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

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Date:   Thu, 19 Jul 2018 11:39:35 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Michel Machado <michel@...irati.com.br>
Cc:     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

(Sorry for missing this email, it is lost in other discussions.)

On Wed, Jul 11, 2018 at 8:25 AM Michel Machado <michel@...irati.com.br> wrote:
>
> On 07/10/2018 10:57 PM, Cong Wang wrote:
> > The dev->tx_queue_len is fundamentally non-sense since now
> > almost every real NIC is multi-queue and qdisc has a completely
> > different sch->limit. This is why I suggested you to simply
> > avoid it in your code.
>
>     Would you be okay with a constant there? If so, we could just put 64
> there. The optimal number is hardware dependent, but we don't know how
> to calculate it.

Yes, sure, fq_codel uses 10240 already. :)


>
> > There is no standard way to use dev->tx_queue_len in kernel,
> > so I can't claim your use is correct or not, but it still looks odd,
> > other qdisc seems just uses as a default, rather than picking
> > the smaller or bigger value as a cap.
>
>     The reason for the `max(qdisc_dev(sch)->tx_queue_len, min_limit)` is
> to make sure that sch->limit is at least 1. We couldn't come up with a
> meaningful behavior for sch->limit being zero, so we defined the basis
> case of skbprio_enqueue() as sch->limit one. If there's a guarantee that
> qdisc_dev(sch)->tx_queue_len is always greater than zero, we don't need
> the max().

I think tx_queue_len could be 0. But again, why do you need to care
about tx_queue_len being 0 or not here?

sch->limit could be 0 too, it means this qdisc should not queue any
packets.

Thanks