netdev - Re: [PATCH net-next] tcp: prepare skbs for better sack shifting

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Date:   Thu, 15 Sep 2016 10:52:21 -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>
Subject: Re: [PATCH net-next] tcp: prepare skbs for better sack shifting

On Thu, Sep 15, 2016 at 9:33 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> With large BDP TCP flows and lossy networks, it is very important
> to keep a low number of skbs in the write queue.
>
> RACK and SACK processing can perform a linear scan of it.
>
> We should avoid putting any payload in skb->head, so that SACK
> shifting can be done if needed.
>
> With this patch, we allow to pack ~0.5 MB per skb instead of
> the 64KB initially cooked at tcp_sendmsg() time.
>
> This gives a reduction of number of skbs in write queue by eight.
> tcp_rack_detect_loss() likes this.
>
> We still allow payload in skb->head for first skb put in the queue,
> to not impact RPC workloads.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>


> ---
>  net/ipv4/tcp.c |   31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index a13fcb369f52fe85def7c9d856259bc0509f3453..7dae800092e62cec330544851289d20a68642561 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1020,17 +1020,31 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset,
>  }
>  EXPORT_SYMBOL(tcp_sendpage);
>
> -static inline int select_size(const struct sock *sk, bool sg)
> +/* Do not bother using a page frag for very small frames.
> + * But use this heuristic only for the first skb in write queue.
> + *
> + * Having no payload in skb->head allows better SACK shifting
> + * in tcp_shift_skb_data(), reducing sack/rack overhead, because
> + * write queue has less skbs.
> + * Each skb can hold up to MAX_SKB_FRAGS * 32Kbytes, or ~0.5 MB.
> + * This also speeds up tso_fragment(), since it wont fallback
> + * to tcp_fragment().
> + */
> +static int linear_payload_sz(bool first_skb)
> +{
> +       if (first_skb)
> +               return SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER);
> +       return 0;
> +}
> +
> +static int select_size(const struct sock *sk, bool sg, bool first_skb)
>  {
>         const struct tcp_sock *tp = tcp_sk(sk);
>         int tmp = tp->mss_cache;
>
>         if (sg) {
>                 if (sk_can_gso(sk)) {
> -                       /* Small frames wont use a full page:
> -                        * Payload will immediately follow tcp header.
> -                        */
> -                       tmp = SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER);
> +                       tmp = linear_payload_sz(first_skb);
>                 } else {
>                         int pgbreak = SKB_MAX_HEAD(MAX_TCP_HEADER);
>
> @@ -1161,6 +1175,8 @@ restart:
>                 }
>
>                 if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) {
> +                       bool first_skb;
> +
>  new_segment:
>                         /* Allocate new segment. If the interface is SG,
>                          * allocate skb fitting to single page.
> @@ -1172,10 +1188,11 @@ new_segment:
>                                 process_backlog = false;
>                                 goto restart;
>                         }
> +                       first_skb = skb_queue_empty(&sk->sk_write_queue);
>                         skb = sk_stream_alloc_skb(sk,
> -                                                 select_size(sk, sg),
> +                                                 select_size(sk, sg, first_skb),
>                                                   sk->sk_allocation,
> -                                                 skb_queue_empty(&sk->sk_write_queue));
> +                                                 first_skb);
>                         if (!skb)
>                                 goto wait_for_memory;
>
>
>