netdev - Re: [PATCH v7 net-next 14/15] tcp: accecn: try to fit AccECN option with SACK

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Message-ID: <5cd44751-19df-4356-a485-a7ba18a05482@redhat.com>
Date: Tue, 20 May 2025 12:03:47 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: chia-yu.chang@...ia-bell-labs.com, linux-doc@...r.kernel.org,
 corbet@....net, horms@...nel.org, dsahern@...nel.org, kuniyu@...zon.com,
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Subject: Re: [PATCH v7 net-next 14/15] tcp: accecn: try to fit AccECN option
 with SACK

On 5/14/25 3:56 PM, chia-yu.chang@...ia-bell-labs.com wrote:
> From: Ilpo Järvinen <ij@...nel.org>
> 
> 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 <ij@...nel.org>
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> ---
>  net/ipv4/tcp_output.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index b630923c4cef..d9d3cc8dbb5b 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -982,8 +982,21 @@ static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
>  		opts->num_accecn_fields--;
>  		size -= TCPOLEN_ACCECN_PERFIELD;
>  	}
> -	if (opts->num_accecn_fields < required)
> +	if (opts->num_accecn_fields < 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);

How deep is the recursion level, worst case? In any case please try to
avoid recursion entirely. Possibly a 'goto' statement would help.

/P