tcp: always timestamp on every skb transmission (7f12422c) · Commits · 戴 / test

Previously TCP skbs are not always timestamped if the transmission failed due to memory or other local issues. This makes deciding when to abort a socket tricky and complicated because the first unacknowledged skb's timestamp may be 0 on TCP timeout. The straight-forward fix is to always timestamp skb on every transmission attempt. Also every skb retransmission needs to be flagged properly to avoid RTT under-estimation. This can happen upon receiving an ACK for the original packet and the a previous (spurious) retransmission has failed. It's worth noting that this reverts to the old time-stamping style before commit ("tcp: update skb->skb_mstamp more carefully") which addresses a problem in computing the elapsed time of a stalled window-probing socket. The problem will be addressed differently in the next patches with a simpler approach. Signed-off-by:

Yuchung Cheng <ycheng@google.com> Signed-off-by:

Eric Dumazet <edumazet@google.com> Reviewed-by:

Neal Cardwell <ncardwell@google.com> Reviewed-by:

Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

+8 −8

Original line number	Diff line number	Diff line
		@@ -980,7 +980,6 @@ static void tcp_update_skb_after_send(struct sock sk, struct sk_buff skb,
		{
		struct tcp_sock *tp = tcp_sk(sk);

		skb->skb_mstamp_ns = tp->tcp_wstamp_ns;
		if (sk->sk_pacing_status != SK_PACING_NONE) {
		unsigned long rate = sk->sk_pacing_rate;

		@@ -1028,7 +1027,9 @@ static int __tcp_transmit_skb(struct sock sk, struct sk_buff skb,

		BUG_ON(!skb \|\| !tcp_skb_pcount(skb));
		tp = tcp_sk(sk);

		prior_wstamp = tp->tcp_wstamp_ns;
		tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);
		skb->skb_mstamp_ns = tp->tcp_wstamp_ns;
		if (clone_it) {
		TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
		- tp->snd_una;
		@@ -1045,11 +1046,6 @@ static int __tcp_transmit_skb(struct sock sk, struct sk_buff skb,
		return -ENOBUFS;
		}

		prior_wstamp = tp->tcp_wstamp_ns;
		tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);

		skb->skb_mstamp_ns = tp->tcp_wstamp_ns;

		inet = inet_sk(sk);
		tcb = TCP_SKB_CB(skb);
		memset(&opts, 0, sizeof(opts));
		@@ -2937,12 +2933,16 @@ int __tcp_retransmit_skb(struct sock sk, struct sk_buff skb, int segs)
		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
		}

		/* To avoid taking spuriously low RTT samples based on a timestamp
		* for a transmit that never happened, always mark EVER_RETRANS
		*/
		TCP_SKB_CB(skb)->sacked \|= TCPCB_EVER_RETRANS;

		if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
		TCP_SKB_CB(skb)->seq, segs, err);

		if (likely(!err)) {
		TCP_SKB_CB(skb)->sacked \|= TCPCB_EVER_RETRANS;
		trace_tcp_retransmit_skb(sk, skb);
		} else if (err != -EBUSY) {
		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);

Admin message