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freebsd-dev/sys/netinet/cc/cc_dctcp.c
Randall Stewart b8d60729de tcp: Congestion control cleanup. 
NOTE: HEADS UP read the note below if your kernel config is not including GENERIC!!

This patch does a bit of cleanup on TCP congestion control modules. There were some rather
interesting surprises that one could get i.e. where you use a socket option to change
from one CC (say cc_cubic) to another CC (say cc_vegas) and you could in theory get
a memory failure and end up on cc_newreno. This is not what one would expect. The
new code fixes this by requiring a cc_data_sz() function so we can malloc with M_WAITOK
and pass in to the init function preallocated memory. The CC init is expected in this
case *not* to fail but if it does and a module does break the
"no fail with memory given" contract we do fall back to the CC that was in place at the time.

This also fixes up a set of common newreno utilities that can be shared amongst other
CC modules instead of the other CC modules reaching into newreno and executing
what they think is a "common and understood" function. Lets put these functions in
cc.c and that way we have a common place that is easily findable by future developers or
bug fixers. This also allows newreno to evolve and grow support for its features i.e. ABE
and HYSTART++ without having to dance through hoops for other CC modules, instead
both newreno and the other modules just call into the common functions if they desire
that behavior or roll there own if that makes more sense.

Note: This commit changes the kernel configuration!! If you are not using GENERIC in
some form you must add a CC module option (one of CC_NEWRENO, CC_VEGAS, CC_CUBIC,
CC_CDG, CC_CHD, CC_DCTCP, CC_HTCP, CC_HD). You can have more than one defined
as well if you desire. Note that if you create a kernel configuration that does not
define a congestion control module and includes INET or INET6 the kernel compile will
break. Also you need to define a default, generic adds 'options CC_DEFAULT=\"newreno\"
but you can specify any string that represents the name of the CC module (same names
that show up in the CC module list under net.inet.tcp.cc). If you fail to add the
options CC_DEFAULT in your kernel configuration the kernel build will also break.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
RELNOTES:YES
Differential Revision: https://reviews.freebsd.org/D32693
2021-11-11 06:28:18 -05:00

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/*-
* Copyright (c) 2007-2008
* Swinburne University of Technology, Melbourne, Australia
* Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
* Copyright (c) 2014 Midori Kato <katoon@sfc.wide.ad.jp>
* Copyright (c) 2014 The FreeBSD Foundation
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/*
* An implementation of the DCTCP algorithm for FreeBSD, based on
* "Data Center TCP (DCTCP)" by M. Alizadeh, A. Greenberg, D. A. Maltz,
* J. Padhye, P. Patel, B. Prabhakar, S. Sengupta, and M. Sridharan.,
* in ACM Conference on SIGCOMM 2010, New York, USA,
* Originally released as the contribution of Microsoft Research project.
*/
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/malloc.h>
#include <sys/module.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <net/vnet.h>
#include <net/route.h>
#include <net/route/nhop.h>
#include <netinet/in_pcb.h>
#include <netinet/tcp.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_var.h>
#include <netinet/cc/cc.h>
#include <netinet/cc/cc_module.h>
#define DCTCP_SHIFT 10
#define MAX_ALPHA_VALUE (1<<DCTCP_SHIFT)
VNET_DEFINE_STATIC(uint32_t, dctcp_alpha) = MAX_ALPHA_VALUE;
#define V_dctcp_alpha VNET(dctcp_alpha)
VNET_DEFINE_STATIC(uint32_t, dctcp_shift_g) = 4;
#define V_dctcp_shift_g VNET(dctcp_shift_g)
VNET_DEFINE_STATIC(uint32_t, dctcp_slowstart) = 0;
#define V_dctcp_slowstart VNET(dctcp_slowstart)
struct dctcp {
uint32_t bytes_ecn; /* # of marked bytes during a RTT */
uint32_t bytes_total; /* # of acked bytes during a RTT */
int alpha; /* the fraction of marked bytes */
int ce_prev; /* CE state of the last segment */
tcp_seq save_sndnxt; /* end sequence number of the current window */
int ece_curr; /* ECE flag in this segment */
int ece_prev; /* ECE flag in the last segment */
uint32_t num_cong_events; /* # of congestion events */
};
static void dctcp_ack_received(struct cc_var *ccv, uint16_t type);
static void dctcp_after_idle(struct cc_var *ccv);
static void dctcp_cb_destroy(struct cc_var *ccv);
static int dctcp_cb_init(struct cc_var *ccv, void *ptr);
static void dctcp_cong_signal(struct cc_var *ccv, uint32_t type);
static void dctcp_conn_init(struct cc_var *ccv);
static void dctcp_post_recovery(struct cc_var *ccv);
static void dctcp_ecnpkt_handler(struct cc_var *ccv);
static void dctcp_update_alpha(struct cc_var *ccv);
static size_t dctcp_data_sz(void);
struct cc_algo dctcp_cc_algo = {
.name = "dctcp",
.ack_received = dctcp_ack_received,
.cb_destroy = dctcp_cb_destroy,
.cb_init = dctcp_cb_init,
.cong_signal = dctcp_cong_signal,
.conn_init = dctcp_conn_init,
.post_recovery = dctcp_post_recovery,
.ecnpkt_handler = dctcp_ecnpkt_handler,
.after_idle = dctcp_after_idle,
.cc_data_sz = dctcp_data_sz,
};
static void
dctcp_ack_received(struct cc_var *ccv, uint16_t type)
{
struct dctcp *dctcp_data;
int bytes_acked = 0;
dctcp_data = ccv->cc_data;
if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT) {
/*
* DCTCP doesn't treat receipt of ECN marked packet as a
* congestion event. Thus, DCTCP always executes the ACK
* processing out of congestion recovery.
*/
if (IN_CONGRECOVERY(CCV(ccv, t_flags))) {
EXIT_CONGRECOVERY(CCV(ccv, t_flags));
newreno_cc_ack_received(ccv, type);
ENTER_CONGRECOVERY(CCV(ccv, t_flags));
} else
newreno_cc_ack_received(ccv, type);
if (type == CC_DUPACK)
bytes_acked = min(ccv->bytes_this_ack, CCV(ccv, t_maxseg));
if (type == CC_ACK)
bytes_acked = ccv->bytes_this_ack;
/* Update total bytes. */
dctcp_data->bytes_total += bytes_acked;
/* Update total marked bytes. */
if (dctcp_data->ece_curr) {
//XXRMS: For fluid-model DCTCP, update
//cwnd here during for RTT fairness
if (!dctcp_data->ece_prev
&& bytes_acked > CCV(ccv, t_maxseg)) {
dctcp_data->bytes_ecn +=
(bytes_acked - CCV(ccv, t_maxseg));
} else
dctcp_data->bytes_ecn += bytes_acked;
dctcp_data->ece_prev = 1;
} else {
if (dctcp_data->ece_prev
&& bytes_acked > CCV(ccv, t_maxseg))
dctcp_data->bytes_ecn += CCV(ccv, t_maxseg);
dctcp_data->ece_prev = 0;
}
dctcp_data->ece_curr = 0;
/*
* Update the fraction of marked bytes at the end of
* current window size.
*/
if (!IN_FASTRECOVERY(CCV(ccv, t_flags)) &&
SEQ_GT(ccv->curack, dctcp_data->save_sndnxt))
dctcp_update_alpha(ccv);
} else
newreno_cc_ack_received(ccv, type);
}
static size_t
dctcp_data_sz(void)
{
return (sizeof(struct dctcp));
}
static void
dctcp_after_idle(struct cc_var *ccv)
{
struct dctcp *dctcp_data;
if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT) {
dctcp_data = ccv->cc_data;
/* Initialize internal parameters after idle time */
dctcp_data->bytes_ecn = 0;
dctcp_data->bytes_total = 0;
dctcp_data->save_sndnxt = CCV(ccv, snd_nxt);
dctcp_data->alpha = V_dctcp_alpha;
dctcp_data->ece_curr = 0;
dctcp_data->ece_prev = 0;
dctcp_data->num_cong_events = 0;
}
newreno_cc_after_idle(ccv);
}
static void
dctcp_cb_destroy(struct cc_var *ccv)
{
free(ccv->cc_data, M_CC_MEM);
}
static int
dctcp_cb_init(struct cc_var *ccv, void *ptr)
{
struct dctcp *dctcp_data;
INP_WLOCK_ASSERT(ccv->ccvc.tcp->t_inpcb);
if (ptr == NULL) {
dctcp_data = malloc(sizeof(struct dctcp), M_CC_MEM, M_NOWAIT|M_ZERO);
if (dctcp_data == NULL)
return (ENOMEM);
} else
dctcp_data = ptr;
/* Initialize some key variables with sensible defaults. */
dctcp_data->bytes_ecn = 0;
dctcp_data->bytes_total = 0;
/*
* When alpha is set to 0 in the beginning, DCTCP sender transfers as
* much data as possible until the value converges which may expand the
* queueing delay at the switch. When alpha is set to 1, queueing delay
* is kept small.
* Throughput-sensitive applications should have alpha = 0
* Latency-sensitive applications should have alpha = 1
*
* Note: DCTCP draft suggests initial alpha to be 1 but we've decided to
* keep it 0 as default.
*/
dctcp_data->alpha = V_dctcp_alpha;
dctcp_data->save_sndnxt = 0;
dctcp_data->ce_prev = 0;
dctcp_data->ece_curr = 0;
dctcp_data->ece_prev = 0;
dctcp_data->num_cong_events = 0;
ccv->cc_data = dctcp_data;
return (0);
}
/*
* Perform any necessary tasks before we enter congestion recovery.
*/
static void
dctcp_cong_signal(struct cc_var *ccv, uint32_t type)
{
struct dctcp *dctcp_data;
u_int cwin, mss;
if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT) {
dctcp_data = ccv->cc_data;
cwin = CCV(ccv, snd_cwnd);
mss = tcp_maxseg(ccv->ccvc.tcp);
switch (type) {
case CC_NDUPACK:
if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
CCV(ccv, snd_ssthresh) =
max(cwin / 2, 2 * mss);
dctcp_data->num_cong_events++;
} else {
/* cwnd has already updated as congestion
* recovery. Reverse cwnd value using
* snd_cwnd_prev and recalculate snd_ssthresh
*/
cwin = CCV(ccv, snd_cwnd_prev);
CCV(ccv, snd_ssthresh) =
max(cwin / 2, 2 * mss);
}
ENTER_RECOVERY(CCV(ccv, t_flags));
}
break;
case CC_ECN:
/*
* Save current snd_cwnd when the host encounters both
* congestion recovery and fast recovery.
*/
CCV(ccv, snd_cwnd_prev) = cwin;
if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
if (V_dctcp_slowstart &&
dctcp_data->num_cong_events++ == 0) {
CCV(ccv, snd_ssthresh) =
max(cwin / 2, 2 * mss);
dctcp_data->alpha = MAX_ALPHA_VALUE;
dctcp_data->bytes_ecn = 0;
dctcp_data->bytes_total = 0;
dctcp_data->save_sndnxt = CCV(ccv, snd_nxt);
} else
CCV(ccv, snd_ssthresh) =
max((cwin - (((uint64_t)cwin *
dctcp_data->alpha) >> (DCTCP_SHIFT+1))),
2 * mss);
CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
ENTER_CONGRECOVERY(CCV(ccv, t_flags));
}
dctcp_data->ece_curr = 1;
break;
case CC_RTO:
CCV(ccv, snd_ssthresh) = max(min(CCV(ccv, snd_wnd),
CCV(ccv, snd_cwnd)) / 2 / mss,
2) * mss;
CCV(ccv, snd_cwnd) = mss;
dctcp_update_alpha(ccv);
dctcp_data->save_sndnxt += CCV(ccv, t_maxseg);
dctcp_data->num_cong_events++;
break;
}
} else
newreno_cc_cong_signal(ccv, type);
}
static void
dctcp_conn_init(struct cc_var *ccv)
{
struct dctcp *dctcp_data;
dctcp_data = ccv->cc_data;
if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT)
dctcp_data->save_sndnxt = CCV(ccv, snd_nxt);
}
/*
* Perform any necessary tasks before we exit congestion recovery.
*/
static void
dctcp_post_recovery(struct cc_var *ccv)
{
newreno_cc_post_recovery(ccv);
if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT)
dctcp_update_alpha(ccv);
}
/*
* Execute an additional ECN processing using ECN field in IP header
* and the CWR bit in TCP header.
*/
static void
dctcp_ecnpkt_handler(struct cc_var *ccv)
{
struct dctcp *dctcp_data;
uint32_t ccflag;
int acknow;
dctcp_data = ccv->cc_data;
ccflag = ccv->flags;
acknow = 0;
/*
* DCTCP responds with an ACK immediately when the CE state
* in between this segment and the last segment has changed.
*/
if (ccflag & CCF_IPHDR_CE) {
if (!dctcp_data->ce_prev) {
acknow = 1;
dctcp_data->ce_prev = 1;
CCV(ccv, t_flags2) |= TF2_ECN_SND_ECE;
}
} else {
if (dctcp_data->ce_prev) {
acknow = 1;
dctcp_data->ce_prev = 0;
CCV(ccv, t_flags2) &= ~TF2_ECN_SND_ECE;
}
}
if ((acknow) || (ccflag & CCF_TCPHDR_CWR)) {
ccv->flags |= CCF_ACKNOW;
} else {
ccv->flags &= ~CCF_ACKNOW;
}
}
/*
* Update the fraction of marked bytes represented as 'alpha'.
* Also initialize several internal parameters at the end of this function.
*/
static void
dctcp_update_alpha(struct cc_var *ccv)
{
struct dctcp *dctcp_data;
int alpha_prev;
dctcp_data = ccv->cc_data;
alpha_prev = dctcp_data->alpha;
dctcp_data->bytes_total = max(dctcp_data->bytes_total, 1);
/*
* Update alpha: alpha = (1 - g) * alpha + g * M.
* Here:
* g is weight factor
* recommaded to be set to 1/16
* small g = slow convergence between competitive DCTCP flows
* large g = impacts low utilization of bandwidth at switches
* M is fraction of marked segments in last RTT
* updated every RTT
* Alpha must be round to 0 - MAX_ALPHA_VALUE.
*/
dctcp_data->alpha = ulmin(alpha_prev - (alpha_prev >> V_dctcp_shift_g) +
((uint64_t)dctcp_data->bytes_ecn << (DCTCP_SHIFT - V_dctcp_shift_g)) /
dctcp_data->bytes_total, MAX_ALPHA_VALUE);
/* Initialize internal parameters for next alpha calculation */
dctcp_data->bytes_ecn = 0;
dctcp_data->bytes_total = 0;
dctcp_data->save_sndnxt = CCV(ccv, snd_nxt);
}
static int
dctcp_alpha_handler(SYSCTL_HANDLER_ARGS)
{
uint32_t new;
int error;
new = V_dctcp_alpha;
error = sysctl_handle_int(oidp, &new, 0, req);
if (error == 0 && req->newptr != NULL) {
if (new > MAX_ALPHA_VALUE)
error = EINVAL;
else
V_dctcp_alpha = new;
}
return (error);
}
static int
dctcp_shift_g_handler(SYSCTL_HANDLER_ARGS)
{
uint32_t new;
int error;
new = V_dctcp_shift_g;
error = sysctl_handle_int(oidp, &new, 0, req);
if (error == 0 && req->newptr != NULL) {
if (new > DCTCP_SHIFT)
error = EINVAL;
else
V_dctcp_shift_g = new;
}
return (error);
}
static int
dctcp_slowstart_handler(SYSCTL_HANDLER_ARGS)
{
uint32_t new;
int error;
new = V_dctcp_slowstart;
error = sysctl_handle_int(oidp, &new, 0, req);
if (error == 0 && req->newptr != NULL) {
if (new > 1)
error = EINVAL;
else
V_dctcp_slowstart = new;
}
return (error);
}
SYSCTL_DECL(_net_inet_tcp_cc_dctcp);
SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, dctcp,
CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
"dctcp congestion control related settings");
SYSCTL_PROC(_net_inet_tcp_cc_dctcp, OID_AUTO, alpha,
CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
&VNET_NAME(dctcp_alpha), 0, &dctcp_alpha_handler, "IU",
"dctcp alpha parameter at start of session");
SYSCTL_PROC(_net_inet_tcp_cc_dctcp, OID_AUTO, shift_g,
CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
&VNET_NAME(dctcp_shift_g), 4, &dctcp_shift_g_handler, "IU",
"dctcp shift parameter");
SYSCTL_PROC(_net_inet_tcp_cc_dctcp, OID_AUTO, slowstart,
CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
&VNET_NAME(dctcp_slowstart), 0, &dctcp_slowstart_handler, "IU",
"half CWND reduction after the first slow start");
DECLARE_CC_MODULE(dctcp, &dctcp_cc_algo);
MODULE_VERSION(dctcp, 2);
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