netdev - Re: [RFC PATCH v2 net-next 4/7] tcp: Make use of MSG_EOR flag in tcp

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Message-ID: <1461021493.10638.131.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Mon, 18 Apr 2016 16:18:13 -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.kdev@...il.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH v2 net-next 4/7] tcp: Make use of MSG_EOR flag in
 tcp_sendmsg

On Mon, 2016-04-18 at 15:46 -0700, Martin KaFai Lau wrote:
> This patch allows the user process to use MSG_EOR during
> tcp_sendmsg to tell the kernel that it is the last byte
> of an application response message.
> 
> It is currently useful when the end-user has turned on any bit of the
> SOF_TIMESTAMPING_TX_RECORD_MASK (either by setsockopt or cmsg).
> The kernel will then mark the newly added tcb->eor_info bit so
> that the shinfo->tskey will not be overwritten (i.e. lost) in
> the later skb append/collapse operation.
> 
> With selective SOF_TIMESTAMPING_TX_ACK (by cmsg) and MSG_EOR (this
> patch), the user application can specially tell which outgoing byte
> it wants to track its ACK and ask the kernel not to lose this
> tracking info in the later skb append/collapse action.
> 
> This patch handles the append case in tcp_sendmsg.  The later
> patches will handle the collapse during retransmission and
> skb slicing in tcp_fragment()/tso_fragment().
> 
> One of our use case is at the webserver.  The webserver tracks
> the HTTP2 response latency by measuring when the webserver sends
> the first byte to the socket till the TCP ACK of the last byte
> is received.  In the cases where we don't have client side
> measurement, measuring from the server side is the only option.
> In the cases we have the client side measurement, the server side
> data can also be used to justify/cross-check-with the client
> side data.
> 
> 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.kdev@...il.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> ---

MSG_EOR should not depend on SKBTX_ANY_TSTAMP

Really, simply using send(fd, ..., len, MSG_EOR) should instruct TCP to
mark the cooked skb as a non candidate for future coalescing.

netperf could then get an option to set this MSG_EOR ;)

I believe Soheil was working on such simple alternative ?