1 /* Protective Load Balancing (PLB) 2 * 3 * PLB was designed to reduce link load imbalance across datacenter 4 * switches. PLB is a host-based optimization; it leverages congestion 5 * signals from the transport layer to randomly change the path of the 6 * connection experiencing sustained congestion. PLB prefers to repath 7 * after idle periods to minimize packet reordering. It repaths by 8 * changing the IPv6 Flow Label on the packets of a connection, which 9 * datacenter switches include as part of ECMP/WCMP hashing. 10 * 11 * PLB is described in detail in: 12 * 13 * Mubashir Adnan Qureshi, Yuchung Cheng, Qianwen Yin, Qiaobin Fu, 14 * Gautam Kumar, Masoud Moshref, Junhua Yan, Van Jacobson, 15 * David Wetherall,Abdul Kabbani: 16 * "PLB: Congestion Signals are Simple and Effective for 17 * Network Load Balancing" 18 * In ACM SIGCOMM 2022, Amsterdam Netherlands. 19 * 20 */ 21 22 #include <net/tcp.h> 23 24 /* Called once per round-trip to update PLB state for a connection. */ tcp_plb_update_state(const struct sock * sk,struct tcp_plb_state * plb,const int cong_ratio)25 void tcp_plb_update_state(const struct sock *sk, struct tcp_plb_state *plb, 26 const int cong_ratio) 27 { 28 struct net *net = sock_net(sk); 29 30 if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled)) 31 return; 32 33 if (cong_ratio >= 0) { 34 if (cong_ratio < READ_ONCE(net->ipv4.sysctl_tcp_plb_cong_thresh)) 35 plb->consec_cong_rounds = 0; 36 else if (plb->consec_cong_rounds < 37 READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds)) 38 plb->consec_cong_rounds++; 39 } 40 } 41 EXPORT_SYMBOL_GPL(tcp_plb_update_state); 42 43 /* Check whether recent congestion has been persistent enough to warrant 44 * a load balancing decision that switches the connection to another path. 45 */ tcp_plb_check_rehash(struct sock * sk,struct tcp_plb_state * plb)46 void tcp_plb_check_rehash(struct sock *sk, struct tcp_plb_state *plb) 47 { 48 struct net *net = sock_net(sk); 49 u32 max_suspend; 50 bool forced_rehash = false, idle_rehash = false; 51 52 if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled)) 53 return; 54 55 forced_rehash = plb->consec_cong_rounds >= 56 READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds); 57 /* If sender goes idle then we check whether to rehash. */ 58 idle_rehash = READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds) && 59 !tcp_sk(sk)->packets_out && 60 plb->consec_cong_rounds >= 61 READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds); 62 63 if (!forced_rehash && !idle_rehash) 64 return; 65 66 /* Note that tcp_jiffies32 can wrap; we detect wraps by checking for 67 * cases where the max suspension end is before the actual suspension 68 * end. We clear pause_until to 0 to indicate there is no recent 69 * RTO event that constrains PLB rehashing. 70 */ 71 max_suspend = 2 * READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ; 72 if (plb->pause_until && 73 (!before(tcp_jiffies32, plb->pause_until) || 74 before(tcp_jiffies32 + max_suspend, plb->pause_until))) 75 plb->pause_until = 0; 76 77 if (plb->pause_until) 78 return; 79 80 sk_rethink_txhash(sk); 81 plb->consec_cong_rounds = 0; 82 tcp_sk(sk)->plb_rehash++; 83 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPLBREHASH); 84 } 85 EXPORT_SYMBOL_GPL(tcp_plb_check_rehash); 86 87 /* Upon RTO, disallow load balancing for a while, to avoid having load 88 * balancing decisions switch traffic to a black-holed path that was 89 * previously avoided with a sk_rethink_txhash() call at RTO time. 90 */ tcp_plb_update_state_upon_rto(struct sock * sk,struct tcp_plb_state * plb)91 void tcp_plb_update_state_upon_rto(struct sock *sk, struct tcp_plb_state *plb) 92 { 93 struct net *net = sock_net(sk); 94 u32 pause; 95 96 if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled)) 97 return; 98 99 pause = READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ; 100 pause += get_random_u32_below(pause); 101 plb->pause_until = tcp_jiffies32 + pause; 102 103 /* Reset PLB state upon RTO, since an RTO causes a sk_rethink_txhash() call 104 * that may switch this connection to a path with completely different 105 * congestion characteristics. 106 */ 107 plb->consec_cong_rounds = 0; 108 } 109 EXPORT_SYMBOL_GPL(tcp_plb_update_state_upon_rto); 110