netdev - Re: [PATCH net v2 0/5] net_sched: Adapt qdiscs for reentrant enqueue cases

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Message-ID: <CAM0EoM=Yh8ynZUMqma02ktnajJWOEEr-+GLgCVZH=EohFu6itQ@mail.gmail.com>
Date: Thu, 24 Apr 2025 11:40:06 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Cong Wang <xiyou.wangcong@...il.com>, Paolo Abeni <pabeni@...hat.com>, 
	Victor Nogueira <victor@...atatu.com>, 
	Linux Kernel Network Developers <netdev@...r.kernel.org>, Jiri Pirko <jiri@...nulli.us>, 
	David Miller <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Toke Høiland-Jørgensen <toke@...hat.com>, 
	Gerrard Tai <gerrard.tai@...rlabs.sg>, Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH net v2 0/5] net_sched: Adapt qdiscs for reentrant enqueue cases

On Thu, Apr 24, 2025 at 11:22 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On Wed, Apr 23, 2025 at 8:24 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Wed, 23 Apr 2025 16:29:38 -0700 Cong Wang wrote:
> > > > +   /*
> > > > +    * If doing duplication then re-insert at top of the
> > > > +    * qdisc tree, since parent queuer expects that only one
> > > > +    * skb will be queued.
> > > > +    */
> > > > +   if (skb2) {
> > > > +           struct Qdisc *rootq = qdisc_root_bh(sch);
> > > > +           u32 dupsave = q->duplicate; /* prevent duplicating a dup... */
> > > > +
> > > > +           q->duplicate = 0;
> > > > +           rootq->enqueue(skb2, rootq, to_free);
> > > > +           q->duplicate = dupsave;
> > > > +           skb2 = NULL;
> > > > +   }
> > > > +
> > > >  finish_segs:
> > > >     if (skb2)
> > > >             __qdisc_drop(skb2, to_free);
> > > >
> > >
> > > Just FYI: I tested this patch, netem duplication still worked, I didn't
> > > see any issue.
> >
> > Does it still work if you have another layer of qdiscs in the middle?
> > It works if say DRR is looking at the netem directly as its child when
> > it does:
> >
> >         first = cl->qdisc->q.qlen
> >
> > but there may be another layer, the cl->qdisc may be something that
> > hasn't incremented its qlen, and something that has netem as its child.
>
> Very strong feeling it wont work in that scenario. We can double check.

Verified the following breaks:

drr
  |
  |
class drr
  |
  |
drr
  |
  |
class drr
  |
  |
netem

It's probably a crazy config - but doesnt matter, the bounty types will use it.

cheers,
jamal

> Regardless - even if it did - what Victor sent is still a fix. Seems
> DRR had that bug originally. And then in the true tradition of
> TheLinuxWay(tm) was cutnpasted into HFSC and then the others followed.
>
> cheers,
> jamal