netdev - Re: [PATCH net] net: tcp: don't allocate fast clones for fastopen SYN

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Date:   Thu, 4 Mar 2021 11:06:26 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Neil Spring <ntspring@...com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net] net: tcp: don't allocate fast clones for fastopen
 SYN

On Thu, 4 Mar 2021 13:51:15 +0100 Eric Dumazet wrote:
> I think we are over thinking this really (especially if the fix needs
> a change in core networking or drivers)
> 
> We can reuse TSQ logic to have a chance to recover when the clone is
> eventually freed.
> This will be more generic, not only for the SYN+data of FastOpen.
> 
> Can you please test the following patch ?

#7 - Eric comes up with something much better :)


But so far doesn't seem to quite do it, I'm looking but maybe you'll
know right away (FWIW testing a v5.6 backport but I don't think TSQ
changed?):

On __tcp_retransmit_skb kretprobe:

==> Hit TFO case ret:-16 ca_state:0 skb:ffff888fdb4bac00!

First hit:
        __tcp_retransmit_skb+1
        tcp_rcv_state_process+2488
        tcp_v6_do_rcv+405
        tcp_v6_rcv+2984
        ip6_protocol_deliver_rcu+180
        ip6_input_finish+17

Successful hit:
        __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

 skb:ffff888fdb4bac00 --- delay:51642us bytes_acked:1