From a842fe1425cb20f457abd3f8ef98b468f83ca98b Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 12 Jun 2019 11:57:25 -0700 Subject: [PATCH] tcp: add optional per socket transmit delay Adding delays to TCP flows is crucial for studying behavior of TCP stacks, including congestion control modules. Linux offers netem module, but it has unpractical constraints : - Need root access to change qdisc - Hard to setup on egress if combined with non trivial qdisc like FQ - Single delay for all flows. EDT (Earliest Departure Time) adoption in TCP stack allows us to enable a per socket delay at a very small cost. Networking tools can now establish thousands of flows, each of them with a different delay, simulating real world conditions. This requires FQ packet scheduler or a EDT-enabled NIC. This patchs adds TCP_TX_DELAY socket option, to set a delay in usec units. unsigned int tx_delay = 10000; /* 10 msec */ setsockopt(fd, SOL_TCP, TCP_TX_DELAY, &tx_delay, sizeof(tx_delay)); Note that FQ packet scheduler limits might need some tweaking : man tc-fq PARAMETERS limit Hard limit on the real queue size. When this limit is reached, new packets are dropped. If the value is lowered, packets are dropped so that the new limit is met. Default is 10000 packets. flow_limit Hard limit on the maximum number of packets queued per flow. Default value is 100. Use of TCP_TX_DELAY option will increase number of skbs in FQ qdisc, so packets would be dropped if any of the previous limit is hit. Use of a jump label makes this support runtime-free, for hosts never using the option. Also note that TSQ (TCP Small Queues) limits are slightly changed with this patch : we need to account that skbs artificially delayed wont stop us providind more skbs to feed the pipe (netem uses skb_orphan_partial() for this purpose, but FQ can not use this trick) Because of that, using big delays might very well trigger old bugs in TSO auto defer logic and/or sndbuf limited detection. Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- include/linux/tcp.h | 2 ++ include/net/tcp.h | 19 +++++++++++++++++++ include/uapi/linux/tcp.h | 3 +++ net/ipv4/tcp.c | 24 ++++++++++++++++++++++++ net/ipv4/tcp_ipv4.c | 10 ++++++---- net/ipv4/tcp_minisocks.c | 2 +- net/ipv4/tcp_output.c | 23 ++++++++++++++++++++--- net/ipv6/tcp_ipv6.c | 1 + 8 files changed, 76 insertions(+), 8 deletions(-) diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 711361a..c23019a 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -245,6 +245,7 @@ struct tcp_sock { syn_smc:1; /* SYN includes SMC */ u32 tlp_high_seq; /* snd_nxt at the time of TLP retransmit. */ + u32 tcp_tx_delay; /* delay (in usec) added to TX packets */ u64 tcp_wstamp_ns; /* departure time for next sent data packet */ u64 tcp_clock_cache; /* cache last tcp_clock_ns() (see tcp_mstamp_refresh()) */ @@ -436,6 +437,7 @@ struct tcp_timewait_sock { u32 tw_last_oow_ack_time; int tw_ts_recent_stamp; + u32 tw_tx_delay; #ifdef CONFIG_TCP_MD5SIG struct tcp_md5sig_key *tw_md5_key; #endif diff --git a/include/net/tcp.h b/include/net/tcp.h index 204328b..49a178b 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -2232,4 +2232,23 @@ void clean_acked_data_disable(struct inet_connection_sock *icsk); void clean_acked_data_flush(void); #endif +DECLARE_STATIC_KEY_FALSE(tcp_tx_delay_enabled); +static inline void tcp_add_tx_delay(struct sk_buff *skb, + const struct tcp_sock *tp) +{ + if (static_branch_unlikely(&tcp_tx_delay_enabled)) + skb->skb_mstamp_ns += (u64)tp->tcp_tx_delay * NSEC_PER_USEC; +} + +static inline void tcp_set_tx_time(struct sk_buff *skb, + const struct sock *sk) +{ + if (static_branch_unlikely(&tcp_tx_delay_enabled)) { + u32 delay = (sk->sk_state == TCP_TIME_WAIT) ? + tcp_twsk(sk)->tw_tx_delay : tcp_sk(sk)->tcp_tx_delay; + + skb->skb_mstamp_ns = tcp_clock_ns() + (u64)delay * NSEC_PER_USEC; + } +} + #endif /* _TCP_H */ diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h index b521464..b3564f8 100644 --- a/include/uapi/linux/tcp.h +++ b/include/uapi/linux/tcp.h @@ -127,6 +127,9 @@ enum { #define TCP_CM_INQ TCP_INQ +#define TCP_TX_DELAY 37 /* delay outgoing packets by XX usec */ + + #define TCP_REPAIR_ON 1 #define TCP_REPAIR_OFF 0 #define TCP_REPAIR_OFF_NO_WP -1 /* Turn off without window probes */ diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index bd0856ac..5542e3d 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -2736,6 +2736,21 @@ static int tcp_repair_options_est(struct sock *sk, return 0; } +DEFINE_STATIC_KEY_FALSE(tcp_tx_delay_enabled); +EXPORT_SYMBOL(tcp_tx_delay_enabled); + +static void tcp_enable_tx_delay(void) +{ + if (!static_branch_unlikely(&tcp_tx_delay_enabled)) { + static int __tcp_tx_delay_enabled = 0; + + if (cmpxchg(&__tcp_tx_delay_enabled, 0, 1) == 0) { + static_branch_enable(&tcp_tx_delay_enabled); + pr_info("TCP_TX_DELAY enabled\n"); + } + } +} + /* * Socket option code for TCP. */ @@ -3087,6 +3102,11 @@ static int do_tcp_setsockopt(struct sock *sk, int level, else tp->recvmsg_inq = val; break; + case TCP_TX_DELAY: + if (val) + tcp_enable_tx_delay(); + tp->tcp_tx_delay = val; + break; default: err = -ENOPROTOOPT; break; @@ -3546,6 +3566,10 @@ static int do_tcp_getsockopt(struct sock *sk, int level, val = tp->fastopen_no_cookie; break; + case TCP_TX_DELAY: + val = tp->tcp_tx_delay; + break; + case TCP_TIMESTAMP: val = tcp_time_stamp_raw() + tp->tsoffset; break; diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index f059fbd..1b7e9e1 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -767,9 +767,11 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb) arg.uid = sock_net_uid(net, sk && sk_fullsock(sk) ? sk : NULL); local_bh_disable(); ctl_sk = this_cpu_read(*net->ipv4.tcp_sk); - if (sk) + if (sk) { ctl_sk->sk_mark = (sk->sk_state == TCP_TIME_WAIT) ? inet_twsk(sk)->tw_mark : sk->sk_mark; + tcp_set_tx_time(skb, sk); + } ip_send_unicast_reply(ctl_sk, skb, &TCP_SKB_CB(skb)->header.h4.opt, ip_hdr(skb)->saddr, ip_hdr(skb)->daddr, @@ -859,9 +861,9 @@ static void tcp_v4_send_ack(const struct sock *sk, arg.uid = sock_net_uid(net, sk_fullsock(sk) ? sk : NULL); local_bh_disable(); ctl_sk = this_cpu_read(*net->ipv4.tcp_sk); - if (sk) - ctl_sk->sk_mark = (sk->sk_state == TCP_TIME_WAIT) ? - inet_twsk(sk)->tw_mark : sk->sk_mark; + ctl_sk->sk_mark = (sk->sk_state == TCP_TIME_WAIT) ? + inet_twsk(sk)->tw_mark : sk->sk_mark; + tcp_set_tx_time(skb, sk); ip_send_unicast_reply(ctl_sk, skb, &TCP_SKB_CB(skb)->header.h4.opt, ip_hdr(skb)->saddr, ip_hdr(skb)->daddr, diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index 11011e8..8bcaf25 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -274,7 +274,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo) tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp; tcptw->tw_ts_offset = tp->tsoffset; tcptw->tw_last_oow_ack_time = 0; - + tcptw->tw_tx_delay = tp->tcp_tx_delay; #if IS_ENABLED(CONFIG_IPV6) if (tw->tw_family == PF_INET6) { struct ipv6_pinfo *np = inet6_sk(sk); diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index f429e85..d954ff9 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1153,6 +1153,8 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, memset(skb->cb, 0, max(sizeof(struct inet_skb_parm), sizeof(struct inet6_skb_parm))); + tcp_add_tx_delay(skb, tp); + err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl); if (unlikely(err > 0)) { @@ -2234,6 +2236,18 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb, sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes); limit <<= factor; + if (static_branch_unlikely(&tcp_tx_delay_enabled) && + tcp_sk(sk)->tcp_tx_delay) { + u64 extra_bytes = (u64)sk->sk_pacing_rate * tcp_sk(sk)->tcp_tx_delay; + + /* TSQ is based on skb truesize sum (sk_wmem_alloc), so we + * approximate our needs assuming an ~100% skb->truesize overhead. + * USEC_PER_SEC is approximated by 2^20. + * do_div(extra_bytes, USEC_PER_SEC/2) is replaced by a right shift. + */ + extra_bytes >>= (20 - 1); + limit += extra_bytes; + } if (refcount_read(&sk->sk_wmem_alloc) > limit) { /* Always send skb if rtx queue is empty. * No need to wait for TX completion to call us back, @@ -3212,6 +3226,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst, int tcp_header_size; struct tcphdr *th; int mss; + u64 now; skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC); if (unlikely(!skb)) { @@ -3243,13 +3258,14 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst, mss = tcp_mss_clamp(tp, dst_metric_advmss(dst)); memset(&opts, 0, sizeof(opts)); + now = tcp_clock_ns(); #ifdef CONFIG_SYN_COOKIES if (unlikely(req->cookie_ts)) skb->skb_mstamp_ns = cookie_init_timestamp(req); else #endif { - skb->skb_mstamp_ns = tcp_clock_ns(); + skb->skb_mstamp_ns = now; if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */ tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb); } @@ -3292,8 +3308,9 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst, rcu_read_unlock(); #endif - /* Do not fool tcpdump (if any), clean our debris */ - skb->tstamp = 0; + skb->skb_mstamp_ns = now; + tcp_add_tx_delay(skb, tp); + return skb; } EXPORT_SYMBOL(tcp_make_synack); diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c index ad70391..5606b21 100644 --- a/net/ipv6/tcp_ipv6.c +++ b/net/ipv6/tcp_ipv6.c @@ -892,6 +892,7 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32 } else { mark = sk->sk_mark; } + tcp_set_tx_time(buff, sk); } fl6.flowi6_mark = IP6_REPLY_MARK(net, skb->mark) ?: mark; fl6.fl6_dport = t1->dest; -- 2.7.4