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From:   Neal Cardwell <ncardwell@google.com>
Date:   Mon, 16 Mar 2020 17:25:41 -0400
Message-ID: <CADVnQy=nhaKY8Ub84TvFH4cFtW92_TU72OxXnV32FzdEGtDCvw@mail.gmail.com>
Subject: Re: [PATCH RESEND net-next v2 2/5] tcp: fix stretch ACK bugs in Scalable
To:     Pengcheng Yang <yangpc@wangsu.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        David Miller <davem@davemloft.net>,
        Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Mar 16, 2020 at 2:37 AM Pengcheng Yang <yangpc@wangsu.com> wrote:
>
> Change Scalable to properly handle stretch ACKs in additive
> increase mode by passing in the count of ACKed packets to
> tcp_cong_avoid_ai().
>
> In addition, because we are now precisely accounting for
> stretch ACKs, including delayed ACKs, we can now change
> TCP_SCALABLE_AI_CNT to 100.
>
> Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
> ---
>  net/ipv4/tcp_scalable.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/net/ipv4/tcp_scalable.c b/net/ipv4/tcp_scalable.c
> index 471571e..6cebf41 100644
> --- a/net/ipv4/tcp_scalable.c
> +++ b/net/ipv4/tcp_scalable.c
> @@ -10,10 +10,9 @@
>  #include <net/tcp.h>
>
>  /* These factors derived from the recommended values in the aer:
> - * .01 and and 7/8. We use 50 instead of 100 to account for
> - * delayed ack.
> + * .01 and and 7/8.
>   */
> -#define TCP_SCALABLE_AI_CNT    50U
> +#define TCP_SCALABLE_AI_CNT    100U
>  #define TCP_SCALABLE_MD_SCALE  3
>
>  static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
> @@ -23,11 +22,13 @@ static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
>         if (!tcp_is_cwnd_limited(sk))
>                 return;
>
> -       if (tcp_in_slow_start(tp))
> -               tcp_slow_start(tp, acked);
> -       else
> -               tcp_cong_avoid_ai(tp, min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT),
> -                                 1);
> +       if (tcp_in_slow_start(tp)) {
> +               acked = tcp_slow_start(tp, acked);
> +               if (!acked)
> +                       return;
> +       }
> +       tcp_cong_avoid_ai(tp, min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT),
> +                         acked);
>  }
>
>  static u32 tcp_scalable_ssthresh(struct sock *sk)
> --

Acked-by: Neal Cardwell <ncardwell@google.com>

thanks,
neal