110 lines
3.7 KiB
C
110 lines
3.7 KiB
C
/* Protective Load Balancing (PLB)
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*
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* PLB was designed to reduce link load imbalance across datacenter
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* switches. PLB is a host-based optimization; it leverages congestion
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* signals from the transport layer to randomly change the path of the
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* connection experiencing sustained congestion. PLB prefers to repath
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* after idle periods to minimize packet reordering. It repaths by
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* changing the IPv6 Flow Label on the packets of a connection, which
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* datacenter switches include as part of ECMP/WCMP hashing.
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*
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* PLB is described in detail in:
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*
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* Mubashir Adnan Qureshi, Yuchung Cheng, Qianwen Yin, Qiaobin Fu,
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* Gautam Kumar, Masoud Moshref, Junhua Yan, Van Jacobson,
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* David Wetherall,Abdul Kabbani:
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* "PLB: Congestion Signals are Simple and Effective for
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* Network Load Balancing"
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* In ACM SIGCOMM 2022, Amsterdam Netherlands.
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*
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*/
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#include <net/tcp.h>
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/* Called once per round-trip to update PLB state for a connection. */
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void tcp_plb_update_state(const struct sock *sk, struct tcp_plb_state *plb,
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const int cong_ratio)
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{
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struct net *net = sock_net(sk);
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if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
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return;
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if (cong_ratio >= 0) {
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if (cong_ratio < READ_ONCE(net->ipv4.sysctl_tcp_plb_cong_thresh))
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plb->consec_cong_rounds = 0;
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else if (plb->consec_cong_rounds <
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READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds))
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plb->consec_cong_rounds++;
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}
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}
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EXPORT_SYMBOL_GPL(tcp_plb_update_state);
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/* Check whether recent congestion has been persistent enough to warrant
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* a load balancing decision that switches the connection to another path.
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*/
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void tcp_plb_check_rehash(struct sock *sk, struct tcp_plb_state *plb)
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{
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struct net *net = sock_net(sk);
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u32 max_suspend;
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bool forced_rehash = false, idle_rehash = false;
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if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
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return;
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forced_rehash = plb->consec_cong_rounds >=
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READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds);
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/* If sender goes idle then we check whether to rehash. */
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idle_rehash = READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds) &&
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!tcp_sk(sk)->packets_out &&
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plb->consec_cong_rounds >=
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READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds);
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if (!forced_rehash && !idle_rehash)
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return;
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/* Note that tcp_jiffies32 can wrap; we detect wraps by checking for
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* cases where the max suspension end is before the actual suspension
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* end. We clear pause_until to 0 to indicate there is no recent
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* RTO event that constrains PLB rehashing.
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*/
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max_suspend = 2 * READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ;
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if (plb->pause_until &&
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(!before(tcp_jiffies32, plb->pause_until) ||
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before(tcp_jiffies32 + max_suspend, plb->pause_until)))
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plb->pause_until = 0;
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if (plb->pause_until)
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return;
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sk_rethink_txhash(sk);
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plb->consec_cong_rounds = 0;
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tcp_sk(sk)->plb_rehash++;
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NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPLBREHASH);
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}
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EXPORT_SYMBOL_GPL(tcp_plb_check_rehash);
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/* Upon RTO, disallow load balancing for a while, to avoid having load
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* balancing decisions switch traffic to a black-holed path that was
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* previously avoided with a sk_rethink_txhash() call at RTO time.
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*/
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void tcp_plb_update_state_upon_rto(struct sock *sk, struct tcp_plb_state *plb)
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{
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struct net *net = sock_net(sk);
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u32 pause;
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if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
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return;
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pause = READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ;
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pause += prandom_u32_max(pause);
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plb->pause_until = tcp_jiffies32 + pause;
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/* Reset PLB state upon RTO, since an RTO causes a sk_rethink_txhash() call
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* that may switch this connection to a path with completely different
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* congestion characteristics.
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*/
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plb->consec_cong_rounds = 0;
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}
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EXPORT_SYMBOL_GPL(tcp_plb_update_state_upon_rto);
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