netdev - Re: [Patch v4 net 0/6] netem: Fix skb duplication logic and prevent infinite loops

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Message-ID: <CAM0EoMnexkyxN83S_cGh+da2kSCg5sB4g-kE5qqnMH6eF8m5gQ@mail.gmail.com>
Date: Mon, 21 Jul 2025 10:27:53 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, will@...lsroot.io, stephen@...workplumber.org
Subject: Re: [Patch v4 net 0/6] netem: Fix skb duplication logic and prevent
 infinite loops

On Mon, Jul 21, 2025 at 10:00 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On Sat, Jul 19, 2025 at 6:04 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > This patchset fixes the infinite loops due to duplication in netem, the
> > real root cause of this problem is enqueuing to the root qdisc, which is
> > now changed to enqueuing to the same qdisc. This is more reasonable,
> > more predictable from users' perspective, less error-proone and more elegant.
> >
> > Please see more details in patch 1/6 which contains two pages of detailed
> > explanation including why it is safe and better.
> >
> > This replaces the patches from William, with much less code and without
> > any workaround. More importantly, this does not break any use case.
> >
>
> Cong, you are changing user expected behavior.
> So instead of sending to the root qdisc, you are looping on the same
> qdisc. I dont recall what the history is for the decision to go back

Digging a bit, check the following history:
Commit ID: d5d75cd6b10d
Commit ID: 0afb51e72855

Sorry - in travel mode so cant look closely..

cheers,
jamal

> to the root qdisc - but one reason that sounds sensible is we want to
> iterate through the tree hierarchy again. Stephen may remember.
> The fact that the qfq issue is hit indicates the change has
> consequences - and given the check a few lines above, more than likely
> you are affecting the qlen by what you did.
>
> cheers,
> jamal
> > All the test cases pass with this patchset.
> >
> > ---
> > v4: Added a fix for qfq qdisc (2/6)
> >     Updated 1/6 patch description
> >     Added a patch to update the enqueue reentrant behaviour tests
> >
> > v3: Fixed the root cause of enqueuing to root
> >     Switched back to netem_skb_cb safely
> >     Added two more test cases
> >
> > v2: Fixed a typo
> >     Improved tdc selftest to check sent bytes
> >
> >
> > Cong Wang (6):
> >   net_sched: Implement the right netem duplication behavior
> >   net_sched: Check the return value of qfq_choose_next_agg()
> >   selftests/tc-testing: Add a nested netem duplicate test
> >   selftests/tc-testing: Add a test case for piro with netem duplicate
> >   selftests/tc-testing: Add a test case for mq with netem duplicate
> >   selftests/tc-testing: Update test cases with netem duplicate
> >
> >  net/sched/sch_netem.c                         |  26 ++--
> >  net/sched/sch_qfq.c                           |   2 +
> >  .../tc-testing/tc-tests/infra/qdiscs.json     | 114 +++++++++++++-----
> >  .../tc-testing/tc-tests/qdiscs/netem.json     |  25 ++++
> >  4 files changed, 127 insertions(+), 40 deletions(-)
> >
> > --
> > 2.34.1
> >