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MFC r342127
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Revert r331567 CC Cubic: fix underflow for cubic_cwnd()

This change is causing TCP connections using cubic to hang. Need to dig more to
find exact cause and fix it.
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@hirenp
hirenp committed Dec 17, 2018
1 parent 784236e commit 38ba964
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Showing 3 changed files with 7 additions and 52 deletions.
2 changes: 0 additions & 2 deletions sys/netinet/cc/cc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -102,8 +102,6 @@ struct cc_var {
#define CCF_ACKNOW 0x0008 /* Will this ack be sent now? */
#define CCF_IPHDR_CE 0x0010 /* Does this packet set CE bit? */
#define CCF_TCPHDR_CWR 0x0020 /* Does this packet set CWR bit? */
#define CCF_MAX_CWND 0x0040 /* Have we reached maximum cwnd? */
#define CCF_CHG_MAX_CWND 0x0080 /* Cubic max_cwnd changed, for K */

/* ACK types passed to the ack_received() hook. */
#define CC_ACK 0x0001 /* Regular in sequence ACK. */
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33 changes: 4 additions & 29 deletions sys/netinet/cc/cc_cubic.c
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Original file line number Diff line number Diff line change
Expand Up @@ -88,8 +88,6 @@ struct cubic {
unsigned long max_cwnd;
/* cwnd at the previous congestion event. */
unsigned long prev_max_cwnd;
/* Cached value for t_maxseg when K was computed */
uint32_t k_maxseg;
/* Number of congestion events. */
uint32_t num_cong_events;
/* Minimum observed rtt in ticks. */
Expand Down Expand Up @@ -126,9 +124,6 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
cubic_data = ccv->cc_data;
cubic_record_rtt(ccv);

if (ccv->flags & CCF_MAX_CWND)
return;

/*
* Regular ACK and we're not in cong/fast recovery and we're cwnd
* limited and we're either not doing ABC or are slow starting or are
Expand Down Expand Up @@ -156,12 +151,6 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
cubic_data->mean_rtt_ticks, cubic_data->max_cwnd,
CCV(ccv, t_maxseg));

if (ccv->flags & CCF_CHG_MAX_CWND || cubic_data->k_maxseg != CCV(ccv, t_maxseg)) {
cubic_data->K = cubic_k(cubic_data->max_cwnd / CCV(ccv, t_maxseg));
cubic_data->k_maxseg = CCV(ccv, t_maxseg);
ccv->flags &= ~(CCF_MAX_CWND|CCF_CHG_MAX_CWND);
}

w_cubic_next = cubic_cwnd(ticks_since_cong +
cubic_data->mean_rtt_ticks, cubic_data->max_cwnd,
CCV(ccv, t_maxseg), cubic_data->K);
Expand All @@ -173,18 +162,13 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
* TCP-friendly region, follow tf
* cwnd growth.
*/
CCV(ccv, snd_cwnd) = ulmin(w_tf, TCP_MAXWIN << CCV(ccv, snd_scale));
CCV(ccv, snd_cwnd) = w_tf;

else if (CCV(ccv, snd_cwnd) < w_cubic_next) {
/*
* Concave or convex region, follow CUBIC
* cwnd growth.
*/
if (w_cubic_next >= TCP_MAXWIN << CCV(ccv, snd_scale)) {
w_cubic_next = TCP_MAXWIN << CCV(ccv, snd_scale);
ccv->flags |= CCF_MAX_CWND;
}
w_cubic_next = ulmin(w_cubic_next, TCP_MAXWIN << CCV(ccv, snd_scale));
if (V_tcp_do_rfc3465)
CCV(ccv, snd_cwnd) = w_cubic_next;
else
Expand All @@ -202,10 +186,8 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
* max_cwnd.
*/
if (cubic_data->num_cong_events == 0 &&
cubic_data->max_cwnd < CCV(ccv, snd_cwnd)) {
cubic_data->max_cwnd < CCV(ccv, snd_cwnd))
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
ccv->flags |= CCF_CHG_MAX_CWND;
}
}
}
}
Expand Down Expand Up @@ -254,7 +236,6 @@ cubic_cong_signal(struct cc_var *ccv, uint32_t type)
cubic_data->num_cong_events++;
cubic_data->prev_max_cwnd = cubic_data->max_cwnd;
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
ccv->flags |= CCF_CHG_MAX_CWND;
}
ENTER_RECOVERY(CCV(ccv, t_flags));
}
Expand All @@ -267,8 +248,6 @@ cubic_cong_signal(struct cc_var *ccv, uint32_t type)
cubic_data->prev_max_cwnd = cubic_data->max_cwnd;
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
cubic_data->t_last_cong = ticks;
ccv->flags |= CCF_CHG_MAX_CWND;
ccv->flags &= ~CCF_MAX_CWND;
CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
ENTER_CONGRECOVERY(CCV(ccv, t_flags));
}
Expand All @@ -285,7 +264,6 @@ cubic_cong_signal(struct cc_var *ccv, uint32_t type)
if (CCV(ccv, t_rxtshift) >= 2) {
cubic_data->num_cong_events++;
cubic_data->t_last_cong = ticks;
ccv->flags &= ~CCF_MAX_CWND;
}
break;
}
Expand All @@ -304,7 +282,6 @@ cubic_conn_init(struct cc_var *ccv)
* get used.
*/
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
ccv->flags |= CCF_CHG_MAX_CWND;
}

static int
Expand All @@ -329,11 +306,9 @@ cubic_post_recovery(struct cc_var *ccv)
pipe = 0;

/* Fast convergence heuristic. */
if (cubic_data->max_cwnd < cubic_data->prev_max_cwnd) {
if (cubic_data->max_cwnd < cubic_data->prev_max_cwnd)
cubic_data->max_cwnd = (cubic_data->max_cwnd * CUBIC_FC_FACTOR)
>> CUBIC_SHIFT;
ccv->flags |= CCF_CHG_MAX_CWND;
}

if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
/*
Expand All @@ -356,7 +331,6 @@ cubic_post_recovery(struct cc_var *ccv)
cubic_data->max_cwnd) >> CUBIC_SHIFT));
}
cubic_data->t_last_cong = ticks;
ccv->flags &= ~CCF_MAX_CWND;

/* Calculate the average RTT between congestion epochs. */
if (cubic_data->epoch_ack_count > 0 &&
Expand All @@ -367,6 +341,7 @@ cubic_post_recovery(struct cc_var *ccv)

cubic_data->epoch_ack_count = 0;
cubic_data->sum_rtt_ticks = 0;
cubic_data->K = cubic_k(cubic_data->max_cwnd / CCV(ccv, t_maxseg));
}

/*
Expand Down
24 changes: 3 additions & 21 deletions sys/netinet/cc/cc_cubic.h
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Original file line number Diff line number Diff line change
Expand Up @@ -41,8 +41,6 @@
#ifndef _NETINET_CC_CUBIC_H_
#define _NETINET_CC_CUBIC_H_

#include <sys/limits.h>

/* Number of bits of precision for fixed point math calcs. */
#define CUBIC_SHIFT 8

Expand Down Expand Up @@ -163,6 +161,8 @@ cubic_k(unsigned long wmax_pkts)
/*
* Compute the new cwnd value using an implementation of eqn 1 from the I-D.
* Thanks to Kip Macy for help debugging this function.
*
* XXXLAS: Characterise bounds for overflow.
*/
static __inline unsigned long
cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss, int64_t K)
Expand All @@ -174,15 +174,6 @@ cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss, int64_t K)
/* t - K, with CUBIC_SHIFT worth of precision. */
cwnd = ((int64_t)(ticks_since_cong << CUBIC_SHIFT) - (K * hz)) / hz;

/* moved this calculation up because it cannot overflow or underflow */
cwnd *= CUBIC_C_FACTOR * smss;

if (cwnd > 2097151) /* 2^21 cubed is long max */
return INT_MAX;

if (cwnd < -2097152) /* -2^21 cubed is long min */
return smss;

/* (t - K)^3, with CUBIC_SHIFT^3 worth of precision. */
cwnd *= (cwnd * cwnd);

Expand All @@ -191,17 +182,8 @@ cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss, int64_t K)
* The down shift by CUBIC_SHIFT_4 is because cwnd has 4 lots of
* CUBIC_SHIFT included in the value. 3 from the cubing of cwnd above,
* and an extra from multiplying through by CUBIC_C_FACTOR.
*
* The original formula was this:
* cwnd = ((cwnd * CUBIC_C_FACTOR * smss) >> CUBIC_SHIFT_4) + wmax;
*
* CUBIC_C_FACTOR and smss factors were moved up to an earlier
* calculation to simplify overflow and underflow detection.
*/
cwnd = (cwnd >> CUBIC_SHIFT_4) + wmax;

if (cwnd < 0)
return 1;
cwnd = ((cwnd * CUBIC_C_FACTOR * smss) >> CUBIC_SHIFT_4) + wmax;

return ((unsigned long)cwnd);
}
Expand Down

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