netdev - Re: Packet gets stuck in NOLOCK pfifo

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Date:   Thu, 3 Sep 2020 10:43:38 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Kehuan Feng <kehuan.feng@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Jike Song <albcamus@...il.com>, Josh Hunt <johunt@...mai.com>,
        Jonas Bonn <jonas.bonn@...rounds.com>,
        Michael Zhivich <mzhivich@...mai.com>,
        David Miller <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

On Thu, Sep 3, 2020 at 1:40 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Wed, 2020-09-02 at 22:01 -0700, Cong Wang wrote:
> > Can you test the attached one-line fix? I think we are overthinking,
> > probably all
> > we need here is a busy wait.
>
> I think that will solve, but I also think that will kill NOLOCK
> performances due to really increased contention.

Yeah, we somehow end up with more locks (seqlock, skb array lock)
for lockless qdisc. What an irony... ;)

>
> At this point I fear we could consider reverting the NOLOCK stuff.
> I personally would hate doing so, but it looks like NOLOCK benefits are
> outweighed by its issues.

I agree, NOLOCK brings more pains than gains. There are many race
conditions hidden in generic qdisc layer, another one is enqueue vs.
reset which is being discussed in another thread.

Thanks.