tcp: TCP Small Queues and strange attractors

[pandora-kernel.git] / net / ipv4 / tcp_output.c

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index ee567e9..e13d778 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -839,26 +839,38 @@ void tcp_wfree(struct sk_buff *skb)
 {
        struct sock *sk = skb->sk;
        struct tcp_sock *tp = tcp_sk(sk);
+       int wmem;
+
+       /* Keep one reference on sk_wmem_alloc.
+        * Will be released by sk_free() from here or tcp_tasklet_func()
+        */
+       wmem = atomic_sub_return(skb->truesize - 1, &sk->sk_wmem_alloc);
+
+       /* If this softirq is serviced by ksoftirqd, we are likely under stress.
+        * Wait until our queues (qdisc + devices) are drained.
+        * This gives :
+        * - less callbacks to tcp_write_xmit(), reducing stress (batches)
+        * - chance for incoming ACK (processed by another cpu maybe)
+        *   to migrate this flow (skb->ooo_okay will be eventually set)
+        */
+       if (wmem >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
+               goto out;
 
        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);
+               return;
        }
+out:
+       sk_free(sk);
 }
 
 /* This routine actually transmits TCP packets queued in by
@@ -914,9 +926,13 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                tcp_ca_event(sk, CA_EVENT_TX_START);
 
        /* if no packet is in qdisc/device queue, then allow XPS to select
-        * another queue.
+        * another queue. We can be called from tcp_tsq_handler()
+        * which holds one reference to sk_wmem_alloc.
+        *
+        * TODO: Ideally, in-flight pure ACK packets should not matter here.
+        * One way to get this would be to set skb->truesize = 2 on them.
         */
-       skb->ooo_okay = sk_wmem_alloc_get(sk) == 0;
+       skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
 
        skb_push(skb, tcp_header_size);
        skb_reset_transport_header(skb);
@@ -2110,10 +2126,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 static bool skb_still_in_host_queue(const struct sock *sk,
                                    const struct sk_buff *skb)
 {
-       const struct sk_buff *fclone = skb + 1;
-
-       if (unlikely(skb->fclone == SKB_FCLONE_ORIG &&
-                    fclone->fclone == SKB_FCLONE_CLONE)) {
+       if (unlikely(skb_fclone_busy(skb))) {
                NET_INC_STATS_BH(sock_net(sk),
                                 LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
                return true;