netdev - Re: [PATCH v4 1/2] tcp: Add TCP_INFO counter for packets received out-of-order

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Date:   Fri, 13 Sep 2019 16:55:46 -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 1/2] tcp: Add TCP_INFO counter for packets received out-of-order

On Fri, Sep 13, 2019 at 3:37 PM Thomas Higdon <tph@...com> wrote:
>
> For receive-heavy cases on the server-side, we want to track the
> connection quality for individual client IPs. This counter, similar to
> the existing system-wide TCPOFOQueue counter in /proc/net/netstat,
> tracks out-of-order packet reception. By providing this counter in
> TCP_INFO, it will allow understanding to what degree receive-heavy
> sockets are experiencing out-of-order delivery and packet drops
> indicating congestion.
>
> Please note that this is similar to the counter in NetBSD TCP_INFO, and
> has the same name.
>
> Signed-off-by: Thomas Higdon <tph@...com>
> ---
>
> no changes from v3
>
>  include/linux/tcp.h      | 2 ++
>  include/uapi/linux/tcp.h | 2 ++
>  net/ipv4/tcp.c           | 2 ++
>  net/ipv4/tcp_input.c     | 1 +
>  4 files changed, 7 insertions(+)
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index f3a85a7fb4b1..a01dc78218f1 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -393,6 +393,8 @@ struct tcp_sock {
>          */
>         struct request_sock *fastopen_rsk;
>         u32     *saved_syn;
> +
> +       u32 rcv_ooopack; /* Received out-of-order packets, for tcpinfo */

Thanks for adding this.

A thought: putting the new rcv_ooopack field here makes struct
tcp_sock bigger, and increases the odds of taking a cache miss
(according to "pahole" this field is the only one in a new cache
line).

I'd suggest putting the new rcv_ooopack field immediately before
rcv_rtt_last_tsecr. That would use up a 4-byte hole, and would place
it in a cache line already used on TCP receivers (for rcv_rtt logic).
This would make it less likely this new field causes more cache misses
or uses more space.

Details: looking at the output of "pahole" for tcp_sock in various cases:

net-next before this patch:
-------------------------------------
...
        u8                         bpf_sock_ops_cb_flags; /*  2076     1 */

        /* XXX 3 bytes hole, try to pack */

        u32                        rcv_rtt_last_tsecr;   /*  2080     4 */

        /* XXX 4 bytes hole, try to pack */

        struct {
                u32                rtt_us;               /*  2088     4 */
                u32                seq;                  /*  2092     4 */
                u64                time;                 /*  2096     8 */
        } rcv_rtt_est;                                   /*  2088    16 */
...
        /* size: 2176, cachelines: 34, members: 134 */
        /* sum members: 2164, holes: 4, sum holes: 12 */
        /* paddings: 3, sum paddings: 12 */


net-next with this patch:
-------------------------------------
...
        u32 *                      saved_syn;            /*  2168     8 */
        /* --- cacheline 34 boundary (2176 bytes) --- */
        u32                        rcv_ooopack;          /*  2176     4 */
...
        /* size: 2184, cachelines: 35, members: 135 */
        /* sum members: 2168, holes: 4, sum holes: 12 */
        /* padding: 4 */
        /* paddings: 3, sum paddings: 12 */
        /* last cacheline: 8 bytes */


net-next with this field in the suggested spot:
-------------------------------------
...
       /* XXX 3 bytes hole, try to pack */

        u32                        rcv_ooopack;          /*  2080     4 */
        u32                        rcv_rtt_last_tsecr;   /*  2084     4 */
        struct {
                u32                rtt_us;               /*  2088     4 */
                u32                seq;                  /*  2092     4 */
                u64                time;                 /*  2096     8 */
        } rcv_rtt_est;                                   /*  2088    16 */
...
        /* size: 2176, cachelines: 34, members: 135 */
        /* sum members: 2168, holes: 3, sum holes: 8 */
        /* paddings: 3, sum paddings: 12 */

neal


neal