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

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Message-ID:
 <AS8PR07MB7973548F209E86B95C4DA53EA398A@AS8PR07MB7973.eurprd07.prod.outlook.com>
Date: Fri, 23 May 2025 17:50:29 +0000
From: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
To: Paolo Abeni <pabeni@...hat.com>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "corbet@....net" <corbet@....net>,
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Subject: RE: [PATCH v7 net-next 14/15] tcp: accecn: try to fit AccECN option
 with SACK

-----Original Message-----
> From: Paolo Abeni <pabeni@...hat.com> 
> Sent: Tuesday, May 20, 2025 12:04 PM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>; linux-doc@...r.kernel.org; corbet@....net; horms@...nel.org; dsahern@...nel.org; kuniyu@...zon.com; bpf@...r.kernel.org; netdev@...r.kernel.org; dave.taht@...il.com; jhs@...atatu.com; kuba@...nel.org; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; edumazet@...gle.com; andrew+netdev@...n.ch; donald.hunter@...il.com; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; ncardwell@...gle.com; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white <g.white@...lelabs.com>; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire@...le.com; rs.ietf@....at; Jason_Livingood@...cast.com; vidhi_goel <vidhi_goel@...le.com>
> Subject: Re: [PATCH v7 net-next 14/15] tcp: accecn: try to fit AccECN option with SACK
> 
> 
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
> 
> 
> 
> 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

Hi Paolo,

Thanks for the feedback, in this case up to 2 extra levels will be needed.
And the recursion will be avoided in the next version.

BRs,
Chia-Yu