netdev - Re: [PATCH net-next v4] tcp: Add RFC4898 tcpEStatsPerfDataSegsOut/In

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Date:	Thu, 10 Mar 2016 09:58:17 -0800
From:	Martin KaFai Lau <kafai@...com>
To:	Eric Dumazet <edumazet@...gle.com>
CC:	netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
	Chris Rapier <rapier@....edu>,
	Marcelo Ricardo Leitner <mleitner@...hat.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next v4] tcp: Add RFC4898 tcpEStatsPerfDataSegsOut/In

On Thu, Mar 10, 2016 at 09:43:18AM -0800, Eric Dumazet wrote:
> On Thu, Mar 10, 2016 at 9:39 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> > On Thu, Mar 10, 2016 at 9:29 AM, Martin KaFai Lau <kafai@...com> wrote:
> >> Per RFC4898, they count segments sent/received
> >> containing a positive length data segment (that includes
> >> retransmission segments carrying data).  Unlike
> >> tcpi_segs_out/in, tcpi_data_segs_out/in excludes segments
> >> carrying no data (e.g. pure ack).
> >>
> >> The patch also updates the segs_in in tcp_fastopen_add_skb()
> >> so that segs_in >= data_segs_in property is kept.  If
> >> tcp_segs_in() helper is used in this fastopen case, tp->segs_in
> >> has to be 0 reset first to avoid double counting.  Also, it has
> >> to be done before __skb_pull(skb, tcp_hdrlen(skb)) while
> >> there is no need to check skb->len since skb has already
> >> been confirmed carrying data.  I found it more confusing
> >> and chose to directly set segs_in and data_segs_in in
> >> this special case.
> >
> > Note that on my TODO list after commit e11ecddf5128011c936cc5360780190cbc901fdc
> > I had the project of pulling TCP headers much earlier in input path
> > so that we do not have all these special cases.
> >
> > Acked-by: Eric Dumazet <edumazet@...gle.com>
>
> Actually, tcp_fastopen_add_skb() can queue a packet with a FIN only,
> but no data.
Thanks for pointing it out.  Didn't know it is allowed and
the above end_seq check could also be +1 by the FIN.

>
> I believe you need to test skb->len before setting tp->data_segs_in
In that case, I will try to 0 reset segs_in with comment explanation and call
tcp_segs_in() before the skb_pull.  I will spin another version.