[PATCH net-next] tcp: fix ABC in tcp_slow_start()

[PATCH net-next] tcp: fix ABC in tcp_slow_start()

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.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.

So to reach the "cwnd += 2" goal, we need to use "cnt = 2*cnt + 1"

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: John Heffner <johnwheffner@gmail.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
---
 net/ipv4/tcp_cong.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 04dbd7a..486379a 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -306,7 +306,7 @@ EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);
  */
 void tcp_slow_start(struct tcp_sock *tp)
 {
-	int cnt; /* increase in packets */
+	unsigned int cnt; /* increase in packets */
 
 	/* RFC3465: ABC Slow start
 	 * Increase only after a full MSS of bytes is acked
@@ -318,16 +318,19 @@ void tcp_slow_start(struct tcp_sock *tp)
 	if (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
 		return;
 
-	if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
+	cnt = tp->snd_cwnd;			/* exponential increase */
+	if (sysctl_tcp_max_ssthresh > 0 &&
+	    tp->snd_cwnd > sysctl_tcp_max_ssthresh)
 		cnt = sysctl_tcp_max_ssthresh >> 1;	/* limited slow start */
-	else
-		cnt = tp->snd_cwnd;			/* exponential increase */
 
 	/* RFC3465: ABC
 	 * We MAY increase by 2 if discovered delayed ack
+	 * The "+ 1" in the expression is needed if we want to increase
+	 * cwnd by 2, because the way is coded the following loop.
 	 */
 	if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
-		cnt <<= 1;
+		cnt = 2*cnt + 1;
+
 	tp->bytes_acked = 0;
 
 	tp->snd_cwnd_cnt += cnt;

Re: [PATCH net-next] tcp: fix ABC in tcp_slow_start()

[John Heffner]

It might be clearer to instead introduce a temporary variable to
calculate the snd_cwnd change in the while loop.  That is:

        unsigned int snd_cwnd_delta = 0;
...
        tp->snd_cwnd_cnt += cnt;
        while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
                tp->snd_cwnd_cnt -= tp->snd_cwnd;
                snd_cwnd_delta++;
        }
        tp->snd_cwnd = min(tp->snd_cwnd + snd_cwnd_delta, tp->snd_cwnd_clamp);



On Fri, Jul 20, 2012 at 1:40 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.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.
>
> So to reach the "cwnd += 2" goal, we need to use "cnt = 2*cnt + 1"
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Tom Herbert <therbert@google.com>
> Cc: Yuchung Cheng <ycheng@google.com>
> Cc: Neal Cardwell <ncardwell@google.com>
> Cc: Nandita Dukkipati <nanditad@google.com>
> Cc: John Heffner <johnwheffner@gmail.com>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> ---
>  net/ipv4/tcp_cong.c |   13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
> index 04dbd7a..486379a 100644
> --- a/net/ipv4/tcp_cong.c
> +++ b/net/ipv4/tcp_cong.c
> @@ -306,7 +306,7 @@ EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);
>   */
>  void tcp_slow_start(struct tcp_sock *tp)
>  {
> -       int cnt; /* increase in packets */
> +       unsigned int cnt; /* increase in packets */
>
>         /* RFC3465: ABC Slow start
>          * Increase only after a full MSS of bytes is acked
> @@ -318,16 +318,19 @@ void tcp_slow_start(struct tcp_sock *tp)
>         if (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
>                 return;
>
> -       if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
> +       cnt = tp->snd_cwnd;                     /* exponential increase */
> +       if (sysctl_tcp_max_ssthresh > 0 &&
> +           tp->snd_cwnd > sysctl_tcp_max_ssthresh)
>                 cnt = sysctl_tcp_max_ssthresh >> 1;     /* limited slow start */
> -       else
> -               cnt = tp->snd_cwnd;                     /* exponential increase */
>
>         /* RFC3465: ABC
>          * We MAY increase by 2 if discovered delayed ack
> +        * The "+ 1" in the expression is needed if we want to increase
> +        * cwnd by 2, because the way is coded the following loop.
>          */
>         if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
> -               cnt <<= 1;
> +               cnt = 2*cnt + 1;
> +
>         tp->bytes_acked = 0;
>
>         tp->snd_cwnd_cnt += cnt;
>
>

Re: [PATCH net-next] tcp: fix ABC in tcp_slow_start()

[Eric Dumazet]

On Fri, 2012-07-20 at 10:41 -0400, John Heffner wrote:
> It might be clearer to instead introduce a temporary variable to
> calculate the snd_cwnd change in the while loop.  That is:
> 
>         unsigned int snd_cwnd_delta = 0;
> ...
>         tp->snd_cwnd_cnt += cnt;
>         while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
>                 tp->snd_cwnd_cnt -= tp->snd_cwnd;
>                 snd_cwnd_delta++;
>         }
>         tp->snd_cwnd = min(tp->snd_cwnd + snd_cwnd_delta, tp->snd_cwnd_clamp);
> 

Good idea, thanks, I'll send a v2