target/linux/generic/backport-4.9/024-5-tcp-tsq-add-a-shortcut-in-tcp_small_queue_check.patch

   1 From 75eefc6c59fd2c5f1ab95a3a113c217237d12a31 Mon Sep 17 00:00:00 2001
   2 From: Eric Dumazet <edumazet@google.com>
   3 Date: Sat, 3 Dec 2016 11:14:54 -0800
   4 Subject: [PATCH 05/10] tcp: tsq: add a shortcut in tcp_small_queue_check()
   5
   6 Always allow the two first skbs in write queue to be sent,
   7 regardless of sk_wmem_alloc/sk_pacing_rate values.
   8
   9 This helps a lot in situations where TX completions are delayed either
  10 because of driver latencies or softirq latencies.
  11
  12 Test is done with no cache line misses.
  13
  14 Signed-off-by: Eric Dumazet <edumazet@google.com>
  15 Signed-off-by: David S. Miller <davem@davemloft.net>
  16 ---
  17  net/ipv4/tcp_output.c | 9 +++++++++
  18  1 file changed, 9 insertions(+)
  19
  20 --- a/net/ipv4/tcp_output.c
  21 +++ b/net/ipv4/tcp_output.c
  22 @@ -2131,6 +2131,15 @@ static bool tcp_small_queue_check(struct
  23         limit <<= factor;
  24
  25         if (atomic_read(&sk->sk_wmem_alloc) > limit) {
  26 +               /* Always send the 1st or 2nd skb in write queue.
  27 +                * No need to wait for TX completion to call us back,
  28 +                * after softirq/tasklet schedule.
  29 +                * This helps when TX completions are delayed too much.
  30 +                */
  31 +               if (skb == sk->sk_write_queue.next ||
  32 +                   skb->prev == sk->sk_write_queue.next)
  33 +                       return false;
  34 +
  35                 set_bit(TSQ_THROTTLED, &tcp_sk(sk)->tsq_flags);
  36                 /* It is possible TX completion already happened
  37                  * before we set TSQ_THROTTLED, so we must