On Thu, Sep 15, 2016 at 9:33 AM, Eric Dumazet <eric.duma...@gmail.com> wrote:
>
> From: Eric Dumazet <eduma...@google.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 <eduma...@google.com>
> Cc: Yuchung Cheng <ych...@google.com>
Acked-by: Yuchung Cheng <ych...@google.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;
>
>
>
