netdev - Re: [RFC PATCH v3 net-next 1/3] tcp: Make use of MSG_EOR in tcp_sendmsg and tcp

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Date:	Wed, 20 Apr 2016 02:21:24 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Martin KaFai Lau <kafai@...com>
Cc:	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Soheil Hassas Yeganeh <soheil@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH v3 net-next 1/3] tcp: Make use of MSG_EOR in
 tcp_sendmsg and tcp_sendpage

On Tue, 2016-04-19 at 23:24 -0700, Martin KaFai Lau wrote:
> This patch adds an eor bit to the TCP_SKB_CB.  When MSG_EOR
> is passed to tcp_sendmsg/tcp_sendpage, 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.
> 
> This patch handles the tcp_sendmsg and tcp_sendpage cases.
> 
> 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).
> 
> 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>
> ---
>  include/net/tcp.h | 3 ++-
>  net/ipv4/tcp.c    | 7 +++++--
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index c0ef054..ac31798 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;
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 4d73858..7df0c1a88 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_CB(skb)->eor) {
>  new_segment:
>  			if (!sk_stream_memory_free(sk))
>  				goto wait_for_sndbuf;
> @@ -960,6 +961,7 @@ new_segment:
>  		size -= copy;
>  		if (!size) {
>  			tcp_tx_timestamp(sk, sk->sk_tsflags, skb);
> +			TCP_SKB_CB(skb)->eor = !!(flags & MSG_EOR);

I am not sure you understood how do_tcp_sendpages() was working.

1) It is called one page at a time, so size would reach zero for every
sent page, and we would have at most 4096 bytes (on x86) per skb, even
if a sendfile() or splice() or vmsplice() is requesting a large size.

2) @flags here does not contain typical MSG_... values,
  but a combination of MSG_MORE and MSG_SENDPAGE_NOTLAST

Since there is no way to add a MSG_EOR yet in the sendfile() and related
functions, you should remove the above line and not claim sendpage()
support in patch changelog/title, since it is not true.

We only support not aggregating data to the last skb in write queue if
eor bit is set on it, thus not breaking sendmsg( ... MSG_EOR) prior
uses.



>  			goto out;
>  		}
>  
> @@ -1156,7 +1158,7 @@ int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>  			copy = max - skb->len;
>  		}
>  
> -		if (copy <= 0) {
> +		if (copy <= 0 || TCP_SKB_CB(skb)->eor) {
>  new_segment:
>  			/* Allocate new segment. If the interface is SG,
>  			 * allocate skb fitting to single page.
> @@ -1250,6 +1252,7 @@ new_segment:
>  		copied += copy;
>  		if (!msg_data_left(msg)) {
>  			tcp_tx_timestamp(sk, sockc.tsflags, skb);
> +			TCP_SKB_CB(skb)->eor = !!(flags & MSG_EOR);

Since this is a rmw, it is probably better to avoid it in common case,
since we know prior value of eor must be 0 at this point.

	if (unlikely(flags & MSG_EOR))
           TCP_SKB_CB(skb)->eor = 1;


>  			goto out;
>  		}
>  

Thanks.