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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 += prandom_u32_max(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);
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