netdev - Re: [RFC PATCH 24/28] tcp: try to fit AccECN option with SACK

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Message-ID: <4ae2c8be-3235-9158-a2a7-7f9d30a20c04@gmail.com>
Date:   Wed, 18 Mar 2020 20:29:34 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
        netdev@...r.kernel.org
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Olivier Tilmans <olivier.tilmans@...ia-bell-labs.com>
Subject: Re: [RFC PATCH 24/28] tcp: try to fit AccECN option with SACK



On 3/18/20 2:43 AM, Ilpo Järvinen wrote:
> From: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>
> 
> As SACK blocks tend to eat all option space when there are
> many holes, it is useful to compromise on sending many SACK
> blocks in every ACK and try to fit AccECN option there
> by reduction the number of SACK blocks. But never go below
> two SACK blocks because of AccECN option.
> 
> As AccECN option is often not put to every ACK, the space
> hijack is usually only temporary.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>
> ---
>  net/ipv4/tcp_output.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 4cc590a47f43..0aec2c57a9cc 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -756,6 +756,21 @@ static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
>  	if (opts->num_ecn_bytes < required)
>  		return 0;

Have you tested this patch ?

(You forgot to remove the prior 2 lines)

>  
> +	if (opts->num_ecn_bytes < required) {
> +		if (opts->num_sack_blocks > 2) {
> +			/* Try to fit the option by removing one SACK block */
> +			opts->num_sack_blocks--;
> +			size = tcp_options_fit_accecn(opts, required,
> +						      remaining + TCPOLEN_SACK_PERBLOCK,
> +						      max_combine_saving);
> +			if (opts->options & OPTION_ACCECN)
> +				return size - TCPOLEN_SACK_PERBLOCK;
> +
> +			opts->num_sack_blocks++;
> +		}
> +		return 0;
> +	}
> +
>  	opts->options |= OPTION_ACCECN;
>  	return size;
>  }
>