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>

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

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	*/

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);
 }
 

net/ipv4/tcp.c

@@ -415,6 +415,7 @@ void tcp_init_sock(struct sock *sk)
 	tp->out_of_order_queue = RB_ROOT;
 	tcp_init_xmit_timers(sk);
 	INIT_LIST_HEAD(&tp->tsq_node);
+	INIT_LIST_HEAD(&tp->tsorted_sent_queue);
 
 	icsk->icsk_rto = TCP_TIMEOUT_INIT;
 	tp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
@@ -881,6 +882,7 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
 			 * available to the caller, no more, no less.
 			 */
 			skb->reserved_tailroom = skb->end - skb->tail - size;
+			INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
 			return skb;
 		}
 		__kfree_skb(skb);

net/ipv4/tcp_input.c

@@ -1593,6 +1593,8 @@ static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
 						tcp_skb_pcount(skb),
 						skb->skb_mstamp);
 			tcp_rate_skb_delivered(sk, skb, state->rate);
+			if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
+				list_del_init(&skb->tcp_tsorted_anchor);
 
 			if (!before(TCP_SKB_CB(skb)->seq,
 				    tcp_highest_sack_seq(tp)))
@@ -3054,8 +3056,11 @@ static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
 
 	shinfo = skb_shinfo(skb);
 	if (!before(shinfo->tskey, prior_snd_una) &&
-	    before(shinfo->tskey, tcp_sk(sk)->snd_una))
-		__skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK);
+	    before(shinfo->tskey, tcp_sk(sk)->snd_una)) {
+		tcp_skb_tsorted_save(skb) {
+			__skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK);
+		} tcp_skb_tsorted_restore(skb);
+	}
 }
 
 /* Remove acknowledged frames from the retransmission queue. If our packet

net/ipv4/tcp_minisocks.c

@@ -446,6 +446,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 		newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
 
 		INIT_LIST_HEAD(&newtp->tsq_node);
+		INIT_LIST_HEAD(&newtp->tsorted_sent_queue);
 
 		tcp_init_wl(newtp, treq->rcv_isn);
 

net/ipv4/tcp_output.c

@@ -971,6 +971,12 @@ static void tcp_internal_pacing(struct sock *sk, const struct sk_buff *skb)
 		      HRTIMER_MODE_ABS_PINNED);
 }
 
+static void tcp_update_skb_after_send(struct tcp_sock *tp, struct sk_buff *skb)
+{
+	skb->skb_mstamp = tp->tcp_mstamp;
+	list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
+}
+
 /* This routine actually transmits TCP packets queued in by
  * tcp_do_sendmsg().  This is used by both the initial
  * transmission and possible later retransmissions.
@@ -1003,10 +1009,14 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 		TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
 			- tp->snd_una;
 		oskb = skb;
-		if (unlikely(skb_cloned(skb)))
-			skb = pskb_copy(skb, gfp_mask);
-		else
-			skb = skb_clone(skb, gfp_mask);
+
+		tcp_skb_tsorted_save(oskb) {
+			if (unlikely(skb_cloned(oskb)))
+				skb = pskb_copy(oskb, gfp_mask);
+			else
+				skb = skb_clone(oskb, gfp_mask);
+		} tcp_skb_tsorted_restore(oskb);
+
 		if (unlikely(!skb))
 			return -ENOBUFS;
 	}
@@ -1127,7 +1137,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 		err = net_xmit_eval(err);
 	}
 	if (!err && oskb) {
-		oskb->skb_mstamp = tp->tcp_mstamp;
+		tcp_update_skb_after_send(tp, oskb);
 		tcp_rate_skb_sent(sk, oskb);
 	}
 	return err;
@@ -1328,6 +1338,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
 	/* Link BUFF into the send queue. */
 	__skb_header_release(buff);
 	tcp_insert_write_queue_after(skb, buff, sk);
+	list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);
 
 	return 0;
 }
@@ -2260,7 +2271,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 
 		if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
 			/* "skb_mstamp" is used as a start point for the retransmit timer */
-			skb->skb_mstamp = tp->tcp_mstamp;
+			tcp_update_skb_after_send(tp, skb);
 			goto repair; /* Skip network transmission */
 		}
 
@@ -2838,11 +2849,14 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		     skb_headroom(skb) >= 0xFFFF)) {
 		struct sk_buff *nskb;
 
-		nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
-		err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
-			     -ENOBUFS;
+		tcp_skb_tsorted_save(skb) {
+			nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
+			err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
+				     -ENOBUFS;
+		} tcp_skb_tsorted_restore(skb);
+
 		if (!err)
-			skb->skb_mstamp = tp->tcp_mstamp;
+			tcp_update_skb_after_send(tp, skb);
 	} else {
 		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
 	}
@@ -3023,6 +3037,7 @@ void tcp_send_fin(struct sock *sk)
 				goto coalesce;
 			return;
 		}
+		INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
 		skb_reserve(skb, MAX_TCP_HEADER);
 		sk_forced_mem_schedule(sk, skb->truesize);
 		/* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
@@ -3078,9 +3093,14 @@ int tcp_send_synack(struct sock *sk)
 	}
 	if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
 		if (skb_cloned(skb)) {
-			struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
+			struct sk_buff *nskb;
+
+			tcp_skb_tsorted_save(skb) {
+				nskb = skb_copy(skb, GFP_ATOMIC);
+			} tcp_skb_tsorted_restore(skb);
 			if (!nskb)
 				return -ENOMEM;
+			INIT_LIST_HEAD(&nskb->tcp_tsorted_anchor);
 			tcp_unlink_write_queue(skb, sk);
 			__skb_header_release(nskb);
 			__tcp_add_write_queue_head(sk, nskb);

net/ipv4/tcp_recovery.c

@@ -45,7 +45,7 @@ static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
 static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	struct sk_buff *skb;
+	struct sk_buff *skb, *n;
 	u32 reo_wnd;
 
 	*reo_timeout = 0;
@@ -58,45 +58,31 @@ static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
 	if ((tp->rack.reord || !tp->lost_out) && tcp_min_rtt(tp) != ~0U)
 		reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
 
-	tcp_for_write_queue(skb, sk) {
+	list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
+				 tcp_tsorted_anchor) {
 		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
+		s32 remaining;
 
-		if (skb == tcp_send_head(sk))
-			break;
-
-		/* Skip ones already (s)acked */
-		if (!after(scb->end_seq, tp->snd_una) ||
-		    scb->sacked & TCPCB_SACKED_ACKED)
+		/* Skip ones marked lost but not yet retransmitted */
+		if ((scb->sacked & TCPCB_LOST) &&
+		    !(scb->sacked & TCPCB_SACKED_RETRANS))
 			continue;
 
-		if (tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
-					tp->rack.end_seq, scb->end_seq)) {
-			/* Step 3 in draft-cheng-tcpm-rack-00.txt:
-			 * A packet is lost if its elapsed time is beyond
-			 * the recent RTT plus the reordering window.
-			 */
-			u32 elapsed = tcp_stamp_us_delta(tp->tcp_mstamp,
-							 skb->skb_mstamp);
-			s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed;
-
-			if (remaining < 0) {
-				tcp_rack_mark_skb_lost(sk, skb);
-				continue;
-			}
-
-			/* Skip ones marked lost but not yet retransmitted */
-			if ((scb->sacked & TCPCB_LOST) &&
-			    !(scb->sacked & TCPCB_SACKED_RETRANS))
-				continue;
+		if (!tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
+					 tp->rack.end_seq, scb->end_seq))
+			break;
 
+		/* A packet is lost if it has not been s/acked beyond
+		 * the recent RTT plus the reordering window.
+		 */
+		remaining = tp->rack.rtt_us + reo_wnd -
+			    tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp);
+		if (remaining < 0) {
+			tcp_rack_mark_skb_lost(sk, skb);
+			list_del_init(&skb->tcp_tsorted_anchor);
+		} else {
 			/* Record maximum wait time (+1 to avoid 0) */
 			*reo_timeout = max_t(u32, *reo_timeout, 1 + remaining);
-
-		} else if (!(scb->sacked & TCPCB_RETRANS)) {
-			/* Original data are sent sequentially so stop early
-			 * b/c the rest are all sent after rack_sent
-			 */
-			break;
 		}
 	}
 }