xref: /openbmc/linux/net/ipv4/tcp_plb.c (revision bbaf1ff0)
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. */
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  */
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  */
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