net: TCP thin linear timeouts

Major changes:
-Possible to disable mechanisms by socket option -Socket option value boundary check

Signed-off-by Andreas Petlund <[email protected]>

---
 include/linux/sysctl.h     |    1 +
 include/linux/tcp.h        |    3 +++
 include/net/tcp.h          |    4 ++++
 net/ipv4/sysctl_net_ipv4.c |    7 +++++++
 net/ipv4/tcp.c             |    7 +++++++
 net/ipv4/tcp_timer.c       |   19 ++++++++++++++++++-
 6 files changed, 40 insertions(+), 1 deletions(-)

include/linux/sysctl.h +1 -0

index 9f236cd..d840d75 100644
@@ -425,6 +425,7 @@ enum
 	NET_TCP_ALLOWED_CONG_CONTROL=123,
 	NET_TCP_MAX_SSTHRESH=124,
 	NET_TCP_FRTO_RESPONSE=125,
+	NET_TCP_FORCE_THIN_LINEAR_TIMEOUTS=126,
 };
 
 enum {

include/linux/tcp.h +3 -0

index 7fee8a4..67da706 100644
@@ -103,6 +103,7 @@ enum {
 #define TCP_CONGESTION		13	/* Congestion control algorithm */
 #define TCP_MD5SIG		14	/* TCP MD5 Signature (RFC2385) */
 #define TCP_COOKIE_TRANSACTIONS	15	/* TCP Cookie Transactions */
+#define TCP_THIN_LT             16      /* Use linear timeouts for thin streams*/
 
 /* for TCP_INFO socket option */
 #define TCPI_OPT_TIMESTAMPS	1

@@ -341,6 +342,8 @@ struct tcp_sock {
 	u16	advmss;		/* Advertised MSS			*/
 	u8	frto_counter;	/* Number of new acks after RTO */
 	u8	nonagle;	/* Disable Nagle algorithm?             */
+	u8      thin_lt     : 1,/* Use linear timeouts for thin streams */
+		thin_undef  : 7;
 
 /* RTT measurement */
 	u32	srtt;		/* smoothed round trip time << 3	*/

include/net/tcp.h +4 -0

index e5e2056..bc5856a 100644
@@ -196,6 +196,9 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCP_NAGLE_CORK		2	/* Socket is corked	    */
 #define TCP_NAGLE_PUSH		4	/* Cork is overridden for already queued data */
 
+/* TCP thin-stream limits */
+#define TCP_THIN_LT_RETRIES     6       /* After 6 linear retries, do exp. backoff */
+
 extern struct inet_timewait_death_row tcp_death_row;
 
 /* sysctl variables for tcp */

@@ -241,6 +244,7 @@ extern int sysctl_tcp_workaround_signed_windows;
 extern int sysctl_tcp_slow_start_after_idle;
 extern int sysctl_tcp_max_ssthresh;
 extern int sysctl_tcp_cookie_size;
+extern int sysctl_tcp_force_thin_linear_timeouts;
 
 extern atomic_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;

net/ipv4/sysctl_net_ipv4.c +7 -0

index 7e3712c..cb2ed35 100644
@@ -576,6 +576,13 @@ static struct ctl_table ipv4_table[] = {
 		.proc_handler	= proc_dointvec
 	},
 	{
+		.procname       = "tcp_force_thin_linear_timeouts",
+		.data           = &sysctl_tcp_force_thin_linear_timeouts,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec
+	},
+	{
 		.procname	= "udp_mem",
 		.data		= &sysctl_udp_mem,
 		.maxlen		= sizeof(sysctl_udp_mem),

net/ipv4/tcp.c +7 -0

index d5d69ea..ce9aeb0 100644
@@ -2229,6 +2229,13 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		}
 		break;
 
+	case TCP_THIN_LT:
+		if (val < 0 || val > 1)
+			err = -EINVAL;
+		else
+			tp->thin_lt = val;
+		break;
+
 	case TCP_CORK:
 		/* When set indicates to always queue non-full frames.
 		 * Later the user clears this option and we transmit

net/ipv4/tcp_timer.c +11 -1

index de7d1bf..a682479 100644
@@ -29,6 +29,7 @@ int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
 int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
 int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
 int sysctl_tcp_orphan_retries __read_mostly;
+int sysctl_tcp_force_thin_linear_timeouts __read_mostly;
 
 static void tcp_write_timer(unsigned long);
 static void tcp_delack_timer(unsigned long);

@@ -415,7 +416,23 @@ void tcp_retransmit_timer(struct sock *sk)
 	icsk->icsk_retransmits++;
 
 out_reset_timer:
-	icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
+	 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
+	 * might be increased if the stream oscillates between thin and thick,
+	 * thus the old value might already be too high compared to the value
+	 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
+	 * backoff. Limit to TCP_THIN_LT_RETRIES before initiating exponential
+	 * backoff behaviour to avoid continue hammering linear-timeout
+	 * retransmissions into a black hole*/
+	if ((tp->thin_lt || sysctl_tcp_force_thin_linear_timeouts) &&
+	    tcp_stream_is_thin(sk) && sk->sk_state == TCP_ESTABLISHED &&

Hi Anreas

Could you include a section in Documentation/networking/ip-sysctl.txt about tcp_force_thin_linear_timeouts setting ?

Also, you should provide some documentation to be included in man pages (man 7 tcp), to Michael Kerrisk ( <[email protected]> ), both for tcp_force_thin_linear_timeouts and TCP_THIN_LT

(Same applies for patch 3/3 and tcp_force_thin_dupack / TCP_THIN_DUPACK )

Last year, I'm pretty sure I was on record as thinking this is only a marginally good idea, that would be better at the application layer.

Also that naming was a bit dicey. Now the names are more descriptive, but the "force" is a bit overkill.

How about?

  NET_TCP_FORCE_THIN_LINEAR_TIMEOUTS    -> NET_TCP_THIN_LINEAR_TIMEOUTS
  TCP_THIN_LT                           -> TCP_THIN_LINEAR_TIMEOUTS
  TCP_THIN_LT_RETRIES                   -> TCP_THIN_LINEAR_RETRIES
  tcp_force_thin_linear_timeouts        -> tcp_thin_linear_timeouts

sysctl_tcp_force_thin_linear_timeouts -> sysctl_tcp_thin_linear_timeouts
tp->thin_lt -> tp->thin_lto

The latter mostly traditional "to" for "timeout", as used most everywhere.

Just for efficiency, I'd reorder this
+ if (sk->sk_state == TCP_ESTABLISHED &&

  +        (tp->thin_lt || sysctl_tcp_force_thin_linear_timeouts) &&
  +        tcp_stream_is_thin(sk) &&
  +        icsk->icsk_retransmits <= TCP_THIN_LT_RETRIES) {