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![Eric Dumazet](/assets/img/avatar_default.png)
We had various bugs over the years with code
breaking the assumption that tp->snd_cwnd is greater
than zero.
Lately, syzbot reported the WARN_ON_ONCE(!tp->prior_cwnd) added
in commit 8b8a321ff7
("tcp: fix zero cwnd in tcp_cwnd_reduction")
can trigger, and without a repro we would have to spend
considerable time finding the bug.
Instead of complaining too late, we want to catch where
and when tp->snd_cwnd is set to an illegal value.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Link: https://lore.kernel.org/r/20220405233538.947344-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
309 lines
8.3 KiB
C
309 lines
8.3 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* TCP Westwood+: end-to-end bandwidth estimation for TCP
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*
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* Angelo Dell'Aera: author of the first version of TCP Westwood+ in Linux 2.4
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*
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* Support at http://c3lab.poliba.it/index.php/Westwood
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* Main references in literature:
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*
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* - Mascolo S, Casetti, M. Gerla et al.
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* "TCP Westwood: bandwidth estimation for TCP" Proc. ACM Mobicom 2001
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*
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* - A. Grieco, s. Mascolo
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* "Performance evaluation of New Reno, Vegas, Westwood+ TCP" ACM Computer
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* Comm. Review, 2004
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*
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* - A. Dell'Aera, L. Grieco, S. Mascolo.
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* "Linux 2.4 Implementation of Westwood+ TCP with Rate-Halving :
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* A Performance Evaluation Over the Internet" (ICC 2004), Paris, June 2004
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*
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* Westwood+ employs end-to-end bandwidth measurement to set cwnd and
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* ssthresh after packet loss. The probing phase is as the original Reno.
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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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/* TCP Westwood structure */
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struct westwood {
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u32 bw_ns_est; /* first bandwidth estimation..not too smoothed 8) */
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u32 bw_est; /* bandwidth estimate */
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u32 rtt_win_sx; /* here starts a new evaluation... */
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u32 bk;
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u32 snd_una; /* used for evaluating the number of acked bytes */
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u32 cumul_ack;
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u32 accounted;
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u32 rtt;
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u32 rtt_min; /* minimum observed RTT */
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u8 first_ack; /* flag which infers that this is the first ack */
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u8 reset_rtt_min; /* Reset RTT min to next RTT sample*/
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};
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/* TCP Westwood functions and constants */
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#define TCP_WESTWOOD_RTT_MIN (HZ/20) /* 50ms */
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#define TCP_WESTWOOD_INIT_RTT (20*HZ) /* maybe too conservative?! */
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/*
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* @tcp_westwood_create
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* This function initializes fields used in TCP Westwood+,
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* it is called after the initial SYN, so the sequence numbers
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* are correct but new passive connections we have no
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* information about RTTmin at this time so we simply set it to
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* TCP_WESTWOOD_INIT_RTT. This value was chosen to be too conservative
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* since in this way we're sure it will be updated in a consistent
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* way as soon as possible. It will reasonably happen within the first
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* RTT period of the connection lifetime.
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*/
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static void tcp_westwood_init(struct sock *sk)
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{
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struct westwood *w = inet_csk_ca(sk);
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w->bk = 0;
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w->bw_ns_est = 0;
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w->bw_est = 0;
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w->accounted = 0;
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w->cumul_ack = 0;
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w->reset_rtt_min = 1;
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w->rtt_min = w->rtt = TCP_WESTWOOD_INIT_RTT;
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w->rtt_win_sx = tcp_jiffies32;
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w->snd_una = tcp_sk(sk)->snd_una;
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w->first_ack = 1;
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}
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/*
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* @westwood_do_filter
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* Low-pass filter. Implemented using constant coefficients.
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*/
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static inline u32 westwood_do_filter(u32 a, u32 b)
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{
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return ((7 * a) + b) >> 3;
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}
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static void westwood_filter(struct westwood *w, u32 delta)
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{
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/* If the filter is empty fill it with the first sample of bandwidth */
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if (w->bw_ns_est == 0 && w->bw_est == 0) {
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w->bw_ns_est = w->bk / delta;
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w->bw_est = w->bw_ns_est;
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} else {
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w->bw_ns_est = westwood_do_filter(w->bw_ns_est, w->bk / delta);
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w->bw_est = westwood_do_filter(w->bw_est, w->bw_ns_est);
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}
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}
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/*
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* @westwood_pkts_acked
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* Called after processing group of packets.
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* but all westwood needs is the last sample of srtt.
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*/
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static void tcp_westwood_pkts_acked(struct sock *sk,
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const struct ack_sample *sample)
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{
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struct westwood *w = inet_csk_ca(sk);
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if (sample->rtt_us > 0)
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w->rtt = usecs_to_jiffies(sample->rtt_us);
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}
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/*
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* @westwood_update_window
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* It updates RTT evaluation window if it is the right moment to do
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* it. If so it calls filter for evaluating bandwidth.
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*/
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static void westwood_update_window(struct sock *sk)
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{
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struct westwood *w = inet_csk_ca(sk);
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s32 delta = tcp_jiffies32 - w->rtt_win_sx;
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/* Initialize w->snd_una with the first acked sequence number in order
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* to fix mismatch between tp->snd_una and w->snd_una for the first
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* bandwidth sample
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*/
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if (w->first_ack) {
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w->snd_una = tcp_sk(sk)->snd_una;
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w->first_ack = 0;
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}
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/*
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* See if a RTT-window has passed.
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* Be careful since if RTT is less than
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* 50ms we don't filter but we continue 'building the sample'.
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* This minimum limit was chosen since an estimation on small
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* time intervals is better to avoid...
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* Obviously on a LAN we reasonably will always have
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* right_bound = left_bound + WESTWOOD_RTT_MIN
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*/
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if (w->rtt && delta > max_t(u32, w->rtt, TCP_WESTWOOD_RTT_MIN)) {
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westwood_filter(w, delta);
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w->bk = 0;
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w->rtt_win_sx = tcp_jiffies32;
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}
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}
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static inline void update_rtt_min(struct westwood *w)
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{
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if (w->reset_rtt_min) {
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w->rtt_min = w->rtt;
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w->reset_rtt_min = 0;
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} else
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w->rtt_min = min(w->rtt, w->rtt_min);
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}
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/*
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* @westwood_fast_bw
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* It is called when we are in fast path. In particular it is called when
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* header prediction is successful. In such case in fact update is
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* straight forward and doesn't need any particular care.
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*/
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static inline void westwood_fast_bw(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct westwood *w = inet_csk_ca(sk);
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westwood_update_window(sk);
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w->bk += tp->snd_una - w->snd_una;
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w->snd_una = tp->snd_una;
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update_rtt_min(w);
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}
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/*
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* @westwood_acked_count
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* This function evaluates cumul_ack for evaluating bk in case of
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* delayed or partial acks.
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*/
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static inline u32 westwood_acked_count(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct westwood *w = inet_csk_ca(sk);
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w->cumul_ack = tp->snd_una - w->snd_una;
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/* If cumul_ack is 0 this is a dupack since it's not moving
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* tp->snd_una.
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*/
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if (!w->cumul_ack) {
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w->accounted += tp->mss_cache;
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w->cumul_ack = tp->mss_cache;
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}
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if (w->cumul_ack > tp->mss_cache) {
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/* Partial or delayed ack */
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if (w->accounted >= w->cumul_ack) {
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w->accounted -= w->cumul_ack;
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w->cumul_ack = tp->mss_cache;
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} else {
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w->cumul_ack -= w->accounted;
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w->accounted = 0;
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}
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}
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w->snd_una = tp->snd_una;
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return w->cumul_ack;
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}
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/*
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* TCP Westwood
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* Here limit is evaluated as Bw estimation*RTTmin (for obtaining it
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* in packets we use mss_cache). Rttmin is guaranteed to be >= 2
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* so avoids ever returning 0.
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*/
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static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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const struct westwood *w = inet_csk_ca(sk);
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return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
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}
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static void tcp_westwood_ack(struct sock *sk, u32 ack_flags)
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{
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if (ack_flags & CA_ACK_SLOWPATH) {
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struct westwood *w = inet_csk_ca(sk);
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westwood_update_window(sk);
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w->bk += westwood_acked_count(sk);
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update_rtt_min(w);
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return;
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}
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westwood_fast_bw(sk);
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}
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static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct westwood *w = inet_csk_ca(sk);
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switch (event) {
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case CA_EVENT_COMPLETE_CWR:
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tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
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tcp_snd_cwnd_set(tp, tp->snd_ssthresh);
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break;
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case CA_EVENT_LOSS:
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tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
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/* Update RTT_min when next ack arrives */
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w->reset_rtt_min = 1;
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break;
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default:
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/* don't care */
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break;
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}
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}
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/* Extract info for Tcp socket info provided via netlink. */
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static size_t tcp_westwood_info(struct sock *sk, u32 ext, int *attr,
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union tcp_cc_info *info)
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{
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const struct westwood *ca = inet_csk_ca(sk);
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if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
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info->vegas.tcpv_enabled = 1;
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info->vegas.tcpv_rttcnt = 0;
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info->vegas.tcpv_rtt = jiffies_to_usecs(ca->rtt);
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info->vegas.tcpv_minrtt = jiffies_to_usecs(ca->rtt_min);
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*attr = INET_DIAG_VEGASINFO;
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return sizeof(struct tcpvegas_info);
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}
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return 0;
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}
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static struct tcp_congestion_ops tcp_westwood __read_mostly = {
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.init = tcp_westwood_init,
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.ssthresh = tcp_reno_ssthresh,
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.cong_avoid = tcp_reno_cong_avoid,
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.undo_cwnd = tcp_reno_undo_cwnd,
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.cwnd_event = tcp_westwood_event,
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.in_ack_event = tcp_westwood_ack,
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.get_info = tcp_westwood_info,
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.pkts_acked = tcp_westwood_pkts_acked,
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.owner = THIS_MODULE,
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.name = "westwood"
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};
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static int __init tcp_westwood_register(void)
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{
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BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
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return tcp_register_congestion_control(&tcp_westwood);
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}
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static void __exit tcp_westwood_unregister(void)
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{
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tcp_unregister_congestion_control(&tcp_westwood);
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}
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module_init(tcp_westwood_register);
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module_exit(tcp_westwood_unregister);
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MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("TCP Westwood+");
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