tcp: new list for sent but unacked skbs for RACK recovery

This patch adds a new queue (list) that tracks the sent but not yet
acked or SACKed skbs for a TCP connection. The list is chronologically
ordered by skb->skb_mstamp (the head is the oldest sent skb).

This list will be used to optimize TCP Rack recovery, which checks
an skb's timestamp to judge if it has been lost and needs to be
retransmitted. Since TCP write queue is ordered by sequence instead
of sent time, RACK has to scan over the write queue to catch all
eligible packets to detect lost retransmission, and iterates through
SACKed skbs repeatedly.

Special cares for rare events:
1. TCP repair fakes skb transmission so the send queue needs adjusted
2. SACK reneging would require re-inserting SACKed skbs into the
   send queue. For now I believe it's not worth the complexity to
   make RACK work perfectly on SACK reneging, so we do nothing here.
3. Fast Open: currently for non-TFO, send-queue correctly queues
   the pure SYN packet. For TFO which queues a pure SYN and
   then a data packet, send-queue only queues the data packet but
   not the pure SYN due to the structure of TFO code. This is okay
   because the SYN receiver would never respond with a SACK on a
   missing SYN (i.e. SYN is never fast-retransmitted by SACK/RACK).

In order to not grow sk_buff, we use an union for the new list and
_skb_refdst/destructor fields. This is a bit complicated because
we need to make sure _skb_refdst and destructor are properly zeroed
before skb is cloned/copied at transmit, and before being freed.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 33 insertions, 3 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ada821466e88..01a985937867 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -617,6 +617,7 @@ typedef unsigned char *sk_buff_data_t;
  *	@nf_trace: netfilter packet trace flag
  *	@protocol: Packet protocol from driver
  *	@destructor: Destruct function
+ *	@tcp_tsorted_anchor: list structure for TCP (tp->tsorted_sent_queue)
  *	@_nfct: Associated connection, if any (with nfctinfo bits)
  *	@nf_bridge: Saved data about a bridged frame - see br_netfilter.c
  *	@skb_iif: ifindex of device we arrived on
@@ -686,8 +687,14 @@ struct sk_buff {
 	 */
 	char			cb[48] __aligned(8);
 
-	unsigned long		_skb_refdst;
-	void			(*destructor)(struct sk_buff *skb);
+	union {
+		struct {
+			unsigned long	_skb_refdst;
+			void		(*destructor)(struct sk_buff *skb);
+		};
+		struct list_head	tcp_tsorted_anchor;
+	};
+
 #ifdef CONFIG_XFRM
 	struct	sec_path	*sp;
 #endif
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 4aa40ef02d32..1d2c44e09e31 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -191,6 +191,7 @@ struct tcp_sock {
 	u32	tsoffset;	/* timestamp offset */
 
 	struct list_head tsq_node; /* anchor in tsq_tasklet.head list */
+	struct list_head tsorted_sent_queue; /* time-sorted sent but un-SACKed skbs */
 
 	u32	snd_wl1;	/* Sequence for window update		*/
 	u32	snd_wnd;	/* The window we expect to receive	*/
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 426c2e986016..3b16f353b539 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1589,14 +1589,34 @@ enum tcp_chrono {
 void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type);
 void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type);
 
+/* This helper is needed, because skb->tcp_tsorted_anchor uses
+ * the same memory storage than skb->destructor/_skb_refdst
+ */
+static inline void tcp_skb_tsorted_anchor_cleanup(struct sk_buff *skb)
+{
+	skb->destructor = NULL;
+	skb->_skb_refdst = 0UL;
+}
+
+#define tcp_skb_tsorted_save(skb) {		\
+	unsigned long _save = skb->_skb_refdst;	\
+	skb->_skb_refdst = 0UL;
+
+#define tcp_skb_tsorted_restore(skb)		\
+	skb->_skb_refdst = _save;		\
+}
+
 /* write queue abstraction */
 static inline void tcp_write_queue_purge(struct sock *sk)
 {
 	struct sk_buff *skb;
 
 	tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
-	while ((skb = __skb_dequeue(&sk->sk_write_queue)) != NULL)
+	while ((skb = __skb_dequeue(&sk->sk_write_queue)) != NULL) {
+		tcp_skb_tsorted_anchor_cleanup(skb);
 		sk_wmem_free_skb(sk, skb);
+	}
+	INIT_LIST_HEAD(&tcp_sk(sk)->tsorted_sent_queue);
 	sk_mem_reclaim(sk);
 	tcp_clear_all_retrans_hints(tcp_sk(sk));
 }
@@ -1711,6 +1731,8 @@ static inline void tcp_insert_write_queue_before(struct sk_buff *new,
 
 static inline void tcp_unlink_write_queue(struct sk_buff *skb, struct sock *sk)
 {
+	list_del(&skb->tcp_tsorted_anchor);
+	tcp_skb_tsorted_anchor_cleanup(skb);
 	__skb_unlink(skb, &sk->sk_write_queue);
 }