netdev - Re: [PATCH net-next] tcp: batch calls to sk_flush

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Date:   Fri, 9 Aug 2019 09:08:38 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: batch calls to sk_flush_backlog()

On Fri, Aug 9, 2019 at 8:04 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Starting from commit d41a69f1d390 ("tcp: make tcp_sendmsg() aware of socket backlog")
> loopback flows got hurt, because for each skb sent, the socket receives an
> immediate ACK and sk_flush_backlog() causes extra work.
>
> Intent was to not let the backlog grow too much, but we went a bit too far.
>
> We can check the backlog every 16 skbs (about 1MB chunks)
> to increase TCP over loopback performance by about 15 %
>
> Note that the call to sk_flush_backlog() handles a single ACK,
> thanks to coalescing done on backlog, but cleans the 16 skbs
> found in rtx rb-tree.
>
> Reported-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Thank you very much, Eric!

> ---
>  net/ipv4/tcp.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index a0a66321c0ee99918b2080219dbaefcf3c398e13..f8fa1686f7f3e64f5d4ea8163e7f87538cc0d672 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1162,7 +1162,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>         struct sockcm_cookie sockc;
>         int flags, err, copied = 0;
>         int mss_now = 0, size_goal, copied_syn = 0;
> -       bool process_backlog = false;
> +       int process_backlog = 0;
>         bool zc = false;
>         long timeo;
>
> @@ -1254,9 +1254,10 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                         if (!sk_stream_memory_free(sk))
>                                 goto wait_for_sndbuf;
>
> -                       if (process_backlog && sk_flush_backlog(sk)) {
> -                               process_backlog = false;
> -                               goto restart;
> +                       if (unlikely(process_backlog >= 16)) {
> +                               process_backlog = 0;
> +                               if (sk_flush_backlog(sk))
> +                                       goto restart;
>                         }
>                         first_skb = tcp_rtx_and_write_queues_empty(sk);
>                         skb = sk_stream_alloc_skb(sk, 0, sk->sk_allocation,
> @@ -1264,7 +1265,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                         if (!skb)
>                                 goto wait_for_memory;
>
> -                       process_backlog = true;
> +                       process_backlog++;
>                         skb->ip_summed = CHECKSUM_PARTIAL;
>
>                         skb_entail(sk, skb);
> --
> 2.23.0.rc1.153.gdeed80330f-goog
>