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-rw-r--r--net/ipv4/tcp_plb.c109
1 files changed, 109 insertions, 0 deletions
diff --git a/net/ipv4/tcp_plb.c b/net/ipv4/tcp_plb.c
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+++ b/net/ipv4/tcp_plb.c
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+/* Protective Load Balancing (PLB)
+ *
+ * PLB was designed to reduce link load imbalance across datacenter
+ * switches. PLB is a host-based optimization; it leverages congestion
+ * signals from the transport layer to randomly change the path of the
+ * connection experiencing sustained congestion. PLB prefers to repath
+ * after idle periods to minimize packet reordering. It repaths by
+ * changing the IPv6 Flow Label on the packets of a connection, which
+ * datacenter switches include as part of ECMP/WCMP hashing.
+ *
+ * PLB is described in detail in:
+ *
+ * Mubashir Adnan Qureshi, Yuchung Cheng, Qianwen Yin, Qiaobin Fu,
+ * Gautam Kumar, Masoud Moshref, Junhua Yan, Van Jacobson,
+ * David Wetherall,Abdul Kabbani:
+ * "PLB: Congestion Signals are Simple and Effective for
+ * Network Load Balancing"
+ * In ACM SIGCOMM 2022, Amsterdam Netherlands.
+ *
+ */
+
+#include <net/tcp.h>
+
+/* Called once per round-trip to update PLB state for a connection. */
+void tcp_plb_update_state(const struct sock *sk, struct tcp_plb_state *plb,
+ const int cong_ratio)
+{
+ struct net *net = sock_net(sk);
+
+ if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
+ return;
+
+ if (cong_ratio >= 0) {
+ if (cong_ratio < READ_ONCE(net->ipv4.sysctl_tcp_plb_cong_thresh))
+ plb->consec_cong_rounds = 0;
+ else if (plb->consec_cong_rounds <
+ READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds))
+ plb->consec_cong_rounds++;
+ }
+}
+EXPORT_SYMBOL_GPL(tcp_plb_update_state);
+
+/* Check whether recent congestion has been persistent enough to warrant
+ * a load balancing decision that switches the connection to another path.
+ */
+void tcp_plb_check_rehash(struct sock *sk, struct tcp_plb_state *plb)
+{
+ struct net *net = sock_net(sk);
+ u32 max_suspend;
+ bool forced_rehash = false, idle_rehash = false;
+
+ if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
+ return;
+
+ forced_rehash = plb->consec_cong_rounds >=
+ READ_ONCE(net->ipv4.sysctl_tcp_plb_rehash_rounds);
+ /* If sender goes idle then we check whether to rehash. */
+ idle_rehash = READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds) &&
+ !tcp_sk(sk)->packets_out &&
+ plb->consec_cong_rounds >=
+ READ_ONCE(net->ipv4.sysctl_tcp_plb_idle_rehash_rounds);
+
+ if (!forced_rehash && !idle_rehash)
+ return;
+
+ /* Note that tcp_jiffies32 can wrap; we detect wraps by checking for
+ * cases where the max suspension end is before the actual suspension
+ * end. We clear pause_until to 0 to indicate there is no recent
+ * RTO event that constrains PLB rehashing.
+ */
+ max_suspend = 2 * READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ;
+ if (plb->pause_until &&
+ (!before(tcp_jiffies32, plb->pause_until) ||
+ before(tcp_jiffies32 + max_suspend, plb->pause_until)))
+ plb->pause_until = 0;
+
+ if (plb->pause_until)
+ return;
+
+ sk_rethink_txhash(sk);
+ plb->consec_cong_rounds = 0;
+ tcp_sk(sk)->plb_rehash++;
+ NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPLBREHASH);
+}
+EXPORT_SYMBOL_GPL(tcp_plb_check_rehash);
+
+/* Upon RTO, disallow load balancing for a while, to avoid having load
+ * balancing decisions switch traffic to a black-holed path that was
+ * previously avoided with a sk_rethink_txhash() call at RTO time.
+ */
+void tcp_plb_update_state_upon_rto(struct sock *sk, struct tcp_plb_state *plb)
+{
+ struct net *net = sock_net(sk);
+ u32 pause;
+
+ if (!READ_ONCE(net->ipv4.sysctl_tcp_plb_enabled))
+ return;
+
+ pause = READ_ONCE(net->ipv4.sysctl_tcp_plb_suspend_rto_sec) * HZ;
+ pause += get_random_u32_below(pause);
+ plb->pause_until = tcp_jiffies32 + pause;
+
+ /* Reset PLB state upon RTO, since an RTO causes a sk_rethink_txhash() call
+ * that may switch this connection to a path with completely different
+ * congestion characteristics.
+ */
+ plb->consec_cong_rounds = 0;
+}
+EXPORT_SYMBOL_GPL(tcp_plb_update_state_upon_rto);