Merge branch 'tcp-improving-RACK-cpu-performance'

Yuchung Cheng says:

====================
tcp: improving RACK cpu performance

This patch set improves the CPU consumption of the RACK TCP loss
recovery algorithm, in particular for high-speed networks. Currently,
for every ACK in recovery RACK can potentially iterate over all sent
packets in the write queue. On large BDP networks with non-trivial
losses the RACK write queue walk CPU usage becomes unreasonably high.

This patch introduces a new queue in TCP that keeps only skbs sent and
not yet (s)acked or marked lost, in time order instead of sequence
order.  With that, RACK can examine this time-sorted list and only
check packets that were sent recently, within the reordering window,
per ACK. This is the fastest way without any write queue walks. The
number of skbs examined per ACK is reduced by orders of magnitude.
====================

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);
 }