Re: [net-next PATCH v2 2/3] net: TCP thin linear timeouts

[Damian Lukowski <damian@tvk.rwth-aachen.de>]

Andreas Petlund schrieb:
> Major change: Limit number of thin linear timeout tries to TCP_THIN_LT_RETRIES (currently 6).
> 
>>From ec71404702149bc9197c749e5d1d68656c87f98f Mon Sep 17 00:00:00 2001
> From: Andreas Petlund <apetlund@simula.no>
> Date: Mon, 8 Feb 2010 14:05:53 +0100
> Subject: [PATCH 2/3] net: TCP thin linear timeouts
> 
> 
> Signed-off-by: Andreas Petlund <apetlund@simula.no>
> ---
>  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             |    5 +++++
>  net/ipv4/tcp_timer.c       |   19 ++++++++++++++++++-
>  6 files changed, 38 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 9f236cd..d840d75 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -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 {
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 7fee8a4..67da706 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -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	*/
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index e5e2056..bc5856a 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -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;
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index 7e3712c..cb2ed35 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -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),
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index d5d69ea..cbc1ee3 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2229,6 +2229,11 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>  		}
>  		break;
>  
> +	case TCP_THIN_LT:
> +		if (val)
> +			tp->thin_lt = 1;
> +		break;
> +
>  	case TCP_CORK:
>  		/* When set indicates to always queue non-full frames.
>  		 * Later the user clears this option and we transmit
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index de7d1bf..f01a585 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -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 &&
> +	    icsk->icsk_retransmits <= TCP_THIN_LT_RETRIES) {
> +		icsk->icsk_backoff = 0;

Hi,
I think, this value should be at least 1, as icsk_backoff
might be decreased to -1 and used for bit-shifting in tcp_v4_err().
A lower boundary check might be even better.

Regards
 Damian


> +		icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
> +	} else {
> +		/* Use normal (exponential) backoff */
> +		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
> +	}
>  	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
>  	if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1))
>  		__sk_dst_reset(sk);