netdev - Re: [PATCH net v2] tcp: Adjust clamping window for applications specifying SO

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Message-ID: <CANn89i+56C=o-7FLxdPhDe4eEPffV_MySOPpmTmmkBNW_Tx99A@mail.gmail.com>
Date: Mon, 29 Jul 2024 08:59:01 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Subash Abhinov Kasiviswanathan <quic_subashab@...cinc.com>
Cc: soheil@...gle.com, ncardwell@...gle.com, yyd@...gle.com, ycheng@...gle.com, 
	davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org, 
	dsahern@...nel.org, pabeni@...hat.com, 
	Sean Tranchetti <quic_stranche@...cinc.com>
Subject: Re: [PATCH net v2] tcp: Adjust clamping window for applications
 specifying SO_RCVBUF

On Fri, Jul 26, 2024 at 10:41 PM Subash Abhinov Kasiviswanathan
<quic_subashab@...cinc.com> wrote:
>
> tp->scaling_ratio is not updated based on skb->len/skb->truesize once
> SO_RCVBUF is set leading to the maximum window scaling to be 25% of
> rcvbuf after
> commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> and 50% of rcvbuf after
> commit 697a6c8cec03 ("tcp: increase the default TCP scaling ratio").
> 50% tries to emulate the behavior of older kernels using
> sysctl_tcp_adv_win_scale with default value.
>
> Systems which were using a different values of sysctl_tcp_adv_win_scale
> in older kernels ended up seeing reduced download speeds in certain
> cases as covered in https://lists.openwall.net/netdev/2024/05/15/13
> While the sysctl scheme is no longer acceptable, the value of 50% is
> a bit conservative when the skb->len/skb->truesize ratio is later
> determined to be ~0.66.
>
> Applications not specifying SO_RCVBUF update the window scaling and
> the receiver buffer every time data is copied to userspace. This
> computation is now used for applications setting SO_RCVBUF to update
> the maximum window scaling while ensuring that the receive buffer
> is within the application specified limit.
>
> Fixes: dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> Signed-off-by: Sean Tranchetti <quic_stranche@...cinc.com>
> Signed-off-by: Subash Abhinov Kasiviswanathan <quic_subashab@...cinc.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks.