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

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Message-ID: <20250721075456.6cf6bf7c@hermes.local>
Date: Mon, 21 Jul 2025 07:54:56 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org,
 will@...lsroot.io
Subject: Re: [Patch v4 net 0/6] netem: Fix skb duplication logic and prevent
 infinite loops

On Mon, 21 Jul 2025 10:00:30 -0400
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
> 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

I don't remember why the original version re-queued to the root.
But probably related to trying to keep proper semantics and accounting
especially when doing rate limits.