netdev - Re: [PATCH v4 2/2] tcp: Add snd_wnd to TCP

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Date:   Fri, 13 Sep 2019 17:02:28 -0400
From:   Neal Cardwell <ncardwell@...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>,
        Dave Taht <dave.taht@...il.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH v4 2/2] tcp: Add snd_wnd to TCP_INFO

On Fri, Sep 13, 2019 at 3:36 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>
> ---
> changes from v3:
>  - changed from rcv_wnd to snd_wnd
>
>  include/uapi/linux/tcp.h | 1 +
>  net/ipv4/tcp.c           | 1 +
>  2 files changed, 2 insertions(+)
>
> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index 20237987ccc8..240654f22d98 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -272,6 +272,7 @@ struct tcp_info {
>         __u32   tcpi_reord_seen;     /* reordering events seen */
>
>         __u32   tcpi_rcv_ooopack;    /* Out-of-order packets received */
> +       __u32   tcpi_snd_wnd;        /* Remote peer's advertised recv window size */
>  };

Thanks for adding this!

My run of ./scripts/checkpatch.pl is showing a warning on this line:

WARNING: line over 80 characters
#19: FILE: include/uapi/linux/tcp.h:273:
+       __u32   tcpi_snd_wnd;        /* Remote peer's advertised recv
window size */

What if the comment is shortened up to fit in 80 columns and the units
(bytes) are added, something like:

        __u32   tcpi_snd_wnd;        /* peer's advertised recv window (bytes) */

neal