Merge branch 'tcp-sack-compression-changes'

Eric Dumazet says:

====================
tcp: sack compression changes

Patch series refines SACK compression.

We had issues with missing SACK when TCP option space is tight.

Uses hrtimer slack to improve performance.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -651,6 +651,14 @@ tcp_comp_sack_delay_ns - LONG INTEGER
 
 	Default : 1,000,000 ns (1 ms)
 
+tcp_comp_sack_slack_ns - LONG INTEGER
+	This sysctl control the slack used when arming the
+	timer used by SACK compression. This gives extra time
+	for small RTT flows, and reduces system overhead by allowing
+	opportunistic reduction of timer interrupts.
+
+	Default : 100,000 ns (100 us)
+
 tcp_comp_sack_nr - INTEGER
 	Max number of SACK that can be compressed.
 	Using 0 disables SACK compression.

include/linux/tcp.h

@@ -268,6 +268,7 @@ struct tcp_sock {
 	} rack;
 	u16	advmss;		/* Advertised MSS			*/
 	u8	compressed_ack;
+	u8	dup_ack_counter;
 	u32	chrono_start;	/* Start time in jiffies of a TCP chrono */
 	u32	chrono_stat[3];	/* Time in jiffies for chrono_stat stats */
 	u8	chrono_type:2,	/* current chronograph type */

include/net/netns/ipv4.h

@@ -173,6 +173,7 @@ struct netns_ipv4 {
 	int sysctl_tcp_rmem[3];
 	int sysctl_tcp_comp_sack_nr;
 	unsigned long sysctl_tcp_comp_sack_delay_ns;
+	unsigned long sysctl_tcp_comp_sack_slack_ns;
 	struct inet_timewait_death_row tcp_death_row;
 	int sysctl_max_syn_backlog;
 	int sysctl_tcp_fastopen;

net/ipv4/sysctl_net_ipv4.c

@@ -1329,6 +1329,13 @@ static struct ctl_table ipv4_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_doulongvec_minmax,
 	},
+	{
+		.procname	= "tcp_comp_sack_slack_ns",
+		.data		= &init_net.ipv4.sysctl_tcp_comp_sack_slack_ns,
+		.maxlen		= sizeof(unsigned long),
+		.mode		= 0644,
+		.proc_handler	= proc_doulongvec_minmax,
+	},
 	{
 		.procname	= "tcp_comp_sack_nr",
 		.data		= &init_net.ipv4.sysctl_tcp_comp_sack_nr,

net/ipv4/tcp_input.c

@@ -4327,6 +4327,33 @@ static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
 	}
 }
 
+static void tcp_sack_compress_send_ack(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!tp->compressed_ack)
+		return;
+
+	if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
+		__sock_put(sk);
+
+	/* Since we have to send one ack finally,
+	 * substract one from tp->compressed_ack to keep
+	 * LINUX_MIB_TCPACKCOMPRESSED accurate.
+	 */
+	NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
+		      tp->compressed_ack - 1);
+
+	tp->compressed_ack = 0;
+	tcp_send_ack(sk);
+}
+
+/* Reasonable amount of sack blocks included in TCP SACK option
+ * The max is 4, but this becomes 3 if TCP timestamps are there.
+ * Given that SACK packets might be lost, be conservative and use 2.
+ */
+#define TCP_SACK_BLOCKS_EXPECTED 2
+
 static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -4339,6 +4366,8 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 
 	for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
 		if (tcp_sack_extend(sp, seq, end_seq)) {
+			if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
+				tcp_sack_compress_send_ack(sk);
 			/* Rotate this_sack to the first one. */
 			for (; this_sack > 0; this_sack--, sp--)
 				swap(*sp, *(sp - 1));
@@ -4348,6 +4377,9 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 		}
 	}
 
+	if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
+		tcp_sack_compress_send_ack(sk);
+
 	/* Could not find an adjacent existing SACK, build a new one,
 	 * put it at the front, and shift everyone else down.  We
 	 * always know there is at least one SACK present already here.
@@ -4355,8 +4387,6 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 	 * If the sack array is full, forget about the last one.
 	 */
 	if (this_sack >= TCP_NUM_SACKS) {
-		if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
-			tcp_send_ack(sk);
 		this_sack--;
 		tp->rx_opt.num_sacks--;
 		sp--;
@@ -5275,15 +5305,13 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 
 	if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
 		tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
-		if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
-			NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
-				      tp->compressed_ack - TCP_FASTRETRANS_THRESH);
-		tp->compressed_ack = 0;
+		tp->dup_ack_counter = 0;
 	}
-
-	if (++tp->compressed_ack <= TCP_FASTRETRANS_THRESH)
+	if (tp->dup_ack_counter < TCP_FASTRETRANS_THRESH) {
+		tp->dup_ack_counter++;
 		goto send_now;
-
+	}
+	tp->compressed_ack++;
 	if (hrtimer_is_queued(&tp->compressed_ack_timer))
 		return;
 
@@ -5296,7 +5324,8 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 	delay = min_t(unsigned long, sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns,
 		      rtt * (NSEC_PER_USEC >> 3)/20);
 	sock_hold(sk);
-	hrtimer_start(&tp->compressed_ack_timer, ns_to_ktime(delay),
+	hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
+			       sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns,
 			       HRTIMER_MODE_REL_PINNED_SOFT);
 }
 

net/ipv4/tcp_ipv4.c

@@ -2780,6 +2780,7 @@ static int __net_init tcp_sk_init(struct net *net)
 		       sizeof(init_net.ipv4.sysctl_tcp_wmem));
 	}
 	net->ipv4.sysctl_tcp_comp_sack_delay_ns = NSEC_PER_MSEC;
+	net->ipv4.sysctl_tcp_comp_sack_slack_ns = 100 * NSEC_PER_USEC;
 	net->ipv4.sysctl_tcp_comp_sack_nr = 44;
 	net->ipv4.sysctl_tcp_fastopen = TFO_CLIENT_ENABLE;
 	spin_lock_init(&net->ipv4.tcp_fastopen_ctx_lock);

net/ipv4/tcp_output.c

@@ -184,10 +184,10 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	if (unlikely(tp->compressed_ack > TCP_FASTRETRANS_THRESH)) {
+	if (unlikely(tp->compressed_ack)) {
 		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
-			      tp->compressed_ack - TCP_FASTRETRANS_THRESH);
-		tp->compressed_ack = TCP_FASTRETRANS_THRESH;
+			      tp->compressed_ack);
+		tp->compressed_ack = 0;
 		if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
 			__sock_put(sk);
 	}

net/ipv4/tcp_timer.c

@@ -753,8 +753,14 @@ static enum hrtimer_restart tcp_compressed_ack_kick(struct hrtimer *timer)
 
 	bh_lock_sock(sk);
 	if (!sock_owned_by_user(sk)) {
-		if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
+		if (tp->compressed_ack) {
+			/* Since we have to send one ack finally,
+			 * substract one from tp->compressed_ack to keep
+			 * LINUX_MIB_TCPACKCOMPRESSED accurate.
+			 */
+			tp->compressed_ack--;
 			tcp_send_ack(sk);
+		}
 	} else {
 		if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED,
 				      &sk->sk_tsq_flags))