tcp: autocork should not hold first packet in write queue (a181ceb5) · Commits · 戴 / test

Willem noticed a TCP_RR regression caused by TCP autocorking on a Mellanox test bed. MLX4_EN_TX_COAL_TIME is 16 us, which can be right above RTT between hosts. We can receive a ACK for a packet still in NIC TX ring buffer or in a softnet completion queue. Fix this by always pushing the skb if it is at the head of write queue. Also, as TX completion is lockless, it's safer to perform sk_wmem_alloc test after setting TSQ_THROTTLED. erd:~# MIB="MIN_LATENCY,MEAN_LATENCY,MAX_LATENCY,P99_LATENCY,STDDEV_LATENCY" erd:~# ./netperf -H remote -t TCP_RR -- -o $MIB | tail -n 1 (repeat 3 times) Before patch : 18,1049.87,41004,39631,6295.47 17,239.52,40804,48,2912.79 18,348.40,40877,54,3573.39 After patch : 18,22.84,4606,38,16.39 17,21.56,2871,36,13.51 17,22.46,2705,37,11.83 Reported-by:

Willem de Bruijn <willemb@google.com> Signed-off-by:

Eric Dumazet <edumazet@google.com> Fixes: ("tcp: auto corking") Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

+10 −4

Original line number	Diff line number	Diff line
		@@ -622,19 +622,21 @@ static inline void tcp_mark_urg(struct tcp_sock *tp, int flags)
		}

		/* If a not yet filled skb is pushed, do not send it if
		* we have packets in Qdisc or NIC queues :
		* we have data packets in Qdisc or NIC queues :
		* Because TX completion will happen shortly, it gives a chance
		* to coalesce future sendmsg() payload into this skb, without
		* need for a timer, and with no latency trade off.
		* As packets containing data payload have a bigger truesize
		* than pure acks (dataless) packets, the last check prevents
		* autocorking if we only have an ACK in Qdisc/NIC queues.
		* than pure acks (dataless) packets, the last checks prevent
		* autocorking if we only have an ACK in Qdisc/NIC queues,
		* or if TX completion was delayed after we processed ACK packet.
		*/
		static bool tcp_should_autocork(struct sock sk, struct sk_buff skb,
		int size_goal)
		{
		return skb->len < size_goal &&
		sysctl_tcp_autocorking &&
		skb != tcp_write_queue_head(sk) &&
		atomic_read(&sk->sk_wmem_alloc) > skb->truesize;
		}

		@@ -660,6 +662,10 @@ static void tcp_push(struct sock *sk, int flags, int mss_now,
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPAUTOCORKING);
		set_bit(TSQ_THROTTLED, &tp->tsq_flags);
		}
		/* It is possible TX completion already happened
		* before we set TSQ_THROTTLED.
		*/
		if (atomic_read(&sk->sk_wmem_alloc) > skb->truesize)
		return;
		}

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