tcp: add hrtimer slack to sack compression

Add a sysctl to control hrtimer slack, default of 100 usec.

This gives the opportunity to reduce system overhead,
and help very short RTT flows.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
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/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

@@ -5324,7 +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);