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

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Date:   Wed, 11 Jul 2018 15:37:56 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Cong Wang <xiyou.wangcong@...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 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)

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.