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

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Date:   Thu, 10 Dec 2020 14:34:23 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>
Cc:     David Miller <davem@...emloft.net>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "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

On Thu, Dec 10, 2020 at 1:49 PM Mohamed Abuelfotoh, Hazem
<abuehaze@...zon.com> wrote:
>
> 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?
>

No need for stable tags , as documented in
https://elixir.bootlin.com/linux/v5.9/source/Documentation/networking/netdev-FAQ.rst#L147


> Thank you.
>
> Hazem
>
> On 09/12/2020, 00:29, "David Miller" <davem@...emloft.net> wrote:
>
>     CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>
>
>
>     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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>
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