netdev - Re: [PATCH v2] tcp: Add TCP_INFO counter for packets received out-of-order

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Message-ID: <CADVnQynvcnMi28DUCz3HPc79nz+7UduDT3S6A+fN49p7KxLDdg@mail.gmail.com>
Date:   Tue, 10 Sep 2019 18:25:14 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Thomas Higdon <tph@...com>, netdev <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Dave Jones <dsj@...com>
Subject: Re: [PATCH v2] tcp: Add TCP_INFO counter for packets received out-of-order

On Tue, Sep 10, 2019 at 4:39 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Sep 10, 2019 at 10:11 PM Thomas Higdon <tph@...com> wrote:
> >
> >
> ...
> > Because an additional 32-bit member in struct tcp_info would cause
> > a hole on 64-bit systems, we reserve a struct member '_reserved'.
> ...
> > diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> > index b3564f85a762..990a5bae3ac1 100644
> > --- a/include/uapi/linux/tcp.h
> > +++ b/include/uapi/linux/tcp.h
> > @@ -270,6 +270,9 @@ struct tcp_info {
> >         __u64   tcpi_bytes_retrans;  /* RFC4898 tcpEStatsPerfOctetsRetrans */
> >         __u32   tcpi_dsack_dups;     /* RFC4898 tcpEStatsStackDSACKDups */
> >         __u32   tcpi_reord_seen;     /* reordering events seen */
> > +
> > +       __u32   _reserved;           /* Reserved for future 32-bit member. */
> > +       __u32   tcpi_rcv_ooopack;    /* Out-of-order packets received */
> >  };
> >
>
> Unfortunately we won't be able to use this hole, because the way the
> TCP_INFO works,
>
> The kernel will report the same size after the reserved field is
> renamed to something else.
>
> User space code is able to detect which fields are there or not based
> on what the kernel
> returns for the size of the structure.

If we are looking for something else useful to use for a __u32 to pair
up with this new field, I would suggest  we export tp->snd_wnd
(send-side receive window) and tp->rcv_wnd (receive-side receive
window) in tcp_info. We could export one of them now, and the other
the next time we need to add a field and need some useful "padding".
These fields could help folks diagnose whether a flow is
receive-window-limited at a given instant, using "ss", etc. I think
there was some interest in this internally in our team a while ago.

neal