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-rw-r--r--net/rxrpc/rtt.c195
1 files changed, 195 insertions, 0 deletions
diff --git a/net/rxrpc/rtt.c b/net/rxrpc/rtt.c
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+// SPDX-License-Identifier: GPL-2.0
+/* RTT/RTO calculation.
+ *
+ * Adapted from TCP for AF_RXRPC by David Howells (dhowells@redhat.com)
+ *
+ * https://tools.ietf.org/html/rfc6298
+ * https://tools.ietf.org/html/rfc1122#section-4.2.3.1
+ * http://ccr.sigcomm.org/archive/1995/jan95/ccr-9501-partridge87.pdf
+ */
+
+#include <linux/net.h>
+#include "ar-internal.h"
+
+#define RXRPC_RTO_MAX ((unsigned)(120 * HZ))
+#define RXRPC_TIMEOUT_INIT ((unsigned)(1*HZ)) /* RFC6298 2.1 initial RTO value */
+#define rxrpc_jiffies32 ((u32)jiffies) /* As rxrpc_jiffies32 */
+#define rxrpc_min_rtt_wlen 300 /* As sysctl_tcp_min_rtt_wlen */
+
+static u32 rxrpc_rto_min_us(struct rxrpc_peer *peer)
+{
+ return 200;
+}
+
+static u32 __rxrpc_set_rto(const struct rxrpc_peer *peer)
+{
+ return _usecs_to_jiffies((peer->srtt_us >> 3) + peer->rttvar_us);
+}
+
+static u32 rxrpc_bound_rto(u32 rto)
+{
+ return min(rto, RXRPC_RTO_MAX);
+}
+
+/*
+ * Called to compute a smoothed rtt estimate. The data fed to this
+ * routine either comes from timestamps, or from segments that were
+ * known _not_ to have been retransmitted [see Karn/Partridge
+ * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
+ * piece by Van Jacobson.
+ * NOTE: the next three routines used to be one big routine.
+ * To save cycles in the RFC 1323 implementation it was better to break
+ * it up into three procedures. -- erics
+ */
+static void rxrpc_rtt_estimator(struct rxrpc_peer *peer, long sample_rtt_us)
+{
+ long m = sample_rtt_us; /* RTT */
+ u32 srtt = peer->srtt_us;
+
+ /* The following amusing code comes from Jacobson's
+ * article in SIGCOMM '88. Note that rtt and mdev
+ * are scaled versions of rtt and mean deviation.
+ * This is designed to be as fast as possible
+ * m stands for "measurement".
+ *
+ * On a 1990 paper the rto value is changed to:
+ * RTO = rtt + 4 * mdev
+ *
+ * Funny. This algorithm seems to be very broken.
+ * These formulae increase RTO, when it should be decreased, increase
+ * too slowly, when it should be increased quickly, decrease too quickly
+ * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
+ * does not matter how to _calculate_ it. Seems, it was trap
+ * that VJ failed to avoid. 8)
+ */
+ if (srtt != 0) {
+ m -= (srtt >> 3); /* m is now error in rtt est */
+ srtt += m; /* rtt = 7/8 rtt + 1/8 new */
+ if (m < 0) {
+ m = -m; /* m is now abs(error) */
+ m -= (peer->mdev_us >> 2); /* similar update on mdev */
+ /* This is similar to one of Eifel findings.
+ * Eifel blocks mdev updates when rtt decreases.
+ * This solution is a bit different: we use finer gain
+ * for mdev in this case (alpha*beta).
+ * Like Eifel it also prevents growth of rto,
+ * but also it limits too fast rto decreases,
+ * happening in pure Eifel.
+ */
+ if (m > 0)
+ m >>= 3;
+ } else {
+ m -= (peer->mdev_us >> 2); /* similar update on mdev */
+ }
+
+ peer->mdev_us += m; /* mdev = 3/4 mdev + 1/4 new */
+ if (peer->mdev_us > peer->mdev_max_us) {
+ peer->mdev_max_us = peer->mdev_us;
+ if (peer->mdev_max_us > peer->rttvar_us)
+ peer->rttvar_us = peer->mdev_max_us;
+ }
+ } else {
+ /* no previous measure. */
+ srtt = m << 3; /* take the measured time to be rtt */
+ peer->mdev_us = m << 1; /* make sure rto = 3*rtt */
+ peer->rttvar_us = max(peer->mdev_us, rxrpc_rto_min_us(peer));
+ peer->mdev_max_us = peer->rttvar_us;
+ }
+
+ peer->srtt_us = max(1U, srtt);
+}
+
+/*
+ * Calculate rto without backoff. This is the second half of Van Jacobson's
+ * routine referred to above.
+ */
+static void rxrpc_set_rto(struct rxrpc_peer *peer)
+{
+ u32 rto;
+
+ /* 1. If rtt variance happened to be less 50msec, it is hallucination.
+ * It cannot be less due to utterly erratic ACK generation made
+ * at least by solaris and freebsd. "Erratic ACKs" has _nothing_
+ * to do with delayed acks, because at cwnd>2 true delack timeout
+ * is invisible. Actually, Linux-2.4 also generates erratic
+ * ACKs in some circumstances.
+ */
+ rto = __rxrpc_set_rto(peer);
+
+ /* 2. Fixups made earlier cannot be right.
+ * If we do not estimate RTO correctly without them,
+ * all the algo is pure shit and should be replaced
+ * with correct one. It is exactly, which we pretend to do.
+ */
+
+ /* NOTE: clamping at RXRPC_RTO_MIN is not required, current algo
+ * guarantees that rto is higher.
+ */
+ peer->rto_j = rxrpc_bound_rto(rto);
+}
+
+static void rxrpc_ack_update_rtt(struct rxrpc_peer *peer, long rtt_us)
+{
+ if (rtt_us < 0)
+ return;
+
+ //rxrpc_update_rtt_min(peer, rtt_us);
+ rxrpc_rtt_estimator(peer, rtt_us);
+ rxrpc_set_rto(peer);
+
+ /* RFC6298: only reset backoff on valid RTT measurement. */
+ peer->backoff = 0;
+}
+
+/*
+ * Add RTT information to cache. This is called in softirq mode and has
+ * exclusive access to the peer RTT data.
+ */
+void rxrpc_peer_add_rtt(struct rxrpc_call *call, enum rxrpc_rtt_rx_trace why,
+ rxrpc_serial_t send_serial, rxrpc_serial_t resp_serial,
+ ktime_t send_time, ktime_t resp_time)
+{
+ struct rxrpc_peer *peer = call->peer;
+ s64 rtt_us;
+
+ rtt_us = ktime_to_us(ktime_sub(resp_time, send_time));
+ if (rtt_us < 0)
+ return;
+
+ spin_lock(&peer->rtt_input_lock);
+ rxrpc_ack_update_rtt(peer, rtt_us);
+ if (peer->rtt_count < 3)
+ peer->rtt_count++;
+ spin_unlock(&peer->rtt_input_lock);
+
+ trace_rxrpc_rtt_rx(call, why, send_serial, resp_serial,
+ peer->srtt_us >> 3, peer->rto_j);
+}
+
+/*
+ * Get the retransmission timeout to set in jiffies, backing it off each time
+ * we retransmit.
+ */
+unsigned long rxrpc_get_rto_backoff(struct rxrpc_peer *peer, bool retrans)
+{
+ u64 timo_j;
+ u8 backoff = READ_ONCE(peer->backoff);
+
+ timo_j = peer->rto_j;
+ timo_j <<= backoff;
+ if (retrans && timo_j * 2 <= RXRPC_RTO_MAX)
+ WRITE_ONCE(peer->backoff, backoff + 1);
+
+ if (timo_j < 1)
+ timo_j = 1;
+
+ return timo_j;
+}
+
+void rxrpc_peer_init_rtt(struct rxrpc_peer *peer)
+{
+ peer->rto_j = RXRPC_TIMEOUT_INIT;
+ peer->mdev_us = jiffies_to_usecs(RXRPC_TIMEOUT_INIT);
+ peer->backoff = 0;
+ //minmax_reset(&peer->rtt_min, rxrpc_jiffies32, ~0U);
+}