netdev - Re: [PATCH net-next 1/1] selftests/net: Add packetdrill packetdrill cases

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Message-ID: <CADVnQynBnqkND3nTS==f6MGy_9yUPBFb3RgBPnEuJ446Hkb-7g@mail.gmail.com>
Date: Sun, 18 Jan 2026 11:10:42 -0500
From: Neal Cardwell <ncardwell@...gle.com>
To: chia-yu.chang@...ia-bell-labs.com
Cc: pabeni@...hat.com, edumazet@...gle.com, parav@...dia.com, 
	linux-doc@...r.kernel.org, corbet@....net, horms@...nel.org, 
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	cheshire@...le.com, rs.ietf@....at, Jason_Livingood@...cast.com, 
	vidhi_goel@...le.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 1/1] selftests/net: Add packetdrill packetdrill cases

On Thu, Jan 8, 2026 at 5:46 PM Neal Cardwell <ncardwell@...gle.com> wrote:
>
> On Thu, Jan 8, 2026 at 10:58 AM <chia-yu.chang@...ia-bell-labs.com> wrote:
> >
> > From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> >
> > Linux Accurate ECN test sets using ACE counters and AccECN options to
> > cover several scenarios: Connection teardown, different ACK conditions,
> > counter wrapping, SACK space grabbing, fallback schemes, negotiation
> > retransmission/reorder/loss, AccECN option drop/loss, different
> > handshake reflectors, data with marking, and different sysctl values.
> >
> > Co-developed-by: Ilpo Järvinen <ij@...nel.org>
> > Signed-off-by: Ilpo Järvinen <ij@...nel.org>
> > Co-developed-by: Neal Cardwell <ncardwell@...gle.com>
> > Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> > ---
>
> Chia-Yu, thank you for posting the packetdrill tests.
>
> A couple thoughts:
>
> (1) These tests are using the experimental AccECN packetdrill support
> that is not in mainline packetdrill yet. Can you please share the
> github URL for the version of packetdrill you used? I will work on
> merging the appropriate experimental AccECN packetdrill support into
> the Google packetdrill mainline branch.

An update on the 3 patches at:

https://github.com/google/packetdrill/pull/96

(1) I have merged the following patch into the google packetdrill repo
to facilitate testing of the AccECN patch series:

"net-test: packetdrill: add Accurate ECN (AccECN) option support"
https://github.com/google/packetdrill/pull/96/changes/f6861f888bc7f1e08026de4825519a95504d1047

(2) The following patch I did not yet merge, because it proposes to
add an odd number of u32 fields to tcp_info, so AFAICT leaves a 4-byte
padding hole at the end of tcp_info:

  net-test: packetdrill: Support AccECN counters through tcpi
  https://github.com/google/packetdrill/pull/96/changes/f43649c87a2aa79a33a78111d3d7e5f027d13a7f

I think we'll need to tweak the AccECN kernel patch series so that it
does not leave a 4-byte padding hole at the end of tcp_info, then
update this packetdrill patch to match the kernel patch.

Let's come up with another useful u32 field we can add to the tcp_info
struct, so that the kernel patch doesn't add a padding hole at the end
of tcp_info.

One idea would be to add another field to represent newer options and
connection features that are enabled. AFAICT all 8 bits of the
tcpi_options field have been used, so we can't use more bits in that
field. I'd suggest we add a u32 tcpi_more_options field before the
tcpi_received_ce field, so we can encode other useful info, like:

+ 1 bit to indicate whether AccECN was negotiated (this can go in a
separate patch)

+ 1 bit to indicate whether TCP_NODELAY was set (since forgetting to
use TCP_NODELAY is a classic cause of performance problems; again this
can go in a separate patch)

(And there will be future bits of info we want to add...)

Also, regarding the comment in this line:
  __u32   tcpi_received_ce;    /* # of CE marks received */

That comment is ambiguous, since it doesn't indicate whether it's
counting (potentially LRO/GRO) skbs or TCP segments. I would suggest
clarifying that this is counting segments:

__u32   tcpi_received_ce;    /* # of CE marked segments received */

(3) The following patch I did not merge, because I'd like to migrate
to having all packetdrill tests for the Linux kernel reside in one
place, in the Linux kernel source tree (not the Google packetdrill
repo):

  net-test: add TCP Accurate ECN cases
  https://github.com/google/packetdrill/pull/96/changes/fe4c7293ea640a4c81178b6c88744d7a5d209fd6

Thanks!
neal