net_sched: add qdisc_dequeue_drop() helper

Some qdisc like cake, codel, fq_codel might drop packets
in their dequeue() method.

This is currently problematic because dequeue() runs with
the qdisc spinlock held. Freeing skbs can be extremely expensive.

Add qdisc_dequeue_drop() method and a new TCQ_F_DEQUEUE_DROPS
so that these qdiscs can opt-in to defer the skb frees
after the socket spinlock is released.

TCQ_F_DEQUEUE_DROPS is an attempt to not penalize other qdiscs
with an extra cache line miss.

Signed-off-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/pkt_sched.h

···
 
 void __qdisc_run(struct Qdisc *q);
 
-static inline void qdisc_run(struct Qdisc *q)
+static inline struct sk_buff *qdisc_run(struct Qdisc *q)
 {
 	if (qdisc_run_begin(q)) {
 		__qdisc_run(q);
-		qdisc_run_end(q);
+		return qdisc_run_end(q);
 	}
+	return NULL;
 }
 
 extern const struct nla_policy rtm_tca_policy[TCA_MAX + 1];

include/net/sch_generic.h

···
 #define TCQ_F_INVISIBLE		0x80 /* invisible by default in dump */
 #define TCQ_F_NOLOCK		0x100 /* qdisc does not require locking */
 #define TCQ_F_OFFLOADED		0x200 /* qdisc is offloaded to HW */
+#define TCQ_F_DEQUEUE_DROPS	0x400 /* ->dequeue() can drop packets in q->to_free */
+
 	u32			limit;
 	const struct Qdisc_ops	*ops;
 	struct qdisc_size_table	__rcu *stab;
···
 
 		/* Note : we only change qstats.backlog in fast path. */
 		struct gnet_stats_queue	qstats;
+
+		struct sk_buff		*to_free;
 	__cacheline_group_end(Qdisc_write);
 
 
···
 	return true;
 }
 
-static inline void qdisc_run_end(struct Qdisc *qdisc)
+static inline struct sk_buff *qdisc_run_end(struct Qdisc *qdisc)
 {
+	struct sk_buff *to_free = NULL;
+
 	if (qdisc->flags & TCQ_F_NOLOCK) {
 		spin_unlock(&qdisc->seqlock);
 
···
 		if (unlikely(test_bit(__QDISC_STATE_MISSED,
 				      &qdisc->state)))
 			__netif_schedule(qdisc);
-	} else {
-		WRITE_ONCE(qdisc->running, false);
+		return NULL;
 	}
+
+	if (qdisc->flags & TCQ_F_DEQUEUE_DROPS) {
+		to_free = qdisc->to_free;
+		if (to_free)
+			qdisc->to_free = NULL;
+	}
+	WRITE_ONCE(qdisc->running, false);
+	return to_free;
 }
 
 static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
···
 		kfree_skb_reason(skb, tcf_get_drop_reason(skb));
 		skb = next;
 	}
+}
+
+static inline void qdisc_dequeue_drop(struct Qdisc *q, struct sk_buff *skb,
+				      enum skb_drop_reason reason)
+{
+	DEBUG_NET_WARN_ON_ONCE(!(q->flags & TCQ_F_DEQUEUE_DROPS));
+	DEBUG_NET_WARN_ON_ONCE(q->flags & TCQ_F_NOLOCK);
+
+	tcf_set_drop_reason(skb, reason);
+	skb->next = q->to_free;
+	q->to_free = skb;
 }
 
 /* Instead of calling kfree_skb() while root qdisc lock is held,

net/core/dev.c

···
 				 struct net_device *dev,
 				 struct netdev_queue *txq)
 {
-	struct sk_buff *next, *to_free = NULL;
+	struct sk_buff *next, *to_free = NULL, *to_free2 = NULL;
 	spinlock_t *root_lock = qdisc_lock(q);
 	struct llist_node *ll_list, *first_n;
 	unsigned long defer_count = 0;
···
 			if (unlikely(!nolock_qdisc_is_empty(q))) {
 				rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
 				__qdisc_run(q);
-				qdisc_run_end(q);
+				to_free2 = qdisc_run_end(q);
 
 				goto free_skbs;
 			}
···
 			    !nolock_qdisc_is_empty(q))
 				__qdisc_run(q);
 
-			qdisc_run_end(q);
-			return NET_XMIT_SUCCESS;
+			to_free2 = qdisc_run_end(q);
+			rc = NET_XMIT_SUCCESS;
+			goto free_skbs;
 		}
 
 		rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
-		qdisc_run(q);
+		to_free2 = qdisc_run(q);
 		goto free_skbs;
 	}
 
···
 		qdisc_bstats_update(q, skb);
 		if (sch_direct_xmit(skb, q, dev, txq, root_lock, true))
 			__qdisc_run(q);
-		qdisc_run_end(q);
+		to_free2 = qdisc_run_end(q);
 		rc = NET_XMIT_SUCCESS;
 	} else {
 		int count = 0;
···
 			rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
 			count++;
 		}
-		qdisc_run(q);
+		to_free2 = qdisc_run(q);
 		if (count != 1)
 			rc = NET_XMIT_SUCCESS;
 	}
···
 
 free_skbs:
 	tcf_kfree_skb_list(to_free);
+	tcf_kfree_skb_list(to_free2);
 	return rc;
 }
 
···
 		rcu_read_lock();
 
 		while (head) {
-			struct Qdisc *q = head;
 			spinlock_t *root_lock = NULL;
+			struct sk_buff *to_free;
+			struct Qdisc *q = head;
 
 			head = head->next_sched;
 
···
 			}
 
 			clear_bit(__QDISC_STATE_SCHED, &q->state);
-			qdisc_run(q);
+			to_free = qdisc_run(q);
 			if (root_lock)
 				spin_unlock(root_lock);
+			tcf_kfree_skb_list(to_free);
 		}
 
 		rcu_read_unlock();