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Date:   Sat, 27 Aug 2016 09:13:58 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: add tcp_add_backlog()

On Sat, Aug 27, 2016 at 7:37 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> When TCP operates in lossy environments (between 1 and 10 % packet
> losses), many SACK blocks can be exchanged, and I noticed we could
> drop them on busy senders, if these SACK blocks have to be queued
> into the socket backlog.
>
> While the main cause is the poor performance of RACK/SACK processing,
I have a patch in preparation for this ;)

> we can try to avoid these drops of valuable information that can lead to
> spurious timeouts and retransmits.
>
> Cause of the drops is the skb->truesize overestimation caused by :
>
> - drivers allocating ~2048 (or more) bytes as a fragment to hold an
>   Ethernet frame.
>
> - various pskb_may_pull() calls bringing the headers into skb->head
>   might have pulled all the frame content, but skb->truesize could
>   not be lowered, as the stack has no idea of each fragment truesize.
>
> The backlog drops are also more visible on bidirectional flows, since
> their sk_rmem_alloc can be quite big.
>
> Let's add some room for the backlog, as only the socket owner
> can selectively take action to lower memory needs, like collapsing
> receive queues or partial ofo pruning.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> ---
>  include/net/tcp.h   |    1 +
>  net/ipv4/tcp_ipv4.c |   33 +++++++++++++++++++++++++++++----
>  net/ipv6/tcp_ipv6.c |    5 +----
>  3 files changed, 31 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 25d64f6de69e1f639ed1531bf2d2df3f00fd76a2..5f5f09f6e019682ef29c864d2f43a8f247fcdd9a 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1163,6 +1163,7 @@ static inline void tcp_prequeue_init(struct tcp_sock *tp)
>  }
>
>  bool tcp_prequeue(struct sock *sk, struct sk_buff *skb);
> +bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb);
>
>  #undef STATE_TRACE
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index ad41e8ecf796bba1bd6d9ed155ca4a57ced96844..53e80cd004b6ce401c3acbb4b243b243c5c3c4a3 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1532,6 +1532,34 @@ bool tcp_prequeue(struct sock *sk, struct sk_buff *skb)
>  }
>  EXPORT_SYMBOL(tcp_prequeue);
>
> +bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
> +{
> +       u32 limit = sk->sk_rcvbuf + sk->sk_sndbuf;
> +
> +       /* Only socket owner can try to collapse/prune rx queues
> +        * to reduce memory overhead, so add a little headroom here.
> +        * Few sockets backlog are possibly concurrently non empty.
> +        */
> +       limit += 64*1024;
Just a thought: only add the headroom if ofo queue exists (e.g., signs
of losses ore recovery).

btw is the added headroom subject to the memory pressure check?

> +
> +       /* In case all data was pulled from skb frags (in __pskb_pull_tail()),
> +        * we can fix skb->truesize to its real value to avoid future drops.
> +        * This is valid because skb is not yet charged to the socket.
> +        * It has been noticed pure SACK packets were sometimes dropped
> +        * (if cooked by drivers without copybreak feature).
> +        */
> +       if (!skb->data_len)
> +               skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
nice!

> +
> +       if (unlikely(sk_add_backlog(sk, skb, limit))) {
> +               bh_unlock_sock(sk);
> +               __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPBACKLOGDROP);
> +               return true;
> +       }
> +       return false;
> +}
> +EXPORT_SYMBOL(tcp_add_backlog);
> +
>  /*
>   *     From tcp_input.c
>   */
> @@ -1662,10 +1690,7 @@ process:
>         if (!sock_owned_by_user(sk)) {
>                 if (!tcp_prequeue(sk, skb))
>                         ret = tcp_v4_do_rcv(sk, skb);
> -       } else if (unlikely(sk_add_backlog(sk, skb,
> -                                          sk->sk_rcvbuf + sk->sk_sndbuf))) {
> -               bh_unlock_sock(sk);
> -               __NET_INC_STATS(net, LINUX_MIB_TCPBACKLOGDROP);
> +       } else if (tcp_add_backlog(sk, skb)) {
>                 goto discard_and_relse;
>         }
>         bh_unlock_sock(sk);
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index e4f55683af314438da8a09473927213a140e6e5c..5bf460bd299ff61ae759e8e545ba6298a1d1373c 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1467,10 +1467,7 @@ process:
>         if (!sock_owned_by_user(sk)) {
>                 if (!tcp_prequeue(sk, skb))
>                         ret = tcp_v6_do_rcv(sk, skb);
> -       } else if (unlikely(sk_add_backlog(sk, skb,
> -                                          sk->sk_rcvbuf + sk->sk_sndbuf))) {
> -               bh_unlock_sock(sk);
> -               __NET_INC_STATS(net, LINUX_MIB_TCPBACKLOGDROP);
> +       } else if (tcp_add_backlog(sk, skb)) {
>                 goto discard_and_relse;
>         }
>         bh_unlock_sock(sk);
>
>

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