Subject: Re: [PATCH v4 net-next 13/16] tcp: allow congestion control to expand send buffer differently

From: Stephen Hemminger <[email-masked]>
Date: Tue, 20 Sep 2016 10:48:01 -0700

On Mon, 19 Sep 2016 23:39:20 -0400
Neal Cardwell <[email-masked]> wrote:

> From: Yuchung Cheng <[email-masked]>
> 
> Currently the TCP send buffer expands to twice cwnd, in order to allow
> limited transmits in the CA_Recovery state. This assumes that cwnd
> does not increase in the CA_Recovery.
> 
> For some congestion control algorithms, like the upcoming BBR module,
> if the losses in recovery do not indicate congestion then we may
> continue to raise cwnd multiplicatively in recovery. In such cases the
> current multiplier will falsely limit the sending rate, much as if it
> were limited by the application.
> 
> This commit adds an optional congestion control callback to use a
> different multiplier to expand the TCP send buffer. For congestion
> control modules that do not specificy this callback, TCP continues to
> use the previous default of 2.
> 
> Signed-off-by: Van Jacobson <[email-masked]>
> Signed-off-by: Neal Cardwell <[email-masked]>
> Signed-off-by: Yuchung Cheng <[email-masked]>
> Signed-off-by: Nandita Dukkipati <[email-masked]>
> Signed-off-by: Eric Dumazet <[email-masked]>
> Signed-off-by: Soheil Hassas Yeganeh <[email-masked]>
> ---
>  include/net/tcp.h    | 2 ++
>  net/ipv4/tcp_input.c | 4 +++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 3492041..1aa9628 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -917,6 +917,8 @@ struct tcp_congestion_ops {
>  	void (*pkts_acked)(struct sock *sk, const struct ack_sample *sample);
>  	/* suggest number of segments for each skb to transmit (optional) */
>  	u32 (*tso_segs_goal)(struct sock *sk);
> +	/* returns the multiplier used in tcp_sndbuf_expand (optional) */
> +	u32 (*sndbuf_expand)(struct sock *sk);
>  	/* get info for inet_diag (optional) */
>  	size_t (*get_info)(struct sock *sk, u32 ext, int *attr,
>  			   union tcp_cc_info *info);
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 17de77d..5af0bf3 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -289,6 +289,7 @@ static bool tcp_ecn_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr
>  static void tcp_sndbuf_expand(struct sock *sk)
>  {
>  	const struct tcp_sock *tp = tcp_sk(sk);
> +	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
>  	int sndmem, per_mss;
>  	u32 nr_segs;
>  
> @@ -309,7 +310,8 @@ static void tcp_sndbuf_expand(struct sock *sk)
>  	 * Cubic needs 1.7 factor, rounded to 2 to include
>  	 * extra cushion (application might react slowly to POLLOUT)
>  	 */
> -	sndmem = 2 * nr_segs * per_mss;
> +	sndmem = ca_ops->sndbuf_expand ? ca_ops->sndbuf_expand(sk) : 2;

You could avoid the conditional (if it mattered) by inheriting a default value
that would mean changing all existing congestion control modules.
So doing it this way makes life easier.

Acked-by: Stephen Hemminger <[email-masked]>