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

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Message-ID: <66475f1e-fc28-4198-ab66-75d4cdb9874c@mojatatu.com>
Date: Thu, 17 Apr 2025 19:13:03 -0300
From: Victor Nogueira <victor@...atatu.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, jhs@...atatu.com, jiri@...nulli.us,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, toke@...hat.com, gerrard.tai@...rlabs.sg,
 pctammela@...atatu.com, Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net v2 0/5] net_sched: Adapt qdiscs for reentrant enqueue
 cases

On 4/17/25 16:23, Cong Wang wrote:
> On Wed, Apr 16, 2025 at 07:24:22AM -0300, Victor Nogueira wrote:
>> As described in Gerrard's report [1], there are cases where netem can
>> make the qdisc enqueue callback reentrant. Some qdiscs (drr, hfsc, ets,
>> qfq) break whenever the enqueue callback has reentrant behaviour.
>> This series addresses these issues by adding extra checks that cater for
>> these reentrant corner cases. This series has passed all relevant test
>> cases in the TDC suite.
>>
>> [1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/
>>
> 
> I am wondering why we need to enqueue the duplicate skb before enqueuing
> the original skb in netem? IOW, why not just swap them?

I thought of doing what, I think, you are suggesting, but I was afraid
of breaking netem. Stephen any comments?

cheers,
Victor