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

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Message-ID: <CANn89iJFBqOkD8G1qQDY-p=Z3jSgtPbeSSoiuiAqWXYgXzi5-A@mail.gmail.com>
Date:	Thu, 10 Mar 2016 09:43:18 -0800
From:	Eric Dumazet <edumazet@...gle.com>
To:	Martin KaFai Lau <kafai@...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 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.

I believe you need to test skb->len before setting tp->data_segs_in