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. */
25void 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}
41EXPORT_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 */
46void 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}
85EXPORT_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 */
91void 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}
109EXPORT_SYMBOL_GPL(tcp_plb_update_state_upon_rto);
110