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

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Message-ID: <9f38f8c4-c2db-8b5d-1fea-4cb33a5838f3@digirati.com.br>
Date:   Wed, 11 Jul 2018 11:24:57 -0400
From:   Michel Machado <michel@...irati.com.br>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Nishanth Devarajan <ndev2021@...il.com>
Cc:     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 07/10/2018 10:57 PM, Cong Wang wrote:
> On Sat, Jul 7, 2018 at 3:14 AM Nishanth Devarajan <ndev2021@...il.com> wrote:
>> diff --git a/Documentation/networking/sch_skbprio.txt b/Documentation/networking/sch_skbprio.txt
>> new file mode 100644
>> index 0000000..3aa4d3e
>> --- /dev/null
>> +++ b/Documentation/networking/sch_skbprio.txt
> 
> We usually document each qdisc behavior in tc man pages in iproute2.
> 
> I don't mind you document it in kernel, but it kinda breaks the tradition.

    We're going to move this text to tc man pages for v4.

>> +static int skbprio_change(struct Qdisc *sch, struct nlattr *opt,
>> +                       struct netlink_ext_ack *extack)
>> +{
>> +       struct skbprio_sched_data *q = qdisc_priv(sch);
>> +       struct tc_skbprio_qopt *ctl = nla_data(opt);
>> +       const unsigned int min_limit = 1;
>> +
>> +       if (ctl->limit == (typeof(ctl->limit))-1)
>> +               sch->limit = max(qdisc_dev(sch)->tx_queue_len, min_limit);
>> +       else if (ctl->limit < min_limit)
>> +               return -EINVAL;
>> +       else
>> +               sch->limit = ctl->limit;
> 
> 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.

> 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().

[ ]'s
Michel Machado