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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);
}
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