On Thu, 4 Mar 2021 22:26:58 +0100 Eric Dumazet wrote:
> It would be nice if tun driver would have the ability to delay TX
> completions by N usecs,
> so that packetdrill tests could be used.
>
> It is probably not too hard to add such a feature.
Add an ioctl to turn off the skb_orphan, queue the skbs in tun_do_read()
to free them from a timer?
> I was testing this (note how I also removed the tcp_rearm_rto(sk) call)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index
> 6f450e577975c7be9537338c8a4c0673d7d36c4c..9ef92ca55e530f76ad793d7342442c4ec06165f7
> 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -6471,11 +6471,10 @@ static bool tcp_rcv_fastopen_synack(struct
> sock *sk, struct sk_buff *synack,
> tcp_fastopen_cache_set(sk, mss, cookie, syn_drop, try_exp);
>
> if (data) { /* Retransmit unacked data in SYN */
> - skb_rbtree_walk_from(data) {
> - if (__tcp_retransmit_skb(sk, data, 1))
> - break;
> - }
> - tcp_rearm_rto(sk);
> + skb_rbtree_walk_from(data)
> + tcp_mark_skb_lost(sk, data);
> +
> + tcp_xmit_retransmit_queue(sk);
> NET_INC_STATS(sock_net(sk),
> LINUX_MIB_TCPFASTOPENACTIVEFAIL);
> return true;
AFAICT this works great now:
==> TFO case ret:-16 (0) ca_state:0 skb:ffff8881d3513800!
FREED swapper/5 -- skb 0xffff8881d3513800 freed after: 1us
-----
First:
__tcp_retransmit_skb+1
tcp_retransmit_skb+18
tcp_xmit_retransmit_queue.part.70+339
tcp_rcv_state_process+2491
tcp_v6_do_rcv+405
tcp_v6_rcv+2984
Second:
__tcp_retransmit_skb+1
tcp_retransmit_skb+18
tcp_xmit_retransmit_queue.part.70+339
tcp_tsq_write.part.71+146
tcp_tsq_handler+53
tcp_tasklet_func+181
sk:0xffff8885adc16f00 skb:ffff8881d3513800 --- 61us acked:1
The other case where we hit RTO after __tcp_retransmit_skb() fails is:
==> non-TFO case ret:-11 (0) ca_state:3 skb:ffff8883d71dd400!
-----
First:
__tcp_retransmit_skb+1
tcp_retransmit_skb+18
tcp_xmit_retransmit_queue.part.70+339
tcp_ack+2270
tcp_rcv_established+303
tcp_v6_do_rcv+190
Second:
__tcp_retransmit_skb+1
tcp_retransmit_skb+18
tcp_retransmit_timer+716
tcp_write_timer_handler+136
tcp_write_timer+141
call_timer_fn+43
sk:0xffff88801772d340 skb:ffff8883d71dd400 --- 51738us acked:47324
Which I believe is this:
if (refcount_read(&sk->sk_wmem_alloc) >
min_t(u32, sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2),
sk->sk_sndbuf))
return -EAGAIN;
Because __tcp_retransmit_skb() seems to bail before
inet6_sk_rebuild_header gets called. Should we arm TSQ here as well?
