Re: [PATCH] tcp: don't use timestamp from repaired skb-s to calculate RTT

[Yuchung Cheng]

On Tue, Aug 12, 2014 at 2:45 AM, Andrey Vagin <avagin@xxxxxxxxxx> wrote:
> We don't know right timestamp for repaired skb-s. Wrong RTT estimations
> isn't good, because some congestion modules heavily depends on it.
>
> This patch adds the TCPCB_REPAIRED flag, which is included in
> TCPCB_RETRANS.
>
> Thanks to Eric for the advice how to fix this issue.
>
> This patch fixes the warning:
> [  879.562947] WARNING: CPU: 0 PID: 2825 at net/ipv4/tcp_input.c:3078 tcp_ack+0x11f5/0x1380()
> [  879.567253] CPU: 0 PID: 2825 Comm: socket-tcpbuf-l Not tainted 3.16.0-next-20140811 #1
> [  879.567829] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> [  879.568177]  0000000000000000 00000000c532680c ffff880039643d00 ffffffff817aa2d2
> [  879.568776]  0000000000000000 ffff880039643d38 ffffffff8109afbd ffff880039d6ba80
> [  879.569386]  ffff88003a449800 000000002983d6bd 0000000000000000 000000002983d6bc
> [  879.569982] Call Trace:
> [  879.570264]  [<ffffffff817aa2d2>] dump_stack+0x4d/0x66
> [  879.570599]  [<ffffffff8109afbd>] warn_slowpath_common+0x7d/0xa0
> [  879.570935]  [<ffffffff8109b0ea>] warn_slowpath_null+0x1a/0x20
> [  879.571292]  [<ffffffff816d0a05>] tcp_ack+0x11f5/0x1380
> [  879.571614]  [<ffffffff816d10bd>] tcp_rcv_established+0x1ed/0x710
> [  879.571958]  [<ffffffff816dc9da>] tcp_v4_do_rcv+0x10a/0x370
> [  879.572315]  [<ffffffff81657459>] release_sock+0x89/0x1d0
> [  879.572642]  [<ffffffff816c81a0>] do_tcp_setsockopt.isra.36+0x120/0x860
> [  879.573000]  [<ffffffff8110a52e>] ? rcu_read_lock_held+0x6e/0x80
> [  879.573352]  [<ffffffff816c8912>] tcp_setsockopt+0x32/0x40
> [  879.573678]  [<ffffffff81654ac4>] sock_common_setsockopt+0x14/0x20
> [  879.574031]  [<ffffffff816537b0>] SyS_setsockopt+0x80/0xf0
> [  879.574393]  [<ffffffff817b40a9>] system_call_fastpath+0x16/0x1b
> [  879.574730] ---[ end trace a17cbc38eb8c5c00 ]---
>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
> ---
>  include/net/tcp.h |  4 +++-
>  net/ipv4/tcp.c    | 16 +++++++++-------
>  2 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index dafa1cb..36f5525 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -705,8 +705,10 @@ struct tcp_skb_cb {
>  #define TCPCB_SACKED_RETRANS   0x02    /* SKB retransmitted            */
>  #define TCPCB_LOST             0x04    /* SKB is lost                  */
>  #define TCPCB_TAGBITS          0x07    /* All tag bits                 */
> +#define TCPCB_REPAIRED         0x10    /* SKB repaired (no skb_mstamp) */
>  #define TCPCB_EVER_RETRANS     0x80    /* Ever retransmitted frame     */
> -#define TCPCB_RETRANS          (TCPCB_SACKED_RETRANS|TCPCB_EVER_RETRANS)
> +#define TCPCB_RETRANS          (TCPCB_SACKED_RETRANS|TCPCB_EVER_RETRANS| \
> +                               TCPCB_REPAIRED)
>
>         __u8            ip_dsfield;     /* IPv4 tos or IPv6 dsfield     */
>         /* 1 byte hole */
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 181b70e..cb5f548 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1188,13 +1188,6 @@ new_segment:
>                                         goto wait_for_memory;
>
>                                 /*
> -                                * All packets are restored as if they have
> -                                * already been sent.
> -                                */
> -                               if (tp->repair)
> -                                       TCP_SKB_CB(skb)->when = tcp_time_stamp;
> -
> -                               /*
>                                  * Check whether we can use HW checksum.
>                                  */
>                                 if (sk->sk_route_caps & NETIF_F_ALL_CSUM)
> @@ -1203,6 +1196,15 @@ new_segment:
>                                 skb_entail(sk, skb);
>                                 copy = size_goal;
>                                 max = size_goal;
> +
> +                               /* All packets are restored as if they have
> +                                * already been sent. skb_mstamp isn't set to
> +                                * avoid wrong rtt estimation.
> +                                */
> +                               if (tp->repair) {
> +                                       TCP_SKB_CB(skb)->sacked |= TCPCB_REPAIRED;
> +                                       TCP_SKB_CB(skb)->when = tcp_time_stamp;
But this still allow RTT samples from TCP timestamp options even if
the packet is marked retransmitted/repaired in tcp_ack_update_rtt()?

> +                               }
>                         }
>
>                         /* Try to append data to the end of skb. */
> --
> 1.9.3
>
> --
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