Commit 46d3ceab authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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tcp: TCP Small Queues

This introduce TSQ (TCP Small Queues)

TSQ goal is to reduce number of TCP packets in xmit queues (qdisc &
device queues), to reduce RTT and cwnd bias, part of the bufferbloat

sk->sk_wmem_alloc not allowed to grow above a given limit,
allowing no more than ~128KB [1] per tcp socket in qdisc/dev layers at a
given time.

TSO packets are sized/capped to half the limit, so that we have two
TSO packets in flight, allowing better bandwidth use.

As a side effect, setting the limit to 40000 automatically reduces the
standard gso max limit (65536) to 40000/2 : It can help to reduce
latencies of high prio packets, having smaller TSO packets.

This means we divert sock_wfree() to a tcp_wfree() handler, to
queue/send following frames when skb_orphan() [2] is called for the
already queued skbs.

Results on my dev machines (tg3/ixgbe nics) are really impressive,
using standard pfifo_fast, and with or without TSO/GSO.

Without reduction of nominal bandwidth, we have reduction of buffering
per bulk sender :
< 1ms on Gbit (instead of 50ms with TSO)
< 8ms on 100Mbit (instead of 132 ms)

I no longer have 4 MBytes backlogged in qdisc by a single netperf
session, and both side socket autotuning no longer use 4 Mbytes.

As skb destructor cannot restart xmit itself ( as qdisc lock might be
taken at this point ), we delegate the work to a tasklet. We use one
tasklest per cpu for performance reasons.

If tasklet finds a socket owned by the user, it sets TSQ_OWNED flag.
This flag is tested in a new protocol method called from release_sock(),
to eventually send new segments.

[1] New /proc/sys/net/ipv4/tcp_limit_output_bytes tunable
[2] skb_orphan() is usually called at TX completion time,
  but some drivers call it in their start_xmit() handler.
  These drivers should at least use BQL, or else a single TCP
  session can still fill the whole NIC TX ring, since TSQ will
  have no effect.

Signed-off-by: default avatarEric Dumazet <>
Cc: Dave Taht <>
Cc: Tom Herbert <>
Cc: Matt Mathis <>
Cc: Yuchung Cheng <>
Cc: Nandita Dukkipati <>
Signed-off-by: default avatarDavid S. Miller <>
parent 2100844c
......@@ -551,6 +551,20 @@ tcp_thin_dupack - BOOLEAN
Default: 0
tcp_limit_output_bytes - INTEGER
Controls TCP Small Queue limit per tcp socket.
TCP bulk sender tends to increase packets in flight until it
gets losses notifications. With SNDBUF autotuning, this can
result in a large amount of packets queued in qdisc/device
on the local machine, hurting latency of other flows, for
typical pfifo_fast qdiscs.
tcp_limit_output_bytes limits the number of bytes on qdisc
or device to reduce artificial RTT/cwnd and reduce bufferbloat.
Note: For GSO/TSO enabled flows, we try to have at least two
packets in flight. Reducing tcp_limit_output_bytes might also
reduce the size of individual GSO packet (64KB being the max)
Default: 131072
UDP variables:
udp_mem - vector of 3 INTEGERs: min, pressure, max
......@@ -339,6 +339,9 @@ struct tcp_sock {
u32 rcv_tstamp; /* timestamp of last received ACK (for keepalives) */
u32 lsndtime; /* timestamp of last sent data packet (for restart window) */
struct list_head tsq_node; /* anchor in tsq_tasklet.head list */
unsigned long tsq_flags;
/* Data for direct copy to user */
struct {
struct sk_buff_head prequeue;
......@@ -494,6 +497,12 @@ struct tcp_sock {
struct tcp_cookie_values *cookie_values;
enum tsq_flags {
TSQ_OWNED, /* tcp_tasklet_func() found socket was locked */
static inline struct tcp_sock *tcp_sk(const struct sock *sk)
return (struct tcp_sock *)sk;
......@@ -858,6 +858,8 @@ struct proto {
int (*backlog_rcv) (struct sock *sk,
struct sk_buff *skb);
void (*release_cb)(struct sock *sk);
/* Keeping track of sk's, looking them up, and port selection methods. */
void (*hash)(struct sock *sk);
void (*unhash)(struct sock *sk);
......@@ -253,6 +253,7 @@ extern int sysctl_tcp_cookie_size;
extern int sysctl_tcp_thin_linear_timeouts;
extern int sysctl_tcp_thin_dupack;
extern int sysctl_tcp_early_retrans;
extern int sysctl_tcp_limit_output_bytes;
extern atomic_long_t tcp_memory_allocated;
extern struct percpu_counter tcp_sockets_allocated;
......@@ -321,6 +322,8 @@ extern struct proto tcp_prot;
extern void tcp_init_mem(struct net *net);
extern void tcp_tasklet_init(void);
extern void tcp_v4_err(struct sk_buff *skb, u32);
extern void tcp_shutdown (struct sock *sk, int how);
......@@ -334,6 +337,7 @@ extern int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
size_t size);
extern int tcp_sendpage(struct sock *sk, struct page *page, int offset,
size_t size, int flags);
extern void tcp_release_cb(struct sock *sk);
extern int tcp_ioctl(struct sock *sk, int cmd, unsigned long arg);
extern int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
const struct tcphdr *th, unsigned int len);
......@@ -2159,6 +2159,10 @@ void release_sock(struct sock *sk)
if (sk->sk_backlog.tail)
if (sk->sk_prot->release_cb)
sk->sk_lock.owned = 0;
if (waitqueue_active(&sk->sk_lock.wq))
......@@ -598,6 +598,13 @@ static struct ctl_table ipv4_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec
.procname = "tcp_limit_output_bytes",
.data = &sysctl_tcp_limit_output_bytes,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = proc_dointvec
.procname = "tcp_dma_copybreak",
......@@ -376,6 +376,7 @@ void tcp_init_sock(struct sock *sk)
icsk->icsk_rto = TCP_TIMEOUT_INIT;
tp->mdev = TCP_TIMEOUT_INIT;
......@@ -796,6 +797,10 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
inet_csk(sk)->icsk_ext_hdr_len -
/* TSQ : try to have two TSO segments in flight */
xmit_size_goal = min_t(u32, xmit_size_goal,
sysctl_tcp_limit_output_bytes >> 1);
xmit_size_goal = tcp_bound_to_half_wnd(tp, xmit_size_goal);
/* We try hard to avoid divides here */
......@@ -3574,4 +3579,5 @@ void __init tcp_init(void)
tcp_secret_primary = &tcp_secret_one;
tcp_secret_retiring = &tcp_secret_two;
tcp_secret_secondary = &tcp_secret_two;
......@@ -2588,6 +2588,7 @@ struct proto tcp_prot = {
.sendmsg = tcp_sendmsg,
.sendpage = tcp_sendpage,
.backlog_rcv = tcp_v4_do_rcv,
.release_cb = tcp_release_cb,
.hash = inet_hash,
.unhash = inet_unhash,
.get_port = inet_csk_get_port,
......@@ -424,6 +424,7 @@ struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req,
treq->snt_isn + 1 + tcp_s_data_size(oldtp);
tcp_init_wl(newtp, treq->rcv_isn);
......@@ -50,6 +50,9 @@ int sysctl_tcp_retrans_collapse __read_mostly = 1;
int sysctl_tcp_workaround_signed_windows __read_mostly = 0;
/* Default TSQ limit of two TSO segments */
int sysctl_tcp_limit_output_bytes __read_mostly = 131072;
/* This limits the percentage of the congestion window which we
* will allow a single TSO frame to consume. Building TSO frames
* which are too large can cause TCP streams to be bursty.
......@@ -65,6 +68,8 @@ int sysctl_tcp_slow_start_after_idle __read_mostly = 1;
int sysctl_tcp_cookie_size __read_mostly = 0; /* TCP_COOKIE_MAX */
static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
int push_one, gfp_t gfp);
/* Account for new data that has been sent to the network. */
static void tcp_event_new_data_sent(struct sock *sk, const struct sk_buff *skb)
......@@ -783,6 +788,140 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
return size;
* TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
* to reduce RTT and bufferbloat.
* We do this using a special skb destructor (tcp_wfree).
* Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
* needs to be reallocated in a driver.
* The invariant being skb->truesize substracted from sk->sk_wmem_alloc
* Since transmit from skb destructor is forbidden, we use a tasklet
* to process all sockets that eventually need to send more skbs.
* We use one tasklet per cpu, with its own queue of sockets.
struct tsq_tasklet {
struct tasklet_struct tasklet;
struct list_head head; /* queue of tcp sockets */
static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);
* One tasklest per cpu tries to send more skbs.
* We run in tasklet context but need to disable irqs when
* transfering tsq->head because tcp_wfree() might
* interrupt us (non NAPI drivers)
static void tcp_tasklet_func(unsigned long data)
struct tsq_tasklet *tsq = (struct tsq_tasklet *)data;
unsigned long flags;
struct list_head *q, *n;
struct tcp_sock *tp;
struct sock *sk;
list_splice_init(&tsq->head, &list);
list_for_each_safe(q, n, &list) {
tp = list_entry(q, struct tcp_sock, tsq_node);
sk = (struct sock *)tp;
if (!sock_owned_by_user(sk)) {
if ((1 << sk->sk_state) &
0, 0,
} else {
/* defer the work to tcp_release_cb() */
set_bit(TSQ_OWNED, &tp->tsq_flags);
clear_bit(TSQ_QUEUED, &tp->tsq_flags);
* tcp_release_cb - tcp release_sock() callback
* @sk: socket
* called from release_sock() to perform protocol dependent
* actions before socket release.
void tcp_release_cb(struct sock *sk)
struct tcp_sock *tp = tcp_sk(sk);
if (test_and_clear_bit(TSQ_OWNED, &tp->tsq_flags)) {
if ((1 << sk->sk_state) &
0, 0,
void __init tcp_tasklet_init(void)
int i;
for_each_possible_cpu(i) {
struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
(unsigned long)tsq);
* Write buffer destructor automatically called from kfree_skb.
* We cant xmit new skbs from this context, as we might already
* hold qdisc lock.
void tcp_wfree(struct sk_buff *skb)
struct sock *sk = skb->sk;
struct tcp_sock *tp = tcp_sk(sk);
if (test_and_clear_bit(TSQ_THROTTLED, &tp->tsq_flags) &&
!test_and_set_bit(TSQ_QUEUED, &tp->tsq_flags)) {
unsigned long flags;
struct tsq_tasklet *tsq;
/* Keep a ref on socket.
* This last ref will be released in tcp_tasklet_func()
atomic_sub(skb->truesize - 1, &sk->sk_wmem_alloc);
/* queue this socket to tasklet queue */
tsq = &__get_cpu_var(tsq_tasklet);
list_add(&tp->tsq_node, &tsq->head);
} else {
/* This routine actually transmits TCP packets queued in by
* tcp_do_sendmsg(). This is used by both the initial
* transmission and possible later retransmissions.
......@@ -844,7 +983,12 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
skb_push(skb, tcp_header_size);
skb_set_owner_w(skb, sk);
skb->sk = sk;
skb->destructor = (sysctl_tcp_limit_output_bytes > 0) ?
tcp_wfree : sock_wfree;
atomic_add(skb->truesize, &sk->sk_wmem_alloc);
/* Build TCP header and checksum it. */
th = tcp_hdr(skb);
......@@ -1780,6 +1924,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
while ((skb = tcp_send_head(sk))) {
unsigned int limit;
tso_segs = tcp_init_tso_segs(sk, skb, mss_now);
......@@ -1800,6 +1945,13 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
/* TSQ : sk_wmem_alloc accounts skb truesize,
* including skb overhead. But thats OK.
if (atomic_read(&sk->sk_wmem_alloc) >= sysctl_tcp_limit_output_bytes) {
set_bit(TSQ_THROTTLED, &tp->tsq_flags);
limit = mss_now;
if (tso_segs > 1 && !tcp_urg_mode(tp))
limit = tcp_mss_split_point(sk, skb, mss_now,
......@@ -1970,6 +1970,7 @@ struct proto tcpv6_prot = {
.sendmsg = tcp_sendmsg,
.sendpage = tcp_sendpage,
.backlog_rcv = tcp_v6_do_rcv,
.release_cb = tcp_release_cb,
.hash = tcp_v6_hash,
.unhash = inet_unhash,
.get_port = inet_csk_get_port,
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