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Date: Tue, 12 Aug 2014 07:53:57 -0700
From: Yuchung Cheng <ycheng@...gle.com>
To: Andrey Vagin <avagin@...nvz.org>
Cc: netdev <netdev@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Pavel Emelyanov <xemul@...allels.com>,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] tcp: don't use timestamp from repaired skb-s to calculate RTT
On Tue, Aug 12, 2014 at 2:45 AM, Andrey Vagin <avagin@...nvz.org> 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@...gle.com>
> Cc: Pavel Emelyanov <xemul@...allels.com>
> Cc: "David S. Miller" <davem@...emloft.net>
> Signed-off-by: Andrey Vagin <avagin@...nvz.org>
> ---
> 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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