netdev - Re: [BUG] net/sched : qlen can not really be per cpu ?

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Message-ID: <CAM_iQpVuaP9kVyvM-s2MFw+cz1tZpp0ehipo7Gm-W8+=UzxuvQ@mail.gmail.com>
Date:   Tue, 26 Feb 2019 15:51:17 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [BUG] net/sched : qlen can not really be per cpu ?

On Tue, Feb 26, 2019 at 3:19 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 02/25/2019 10:42 PM, Eric Dumazet wrote:
> > HTB + pfifo_fast as a leaf qdisc hits badly the following warning in htb_activate() :
> >
> > WARN_ON(cl->level || !cl->leaf.q || !cl->leaf.q->q.qlen);
> >
> > This is because pfifo_fast does not update sch->q.qlen, but per cpu counters.
> > So cl->leaf.q->q.qlen is zero.
> >
> > HFSC, CBQ, DRR, QFQ  have the same problem.
> >
> > Any ideas how we can fix this ?
>
> What about something simple for stable ?
> ( I yet have to boot/test this )

Is merely updating qlen sufficient for fixing it?

I thought it is because of the lack of qdisc_tree_reduce_backlog()
in pfifo_fast.