[TCP]: Eliminate redundant computations in tcp_write_xmit().

tcp_snd_test() is run for every packet output by a single
call to tcp_write_xmit(), but this is not necessary.

For one, the congestion window space needs to only be
calculated one time, then used throughout the duration
of the loop.

This cleanup also makes experimenting with different TSO
packetization schemes much easier.

Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8327e5e86d15945232c921cacf736b60e9c8a3f2..0a4cd24b65786379fbc06c7bf9264b6a667cb93e 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -887,6 +887,7 @@ static int tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle)
 {
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
+       unsigned int tso_segs, cwnd_quota;
        int sent_pkts;
 
        /* If we are closed, the bytes will have to remain here.
@@ -896,19 +897,31 @@ static int tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle)
        if (unlikely(sk->sk_state == TCP_CLOSE))
                return 0;
 
+       skb = sk->sk_send_head;
+       if (unlikely(!skb))
+               return 0;
+
+       tso_segs = tcp_init_tso_segs(sk, skb);
+       cwnd_quota = tcp_cwnd_test(tp, skb);
        sent_pkts = 0;
-       while ((skb = sk->sk_send_head) &&
-              tcp_snd_test(sk, skb, mss_now,
-                           tcp_skb_is_last(sk, skb) ? nonagle :
-                           TCP_NAGLE_PUSH)) {
-               if (skb->len > mss_now) {
-                       if (tcp_fragment(sk, skb, mss_now))
+
+       while (cwnd_quota >= tso_segs) {
+               if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
+                                            (tcp_skb_is_last(sk, skb) ?
+                                             nonagle : TCP_NAGLE_PUSH))))
+                       break;
+
+               if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
+                       break;
+
+               if (unlikely(skb->len > mss_now)) {
+                       if (unlikely(tcp_fragment(sk, skb,  mss_now)))
                                break;
                }
 
                TCP_SKB_CB(skb)->when = tcp_time_stamp;
                tcp_tso_set_push(skb);
-               if (tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC)))
+               if (unlikely(tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC))))
                        break;
 
                /* Advance the send_head.  This one is sent out.
@@ -917,10 +930,19 @@ static int tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle)
                update_send_head(sk, tp, skb);
 
                tcp_minshall_update(tp, mss_now, skb);
-               sent_pkts = 1;
+               sent_pkts++;
+
+               /* Do not optimize this to use tso_segs. If we chopped up
+                * the packet above, tso_segs will no longer be valid.
+                */
+               cwnd_quota -= tcp_skb_pcount(skb);
+               skb = sk->sk_send_head;
+               if (!skb)
+                       break;
+               tso_segs = tcp_init_tso_segs(sk, skb);
        }
 
-       if (sent_pkts) {
+       if (likely(sent_pkts)) {
                tcp_cwnd_validate(sk, tp);
                return 0;
        }