netdev - Re: [PATCH v5 2/2] tcp: Add snd_wnd to TCP

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Date:   Fri, 13 Sep 2019 16:36:04 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Thomas Higdon <tph@...com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Dave Jones <dsj@...com>, Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Dave Taht <dave.taht@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH v5 2/2] tcp: Add snd_wnd to TCP_INFO

On Fri, Sep 13, 2019 at 4:23 PM Thomas Higdon <tph@...com> wrote:
>
> Neal Cardwell mentioned that snd_wnd would be useful for diagnosing TCP
> performance problems --
> > (1) Usually when we're diagnosing TCP performance problems, we do so
> > from the sender, since the sender makes most of the
> > performance-critical decisions (cwnd, pacing, TSO size, TSQ, etc).
> > From the sender-side the thing that would be most useful is to see
> > tp->snd_wnd, the receive window that the receiver has advertised to
> > the sender.
>
> This serves the purpose of adding an additional __u32 to avoid the
> would-be hole caused by the addition of the tcpi_rcvi_ooopack field.
>
> Signed-off-by: Thomas Higdon <tph@...com>
> ---
Acked-by: Yuchung Cheng <ycheng@...gle.com>

> changes since v4:
>  - clarify comment
>  include/uapi/linux/tcp.h | 4 ++++
>  net/ipv4/tcp.c           | 1 +
>  2 files changed, 5 insertions(+)
>
> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index 20237987ccc8..81e697978e8b 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -272,6 +272,10 @@ struct tcp_info {
>         __u32   tcpi_reord_seen;     /* reordering events seen */
>
>         __u32   tcpi_rcv_ooopack;    /* Out-of-order packets received */
> +
> +       __u32   tcpi_snd_wnd;        /* peer's advertised receive window after
> +                                     * scaling (bytes)
> +                                     */
>  };
>
>  /* netlink attributes types for SCM_TIMESTAMPING_OPT_STATS */
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 4cf58208270e..79c325a07ba5 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -3297,6 +3297,7 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
>         info->tcpi_dsack_dups = tp->dsack_dups;
>         info->tcpi_reord_seen = tp->reord_seen;
>         info->tcpi_rcv_ooopack = tp->rcv_ooopack;
> +       info->tcpi_snd_wnd = tp->snd_wnd;
>         unlock_sock_fast(sk, slow);
>  }
>  EXPORT_SYMBOL_GPL(tcp_get_info);
> --
> 2.17.1
>