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Commit 4faf7839 authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

tcp: fix cwnd undo in Reno and HTCP congestion controls 

Using ssthresh to revert cwnd is less reliable when ssthresh is
bounded to 2 packets. This patch uses an existing variable in TCP
"prior_cwnd" that snapshots the cwnd right before entering fast
recovery and RTO recovery in Reno.  This fixes the issue discussed
in netdev thread: "A buggy behavior for Linux TCP Reno and HTCP"
https://www.spinics.net/lists/netdev/msg444955.html



Suggested-by: default avatarNeal Cardwell <ncardwell@google.com>
Reported-by: default avatarWei Sun <unlcsewsun@gmail.com>
Signed-off-by: default avatarYuchung Cheng <ncardwell@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 10377ba7

include/linux/tcp.h

+1 −1
Original line number Diff line number Diff line
@@ -258,7 +258,7 @@ struct tcp_sock { 
	u32	snd_cwnd_clamp; /* Do not allow snd_cwnd to grow above this */

	u32	snd_cwnd_used;

	u32	snd_cwnd_stamp;

	u32	prior_cwnd;	/* Congestion window at start of Recovery. */

	u32	prior_cwnd;	/* cwnd right before starting loss recovery */

	u32	prr_delivered;	/* Number of newly delivered packets to

				 * receiver in Recovery. */

	u32	prr_out;	/* Total number of pkts sent during Recovery. */

net/ipv4/tcp_cong.c

+1 −1
Original line number Diff line number Diff line
@@ -456,7 +456,7 @@ u32 tcp_reno_undo_cwnd(struct sock *sk) 
{

	const struct tcp_sock *tp = tcp_sk(sk);



	return max(tp->snd_cwnd, tp->snd_ssthresh << 1);

	return max(tp->snd_cwnd, tp->prior_cwnd);

}

EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);

net/ipv4/tcp_htcp.c

+1 −2
Original line number Diff line number Diff line
@@ -66,7 +66,6 @@ static inline void htcp_reset(struct htcp *ca) 


static u32 htcp_cwnd_undo(struct sock *sk)

{

	const struct tcp_sock *tp = tcp_sk(sk);

	struct htcp *ca = inet_csk_ca(sk);



	if (ca->undo_last_cong) {
@@ -76,7 +75,7 @@ static u32 htcp_cwnd_undo(struct sock *sk) 
		ca->undo_last_cong = 0;

	}



	return max(tp->snd_cwnd, (tp->snd_ssthresh << 7) / ca->beta);

	return tcp_reno_undo_cwnd(sk);

}



static inline void measure_rtt(struct sock *sk, u32 srtt)

net/ipv4/tcp_input.c

+1 −0
Original line number Diff line number Diff line
@@ -1950,6 +1950,7 @@ void tcp_enter_loss(struct sock *sk) 
	    !after(tp->high_seq, tp->snd_una) ||

	    (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {

		tp->prior_ssthresh = tcp_current_ssthresh(sk);

		tp->prior_cwnd = tp->snd_cwnd;

		tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);

		tcp_ca_event(sk, CA_EVENT_LOSS);

		tcp_init_undo(tp);