netdev - Re: [PATCH v1 2/2] tcp: Ignore OOO handling for TCP ACKs

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Message-ID: <CANn89iLZZAA6N5wzzP_ZR2u-shHLxknobxt+5CixA92rv7udcw@mail.gmail.com>
Date:   Thu, 20 Oct 2022 11:57:18 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kamaljit Singh <kamaljit.singh1@....com>
Cc:     davem@...emloft.net, yoshfuji@...ux-ipv6.org, dsahern@...nel.org,
        kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
        Niklas.Cassel@....com, Damien.LeMoal@....com
Subject: Re: [PATCH v1 2/2] tcp: Ignore OOO handling for TCP ACKs

On Thu, Oct 20, 2022 at 11:22 AM Kamaljit Singh <kamaljit.singh1@....com> wrote:
>
> Even with the TCP window fix to tcp_acceptable_seq(), occasional
> out-of-order host ACKs were still seen under heavy write workloads thus
> Impacting performance.  By removing the OoO optionality for ACKs in
> __tcp_transmit_skb() that issue seems to be fixed as well.

This is highly suspect/bogus.

 Please give which driver is used here.

>
> Signed-off-by: Kamaljit Singh <kamaljit.singh1@....com>
> ---
>  net/ipv4/tcp_output.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 322e061edb72..1cd77493f32c 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1307,7 +1307,10 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
>          * TODO: Ideally, in-flight pure ACK packets should not matter here.
>          * One way to get this would be to set skb->truesize = 2 on them.
>          */
> -       skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
> +       if (likely(tcb->tcp_flags & TCPHDR_ACK))
> +               skb->ooo_okay = 0;
> +       else
> +               skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
>

This is absolutely wrong and would impact performance quite a lot.

You are basically removing all possibilities for ackets of a TCP flow
to be directed to a new queue, say if use thread has migrated to
another cpu.

After 3WHS, all packets get ACK set.