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

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Date:	Mon, 18 Apr 2016 20:25:39 -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 20:18 -0700, Martin KaFai Lau wrote:
> On Mon, Apr 18, 2016 at 07:50:41PM -0700, Eric Dumazet wrote:
> > I believe it is slightly wrong (to do the goto new_segment if there is
> > no data to send)
> Aha. Thanks for pointing it out.
> 
> >
> > I would instead use this fast path, doing the test _when_ we already
> > have an skb to test for.
> The v1 was doing a check in the loop but the feedback was, instead
> of doing this unlikely(test) repeatedly in the loop, do it before
> entering the loop and do a goto new_segment if needed.
> 
> I agree that doing it in the loop is easier to follow/read
> and checking TCP_SKB_CB(skb)->eor is cheaper than my v1.
> I will respin with your suggestion.

I do not remember the suggestion. It is better to do the test in the
loop, as you could have multiple threads doing a sendmsg() on the same
socket, and eventually sleeping in sk_stream_wait_memory()

A bit convoluted, but who knows what can be done by some applications ;)