	TCQ_F_BUILTIN,
 	.ops		=	&noop_qdisc_ops,
-	.list		=	LIST_HEAD_INIT(noop_qdisc.list),
 	.q.lock		=	__SPIN_LOCK_UNLOCKED(noop_qdisc.q.lock),
 	.dev_queue	=	&noop_netdev_queue,
 	.running	=	SEQCNT_ZERO(noop_qdisc.running),
@@ -613,7 +612,6 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
 		sch = (struct Qdisc *) QDISC_ALIGN((unsigned long) p);
 		sch->padded = (char *) sch - (char *) p;
 	}
-	INIT_LIST_HEAD(&sch->list);
 	skb_queue_head_init(&sch->q);
 
 	spin_lock_init(&sch->busylock);
@@ -700,7 +698,7 @@ void qdisc_destroy(struct Qdisc *qdisc)
 		return;
 
 #ifdef CONFIG_NET_SCHED
-	qdisc_list_del(qdisc);
+	qdisc_hash_del(qdisc);
 
 	qdisc_put_stab(rtnl_dereference(qdisc->stab));
 #endif
@@ -788,6 +786,10 @@ static void attach_default_qdiscs(struct net_device *dev)
 			qdisc->ops->attach(qdisc);
 		}
 	}
+#ifdef CONFIG_NET_SCHED
+	if (dev->qdisc)
+		qdisc_hash_add(dev->qdisc);
+#endif
 }
 
 static void transition_one_qdisc(struct net_device *dev,