netdev - Re: [PATCH v2 net-next] tcp: fix ABC in tcp_slow

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Message-ID: <CAK6E8=dkwruGMtW2uQ--xZ_so9aZQt-KcmHsy2yE5GApKphrJQ@mail.gmail.com>
Date:	Fri, 20 Jul 2012 08:07:37 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	John Heffner <johnwheffner@...il.com>,
	Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: fix ABC in tcp_slow_start()

On Fri, Jul 20, 2012 at 8:02 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> When/if sysctl_tcp_abc > 1, we expect to increase cwnd by 2 if the
> received ACK acknowledges more than 2*MSS bytes, in tcp_slow_start()
>
> Problem is this RFC 3465 statement is not correctly coded, as
> the while () loop increases snd_cwnd one by one.
>
> Add a new variable to avoid this off-by one error.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>
> Cc: Tom Herbert <therbert@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Nandita Dukkipati <nanditad@...gle.com>
> Cc: John Heffner <johnwheffner@...il.com>
> Cc: Stephen Hemminger <shemminger@...tta.com>
> ---
> v2: added John suggestion
>
>  net/ipv4/tcp_cong.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
> index 04dbd7a..4d4db16 100644
> --- a/net/ipv4/tcp_cong.c
> +++ b/net/ipv4/tcp_cong.c
> @@ -307,6 +307,7 @@ EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);
>  void tcp_slow_start(struct tcp_sock *tp)
>  {
>         int cnt; /* increase in packets */
> +       unsigned int delta = 0;
>
>         /* RFC3465: ABC Slow start
>          * Increase only after a full MSS of bytes is acked
> @@ -333,9 +334,9 @@ void tcp_slow_start(struct tcp_sock *tp)
>         tp->snd_cwnd_cnt += cnt;
>         while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
>                 tp->snd_cwnd_cnt -= tp->snd_cwnd;
> -               if (tp->snd_cwnd < tp->snd_cwnd_clamp)
> -                       tp->snd_cwnd++;
> +               delta++;
Nice! this also removes wasteful iteration when clamp << cwnd_cnt.
>         }
> +       tp->snd_cwnd = min(tp->snd_cwnd + delta, tp->snd_cwnd_clamp);
>  }
>  EXPORT_SYMBOL_GPL(tcp_slow_start);
>
>
>
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