netdev - [PATCH v2 net-next 5/8] tcp: tsq: add a shortcut in tcp_small_queue

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Date:   Sat,  3 Dec 2016 11:14:54 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH v2 net-next 5/8] tcp: tsq: add a shortcut in tcp_small_queue_check()

Always allow the two first skbs in write queue to be sent,
regardless of sk_wmem_alloc/sk_pacing_rate values.

This helps a lot in situations where TX completions are delayed either
because of driver latencies or softirq latencies.

Test is done with no cache line misses.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_output.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 0db63efe5b8b..d5c46749adab 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2091,6 +2091,15 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
 	limit <<= factor;
 
 	if (atomic_read(&sk->sk_wmem_alloc) > limit) {
+		/* Always send the 1st or 2nd skb in write queue.
+		 * No need to wait for TX completion to call us back,
+		 * after softirq/tasklet schedule.
+		 * This helps when TX completions are delayed too much.
+		 */
+		if (skb == sk->sk_write_queue.next ||
+		    skb->prev == sk->sk_write_queue.next)
+			return false;
+
 		set_bit(TSQ_THROTTLED, &tcp_sk(sk)->tsq_flags);
 		/* It is possible TX completion already happened
 		 * before we set TSQ_THROTTLED, so we must
-- 
2.8.0.rc3.226.g39d4020