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Date:   Tue, 8 Dec 2020 15:45:52 -0500
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>
Subject: Re: [PATCH net] tcp: select sane initial rcvq_space.space for big MSS

On Tue, Dec 8, 2020 at 11:21 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> Before commit a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
> small tcp_rmem[1] values were overridden by tcp_fixup_rcvbuf() to accommodate various MSS.
>
> This is no longer the case, and Hazem Mohamed Abuelfotoh reported
> that DRS would not work for MTU 9000 endpoints receiving regular (1500 bytes) frames.
>
> Root cause is that tcp_init_buffer_space() uses tp->rcv_wnd for upper limit
> of rcvq_space.space computation, while it can select later a smaller
> value for tp->rcv_ssthresh and tp->window_clamp.
>
> ss -temoi on receiver would show :
>
> skmem:(r0,rb131072,t0,tb46080,f0,w0,o0,bl0,d0) rcv_space:62496 rcv_ssthresh:56596
>
> This means that TCP can not increase its window in tcp_grow_window(),
> and that DRS can never kick.
>
> Fix this by making sure that rcvq_space.space is not bigger than number of bytes
> that can be held in TCP receive queue.
>
> People unable/unwilling to change their kernel can work around this issue by
> selecting a bigger tcp_rmem[1] value as in :
>
> echo "4096 196608 6291456" >/proc/sys/net/ipv4/tcp_rmem
>
> Based on an initial report and patch from Hazem Mohamed Abuelfotoh
>  https://lore.kernel.org/netdev/20201204180622.14285-1-abuehaze@amazon.com/
>
> Fixes: a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
> Fixes: 041a14d26715 ("tcp: start receiver buffer autotuning sooner")
> Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Nice catch and fix!  Thanks Eric!


> ---
>  net/ipv4/tcp_input.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 389d1b34024854a9bdcbe861d4820d1bfb495e24..ef4bdb038a4bbbd949868a01dc855bba0e90b9ca 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -510,7 +510,6 @@ static void tcp_init_buffer_space(struct sock *sk)
>         if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
>                 tcp_sndbuf_expand(sk);
>
> -       tp->rcvq_space.space = min_t(u32, tp->rcv_wnd, TCP_INIT_CWND * tp->advmss);
>         tcp_mstamp_refresh(tp);
>         tp->rcvq_space.time = tp->tcp_mstamp;
>         tp->rcvq_space.seq = tp->copied_seq;
> @@ -534,6 +533,8 @@ static void tcp_init_buffer_space(struct sock *sk)
>
>         tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
>         tp->snd_cwnd_stamp = tcp_jiffies32;
> +       tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,
> +                                   (u32)TCP_INIT_CWND * tp->advmss);
>  }
>
>  /* 4. Recalculate window clamp after socket hit its memory bounds. */
> --
> 2.29.2.576.ga3fc446d84-goog
>

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