netdev - Re: [PATCH v3 net-next] net/sched: add skbprio scheduler

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Date:   Thu, 12 Jul 2018 22:07:30 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Michel Machado <michel@...irati.com.br>,
        Nishanth Devarajan <ndev2021@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cody Doucette <doucette@...edu>
Subject: Re: [PATCH v3 net-next] net/sched: add skbprio scheduler

On Wed, Jul 11, 2018 at 11:37 AM Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
>
> On Tue, Jul 10, 2018 at 07:32:43PM -0700, Cong Wang wrote:
> > On Mon, Jul 9, 2018 at 12:53 PM Marcelo Ricardo Leitner
> > <marcelo.leitner@...il.com> wrote:
> > >
> > > On Mon, Jul 09, 2018 at 02:18:33PM -0400, Michel Machado wrote:
> > > >
> > > >    2. sch_prio.c does not have a global limit on the number of packets on
> > > > all its queues, only a limit per queue.
> > >
> > > It can be useful to sch_prio.c as well, why not?
> > > prio_enqueue()
> > > {
> > > ...
> > > +       if (count > sch->global_limit)
> > > +               prio_tail_drop(sch);   /* to be implemented */
> > >         ret = qdisc_enqueue(skb, qdisc, to_free);
> > >
> >
> > Isn't the whole point of sch_prio offloading the queueing to
> > each class? If you need a limit, there is one for each child
> > qdisc if you use for example pfifo or bfifo (depending on you
> > want to limit bytes or packets).
>
> Yes, but Michel wants to drop from other lower priorities if needed,
> and that's not possible if you handle the limit already in a child
> qdisc as they don't know about their siblings. The idea in the example
> above is to discard it from whatever lower priority is needed, then
> queue it. (ok, the example missed to check the priority level)

So it disproves your point of adding a flag to sch_prio, right?

Also, you have to re-introduce qdisc->ops->drop() if you really want
to go this direction.

>
> As for the different units, sch_prio holds a count of how many packets
> are queued on its children, and that's what would be used for the limit.
>
> >
> > Also, what's your plan for backward compatibility here?
>
> say:
>   if (sch->global_limit && count > sch->global_limit)
> as in, only do the limit check/enforcing if needed.

Obviously doesn't work, users could pass 0 to effectively
disable the qdisc from enqueue'ing any packet.