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
problem.

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: Eric Dumazet <edumazet@google.com>
Cc: Dave Taht <dave.taht@bufferbloat.net>
Cc: Tom Herbert <therbert@google.com>
Cc: Matt Mathis <mattmathis@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 153 insertions, 1 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c465d3e51e28..03854abfd9d8 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -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 */
 EXPORT_SYMBOL_GPL(sysctl_tcp_cookie_size);
 
+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;
 }
 
+
+/* TCP SMALL QUEUES (TSQ)
+ *
+ * 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;
+        LIST_HEAD(list);
+        unsigned long flags;
+        struct list_head *q, *n;
+        struct tcp_sock *tp;
+        struct sock *sk;
+
+        local_irq_save(flags);
+        list_splice_init(&tsq->head, &list);
+        local_irq_restore(flags);
+
+        list_for_each_safe(q, n, &list) {
+                tp = list_entry(q, struct tcp_sock, tsq_node);
+                list_del(&tp->tsq_node);
+
+                sk = (struct sock *)tp;
+                bh_lock_sock(sk);
+
+                if (!sock_owned_by_user(sk)) {
+                        if ((1 << sk->sk_state) &
+                            (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 |
+                             TCPF_CLOSING | TCPF_CLOSE_WAIT))
+                                tcp_write_xmit(sk,
+                                               tcp_current_mss(sk),
+                                               0, 0,
+                                               GFP_ATOMIC);
+                } else {
+                        /* defer the work to tcp_release_cb() */
+                        set_bit(TSQ_OWNED, &tp->tsq_flags);
+                }
+                bh_unlock_sock(sk);
+
+                clear_bit(TSQ_QUEUED, &tp->tsq_flags);
+                sk_free(sk);
+        }
+}
+
+/**
+ * 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) &
+                    (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 |
+                     TCPF_CLOSING | TCPF_CLOSE_WAIT))
+                        tcp_write_xmit(sk,
+                                       tcp_current_mss(sk),
+                                       0, 0,
+                                       GFP_ATOMIC);
+        }
+}
+EXPORT_SYMBOL(tcp_release_cb);
+
+void __init tcp_tasklet_init(void)
+{
+        int i;
+
+        for_each_possible_cpu(i) {
+                struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
+
+                INIT_LIST_HEAD(&tsq->head);
+                tasklet_init(&tsq->tasklet,
+                             tcp_tasklet_func,
+                             (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 */
+                local_irq_save(flags);
+                tsq = &__get_cpu_var(tsq_tasklet);
+                list_add(&tp->tsq_node, &tsq->head);
+                tasklet_schedule(&tsq->tasklet);
+                local_irq_restore(flags);
+        } else {
+                sock_wfree(skb);
+        }
+}
+
 /* 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_reset_transport_header(skb);
-        skb_set_owner_w(skb, sk);
+
+        skb_orphan(skb);
+        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);
                 BUG_ON(!tso_segs);
 
@@ -1800,6 +1945,13 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                                 break;
                 }
 
+                /* 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);
+                        break;
+                }
                 limit = mss_now;
                 if (tso_segs > 1 && !tcp_urg_mode(tp))
                         limit = tcp_mss_split_point(sk, skb, mss_now,