netdev - Re: [PATCH v4 net-next 1/3] tcp: Make use of MSG_EOR in tcp

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Date:	Mon, 25 Apr 2016 20:48:37 -0400
From:	Soheil Hassas Yeganeh <soheil@...gle.com>
To:	Martin KaFai Lau <kafai@...com>
Cc:	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 net-next 1/3] tcp: Make use of MSG_EOR in tcp_sendmsg

On Mon, Apr 25, 2016 at 5:44 PM, Martin KaFai Lau <kafai@...com> wrote:
> This patch adds an eor bit to the TCP_SKB_CB.  When MSG_EOR
> is passed to tcp_sendmsg, the eor bit will be set at the skb
> containing the last byte of the userland's msg.  The eor bit
> will prevent data from appending to that skb in the future.
>
> The change in do_tcp_sendpages is to honor the eor set
> during the previous tcp_sendmsg(MSG_EOR) call.
>
> This patch handles the tcp_sendmsg case.  The followup patches
> will handle other skb coalescing and fragment cases.
>
> One potential use case is to use MSG_EOR with
> SOF_TIMESTAMPING_TX_ACK to get a more accurate
> TCP ack timestamping on application protocol with
> multiple outgoing response messages (e.g. HTTP2).
>
> Packetdrill script for testing:
> ~~~~~~
> +0 `sysctl -q -w net.ipv4.tcp_min_tso_segs=10`
> +0 `sysctl -q -w net.ipv4.tcp_no_metrics_save=1`
> +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
> +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> +0 bind(3, ..., ...) = 0
> +0 listen(3, 1) = 0
>
> 0.100 < S 0:0(0) win 32792 <mss 1460,sackOK,nop,nop,nop,wscale 7>
> 0.100 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 7>
> 0.200 < . 1:1(0) ack 1 win 257
> 0.200 accept(3, ..., ...) = 4
> +0 setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0
>
> 0.200 write(4, ..., 14600) = 14600
> 0.200 sendto(4, ..., 730, MSG_EOR, ..., ...) = 730
> 0.200 sendto(4, ..., 730, MSG_EOR, ..., ...) = 730
>
> 0.200 > .  1:7301(7300) ack 1
> 0.200 > P. 7301:14601(7300) ack 1
>
> 0.300 < . 1:1(0) ack 14601 win 257
> 0.300 > P. 14601:15331(730) ack 1
> 0.300 > P. 15331:16061(730) ack 1
>
> 0.400 < . 1:1(0) ack 16061 win 257
> 0.400 close(4) = 0
> 0.400 > F. 16061:16061(0) ack 1
> 0.400 < F. 1:1(0) ack 16062 win 257
> 0.400 > . 16062:16062(0) ack 2
>
> Signed-off-by: Martin KaFai Lau <kafai@...com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Suggested-by: Eric Dumazet <edumazet@...gle.com>

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

> ---
>  include/net/tcp.h | 8 +++++++-
>  net/ipv4/tcp.c    | 7 +++++--
>  2 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 7f2553d..ce08038 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -762,7 +762,8 @@ struct tcp_skb_cb {
>
>         __u8            ip_dsfield;     /* IPv4 tos or IPv6 dsfield     */
>         __u8            txstamp_ack:1,  /* Record TX timestamp for ack? */
> -                       unused:7;
> +                       eor:1,          /* Is skb MSG_EOR marked? */
> +                       unused:6;
>         __u32           ack_seq;        /* Sequence number ACK'd        */
>         union {
>                 struct inet_skb_parm    h4;
> @@ -809,6 +810,11 @@ static inline int tcp_skb_mss(const struct sk_buff *skb)
>         return TCP_SKB_CB(skb)->tcp_gso_size;
>  }
>
> +static inline bool tcp_skb_can_collapse_to(const struct sk_buff *skb)
> +{
> +       return likely(!TCP_SKB_CB(skb)->eor);
> +}
> +
>  /* Events passed to congestion control interface */
>  enum tcp_ca_event {
>         CA_EVENT_TX_START,      /* first transmit when no packets in flight */
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 4d73858..ea5364b 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -908,7 +908,8 @@ static ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
>                 int copy, i;
>                 bool can_coalesce;
>
> -               if (!tcp_send_head(sk) || (copy = size_goal - skb->len) <= 0) {
> +               if (!tcp_send_head(sk) || (copy = size_goal - skb->len) <= 0 ||
> +                   !tcp_skb_can_collapse_to(skb)) {
>  new_segment:
>                         if (!sk_stream_memory_free(sk))
>                                 goto wait_for_sndbuf;
> @@ -1156,7 +1157,7 @@ int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>                         copy = max - skb->len;
>                 }
>
> -               if (copy <= 0) {
> +               if (copy <= 0 || !tcp_skb_can_collapse_to(skb)) {
>  new_segment:
>                         /* Allocate new segment. If the interface is SG,
>                          * allocate skb fitting to single page.
> @@ -1250,6 +1251,8 @@ new_segment:
>                 copied += copy;
>                 if (!msg_data_left(msg)) {
>                         tcp_tx_timestamp(sk, sockc.tsflags, skb);
> +                       if (unlikely(flags & MSG_EOR))
> +                               TCP_SKB_CB(skb)->eor = 1;
>                         goto out;
>                 }
>
> --
> 2.5.1
>