netdev - Re: [PATCH net] tcp: select sane initial rcvq

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Message-ID: <A65467F2-CC10-4FAF-A728-0969FD17E780@amazon.com>
Date:   Thu, 10 Dec 2020 12:49:18 +0000
From:   "Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>
To:     David Miller <davem@...emloft.net>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>
CC:     "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "soheil@...gle.com" <soheil@...gle.com>,
        "ncardwell@...gle.com" <ncardwell@...gle.com>,
        "ycheng@...gle.com" <ycheng@...gle.com>
Subject: Re: [PATCH net] tcp: select sane initial rcvq_space.space for big MSS

Hi Eric,

I don't see the patch in the stable queue. Can we add it  to stable so we can cherry pick it  in Amazon Linux kernel?

Thank you.

Hazem

On 09/12/2020, 00:29, "David Miller" <davem@...emloft.net> wrote:

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    From: Eric Dumazet <eric.dumazet@...il.com>
    Date: Tue,  8 Dec 2020 08:21:31 -0800

    > 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>

    Applied, thanks Eric.




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