b9ce204f0a
Commit 164891aadf broke RTT
sampling of congestion control modules. Inaccurate timestamps
could be fed to them without providing any way for them to
identify such cases. Previously RTT sampler was called only if
FLAG_RETRANS_DATA_ACKED was not set filtering inaccurate
timestamps nicely. In addition, the new behavior could give an
invalid timestamp (zero) to RTT sampler if only skbs with
TCPCB_RETRANS were ACKed. This solves both problems.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
388 lines
12 KiB
C
388 lines
12 KiB
C
/*
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* TCP Vegas congestion control
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*
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* This is based on the congestion detection/avoidance scheme described in
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* Lawrence S. Brakmo and Larry L. Peterson.
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* "TCP Vegas: End to end congestion avoidance on a global internet."
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* IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
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* October 1995. Available from:
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* ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
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*
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* See http://www.cs.arizona.edu/xkernel/ for their implementation.
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* The main aspects that distinguish this implementation from the
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* Arizona Vegas implementation are:
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* o We do not change the loss detection or recovery mechanisms of
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* Linux in any way. Linux already recovers from losses quite well,
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* using fine-grained timers, NewReno, and FACK.
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* o To avoid the performance penalty imposed by increasing cwnd
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* only every-other RTT during slow start, we increase during
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* every RTT during slow start, just like Reno.
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* o Largely to allow continuous cwnd growth during slow start,
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* we use the rate at which ACKs come back as the "actual"
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* rate, rather than the rate at which data is sent.
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* o To speed convergence to the right rate, we set the cwnd
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* to achieve the right ("actual") rate when we exit slow start.
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* o To filter out the noise caused by delayed ACKs, we use the
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* minimum RTT sample observed during the last RTT to calculate
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* the actual rate.
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* o When the sender re-starts from idle, it waits until it has
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* received ACKs for an entire flight of new data before making
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* a cwnd adjustment decision. The original Vegas implementation
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* assumed senders never went idle.
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*/
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/skbuff.h>
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#include <linux/inet_diag.h>
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#include <net/tcp.h>
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#include "tcp_vegas.h"
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/* Default values of the Vegas variables, in fixed-point representation
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* with V_PARAM_SHIFT bits to the right of the binary point.
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*/
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#define V_PARAM_SHIFT 1
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static int alpha = 2<<V_PARAM_SHIFT;
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static int beta = 4<<V_PARAM_SHIFT;
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static int gamma = 1<<V_PARAM_SHIFT;
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module_param(alpha, int, 0644);
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MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)");
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module_param(beta, int, 0644);
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MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)");
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module_param(gamma, int, 0644);
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MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
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/* There are several situations when we must "re-start" Vegas:
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*
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* o when a connection is established
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* o after an RTO
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* o after fast recovery
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* o when we send a packet and there is no outstanding
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* unacknowledged data (restarting an idle connection)
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*
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* In these circumstances we cannot do a Vegas calculation at the
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* end of the first RTT, because any calculation we do is using
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* stale info -- both the saved cwnd and congestion feedback are
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* stale.
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*
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* Instead we must wait until the completion of an RTT during
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* which we actually receive ACKs.
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*/
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static void vegas_enable(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct vegas *vegas = inet_csk_ca(sk);
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/* Begin taking Vegas samples next time we send something. */
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vegas->doing_vegas_now = 1;
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/* Set the beginning of the next send window. */
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vegas->beg_snd_nxt = tp->snd_nxt;
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vegas->cntRTT = 0;
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vegas->minRTT = 0x7fffffff;
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}
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/* Stop taking Vegas samples for now. */
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static inline void vegas_disable(struct sock *sk)
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{
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struct vegas *vegas = inet_csk_ca(sk);
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vegas->doing_vegas_now = 0;
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}
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void tcp_vegas_init(struct sock *sk)
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{
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struct vegas *vegas = inet_csk_ca(sk);
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vegas->baseRTT = 0x7fffffff;
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vegas_enable(sk);
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}
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EXPORT_SYMBOL_GPL(tcp_vegas_init);
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/* Do RTT sampling needed for Vegas.
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* Basically we:
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* o min-filter RTT samples from within an RTT to get the current
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* propagation delay + queuing delay (we are min-filtering to try to
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* avoid the effects of delayed ACKs)
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* o min-filter RTT samples from a much longer window (forever for now)
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* to find the propagation delay (baseRTT)
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*/
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void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, ktime_t last)
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{
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struct vegas *vegas = inet_csk_ca(sk);
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u32 vrtt;
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if (ktime_equal(last, net_invalid_timestamp()))
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return;
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/* Never allow zero rtt or baseRTT */
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vrtt = ktime_to_us(net_timedelta(last)) + 1;
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/* Filter to find propagation delay: */
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if (vrtt < vegas->baseRTT)
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vegas->baseRTT = vrtt;
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/* Find the min RTT during the last RTT to find
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* the current prop. delay + queuing delay:
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*/
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vegas->minRTT = min(vegas->minRTT, vrtt);
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vegas->cntRTT++;
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}
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EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);
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void tcp_vegas_state(struct sock *sk, u8 ca_state)
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{
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if (ca_state == TCP_CA_Open)
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vegas_enable(sk);
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else
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vegas_disable(sk);
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}
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EXPORT_SYMBOL_GPL(tcp_vegas_state);
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/*
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* If the connection is idle and we are restarting,
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* then we don't want to do any Vegas calculations
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* until we get fresh RTT samples. So when we
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* restart, we reset our Vegas state to a clean
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* slate. After we get acks for this flight of
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* packets, _then_ we can make Vegas calculations
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* again.
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*/
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void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
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{
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if (event == CA_EVENT_CWND_RESTART ||
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event == CA_EVENT_TX_START)
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tcp_vegas_init(sk);
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}
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EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
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static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack,
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u32 seq_rtt, u32 in_flight, int flag)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct vegas *vegas = inet_csk_ca(sk);
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if (!vegas->doing_vegas_now)
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return tcp_reno_cong_avoid(sk, ack, seq_rtt, in_flight, flag);
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/* The key players are v_beg_snd_una and v_beg_snd_nxt.
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*
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* These are so named because they represent the approximate values
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* of snd_una and snd_nxt at the beginning of the current RTT. More
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* precisely, they represent the amount of data sent during the RTT.
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* At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
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* we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
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* bytes of data have been ACKed during the course of the RTT, giving
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* an "actual" rate of:
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*
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* (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
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*
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* Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
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* because delayed ACKs can cover more than one segment, so they
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* don't line up nicely with the boundaries of RTTs.
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*
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* Another unfortunate fact of life is that delayed ACKs delay the
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* advance of the left edge of our send window, so that the number
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* of bytes we send in an RTT is often less than our cwnd will allow.
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* So we keep track of our cwnd separately, in v_beg_snd_cwnd.
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*/
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if (after(ack, vegas->beg_snd_nxt)) {
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/* Do the Vegas once-per-RTT cwnd adjustment. */
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u32 old_wnd, old_snd_cwnd;
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/* Here old_wnd is essentially the window of data that was
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* sent during the previous RTT, and has all
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* been acknowledged in the course of the RTT that ended
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* with the ACK we just received. Likewise, old_snd_cwnd
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* is the cwnd during the previous RTT.
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*/
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old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) /
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tp->mss_cache;
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old_snd_cwnd = vegas->beg_snd_cwnd;
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/* Save the extent of the current window so we can use this
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* at the end of the next RTT.
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*/
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vegas->beg_snd_una = vegas->beg_snd_nxt;
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vegas->beg_snd_nxt = tp->snd_nxt;
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vegas->beg_snd_cwnd = tp->snd_cwnd;
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/* We do the Vegas calculations only if we got enough RTT
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* samples that we can be reasonably sure that we got
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* at least one RTT sample that wasn't from a delayed ACK.
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* If we only had 2 samples total,
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* then that means we're getting only 1 ACK per RTT, which
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* means they're almost certainly delayed ACKs.
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* If we have 3 samples, we should be OK.
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*/
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if (vegas->cntRTT <= 2) {
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/* We don't have enough RTT samples to do the Vegas
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* calculation, so we'll behave like Reno.
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*/
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tcp_reno_cong_avoid(sk, ack, seq_rtt, in_flight, flag);
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} else {
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u32 rtt, target_cwnd, diff;
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/* We have enough RTT samples, so, using the Vegas
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* algorithm, we determine if we should increase or
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* decrease cwnd, and by how much.
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*/
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/* Pluck out the RTT we are using for the Vegas
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* calculations. This is the min RTT seen during the
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* last RTT. Taking the min filters out the effects
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* of delayed ACKs, at the cost of noticing congestion
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* a bit later.
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*/
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rtt = vegas->minRTT;
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/* Calculate the cwnd we should have, if we weren't
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* going too fast.
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*
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* This is:
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* (actual rate in segments) * baseRTT
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* We keep it as a fixed point number with
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* V_PARAM_SHIFT bits to the right of the binary point.
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*/
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target_cwnd = ((old_wnd * vegas->baseRTT)
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<< V_PARAM_SHIFT) / rtt;
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/* Calculate the difference between the window we had,
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* and the window we would like to have. This quantity
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* is the "Diff" from the Arizona Vegas papers.
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*
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* Again, this is a fixed point number with
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* V_PARAM_SHIFT bits to the right of the binary
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* point.
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*/
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diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
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if (tp->snd_cwnd <= tp->snd_ssthresh) {
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/* Slow start. */
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if (diff > gamma) {
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/* Going too fast. Time to slow down
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* and switch to congestion avoidance.
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*/
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tp->snd_ssthresh = 2;
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/* Set cwnd to match the actual rate
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* exactly:
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* cwnd = (actual rate) * baseRTT
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* Then we add 1 because the integer
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* truncation robs us of full link
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* utilization.
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*/
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tp->snd_cwnd = min(tp->snd_cwnd,
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(target_cwnd >>
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V_PARAM_SHIFT)+1);
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}
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tcp_slow_start(tp);
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} else {
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/* Congestion avoidance. */
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u32 next_snd_cwnd;
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/* Figure out where we would like cwnd
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* to be.
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*/
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if (diff > beta) {
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/* The old window was too fast, so
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* we slow down.
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*/
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next_snd_cwnd = old_snd_cwnd - 1;
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} else if (diff < alpha) {
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/* We don't have enough extra packets
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* in the network, so speed up.
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*/
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next_snd_cwnd = old_snd_cwnd + 1;
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} else {
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/* Sending just as fast as we
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* should be.
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*/
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next_snd_cwnd = old_snd_cwnd;
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}
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/* Adjust cwnd upward or downward, toward the
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* desired value.
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*/
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if (next_snd_cwnd > tp->snd_cwnd)
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tp->snd_cwnd++;
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else if (next_snd_cwnd < tp->snd_cwnd)
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tp->snd_cwnd--;
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}
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if (tp->snd_cwnd < 2)
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tp->snd_cwnd = 2;
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else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
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tp->snd_cwnd = tp->snd_cwnd_clamp;
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}
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/* Wipe the slate clean for the next RTT. */
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vegas->cntRTT = 0;
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vegas->minRTT = 0x7fffffff;
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}
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/* Use normal slow start */
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else if (tp->snd_cwnd <= tp->snd_ssthresh)
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tcp_slow_start(tp);
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}
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/* Extract info for Tcp socket info provided via netlink. */
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void tcp_vegas_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
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{
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const struct vegas *ca = inet_csk_ca(sk);
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if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
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struct tcpvegas_info info = {
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.tcpv_enabled = ca->doing_vegas_now,
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.tcpv_rttcnt = ca->cntRTT,
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.tcpv_rtt = ca->baseRTT,
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.tcpv_minrtt = ca->minRTT,
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};
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nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
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}
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}
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EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
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static struct tcp_congestion_ops tcp_vegas = {
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.flags = TCP_CONG_RTT_STAMP,
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.init = tcp_vegas_init,
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.ssthresh = tcp_reno_ssthresh,
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.cong_avoid = tcp_vegas_cong_avoid,
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.min_cwnd = tcp_reno_min_cwnd,
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.pkts_acked = tcp_vegas_pkts_acked,
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.set_state = tcp_vegas_state,
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.cwnd_event = tcp_vegas_cwnd_event,
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.get_info = tcp_vegas_get_info,
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.owner = THIS_MODULE,
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.name = "vegas",
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};
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static int __init tcp_vegas_register(void)
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{
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BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
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tcp_register_congestion_control(&tcp_vegas);
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return 0;
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}
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static void __exit tcp_vegas_unregister(void)
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{
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tcp_unregister_congestion_control(&tcp_vegas);
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}
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module_init(tcp_vegas_register);
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module_exit(tcp_vegas_unregister);
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MODULE_AUTHOR("Stephen Hemminger");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("TCP Vegas");
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