netdev - Re: [PATCH net-next] tcp: remove a bogus TSO split

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Date:	Fri, 13 Dec 2013 06:54:09 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Van Jacobson <vanj@...gle.com>
Subject: Re: [PATCH net-next] tcp: remove a bogus TSO split

On Fri, 2013-12-13 at 09:15 -0500, Neal Cardwell wrote:

> Seems like a nice improvement, but if we apply this patch then AFAICT
> to get the Nagle-enabled case right we also have to update
> tcp_minshall_update() to notice these new non-MSS-aligned segments
> going out, and count those as non-full-size segments for the
> minshall-nagle check (to ensure we have no more than one outstanding
> un-ACKed sub-MSS packet). Maybe something like (please excuse the
> formatting):
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 70e55d2..a2ec237 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -980,7 +980,8 @@ bool tcp_is_cwnd_limited(const struct sock *sk,
> u32 in_flight);
>  static inline void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss,
>                                        const struct sk_buff *skb)
>  {
> -       if (skb->len < mss)
> +       if (skb->len < mss ||
> +           tcp_skb_pcount(skb) * tcp_skb_mss(skb) > skb->len)
>                 tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
>  }

Very good point Neal, but dont you think tcp_skb_mss(skb) is equal to
mss at this point ? (We normally have synced with tso_segs =
tcp_init_tso_segs(sk, skb, mss_now);)

(Just trying to make this code more understandable...)

Also I think we should move this helper out of include/net/tcp.h, we
only use it from tcp_output.c

I'll submit a v2, rewording the comment in front of
tcp_mss_split_point()

Thanks


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