Subject: Re: [PATCH net-next v8 1/7] sched: Add Common Applications Kept Enhanced (cake) qdisc

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 07 May 2018 20:37:01 +0200

Cong Wang <[email-masked]> writes:

> On Fri, May 4, 2018 at 12:10 PM, Toke Høiland-Jørgensen <[email-masked]> wrote:
>> Thank you for the review! A few comments below, I'll fix the rest.
>>
>>> [...]
>>>
>>> So sch_cake doesn't accept normal tc filters? Is this intentional?
>>> If so, why?
>>
>> For two reasons:
>>
>> - The two-level scheduling used in CAKE (tins / diffserv classes, and
>>   flow hashing) does not map in an obvious way to the classification
>>   index of tc filters.
>
> Sounds like you need to extend struct tcf_result?

Well, the obvious way to support filters would be to have skb->priority
override the diffserv mapping if set, and have the filter classification
result select the queue within that tier. That would probably be doable,
but see below.

>> - No one has asked for it. We have done our best to accommodate the
>>   features people want in a home router qdisc directly in CAKE, and the
>>   ability to integrate tc filters has never been requested.
>
> It is not hard to integrate, basically you need to call
> tcf_classify(). Although it is not mandatory, it is odd to merge a
> qdisc doesn't work with existing tc filters (and actions too).

I looked at the fq_codel code to do this. Is it possible to support
filtering without implementing Qdisc_class_ops? If so, I'll give it a
shot; but implementing the class ops is more than I can commit to...

>>>> +static int cake_init(struct Qdisc *sch, struct nlattr *opt,
>>>> +                    struct netlink_ext_ack *extack)
>>>> +{
>>>> +       struct cake_sched_data *q = qdisc_priv(sch);
>>>> +       int i, j;
>>>> +
>>>> +       sch->limit = 10240;
>>>> +       q->tin_mode = CAKE_DIFFSERV_BESTEFFORT;
>>>> +       q->flow_mode  = CAKE_FLOW_TRIPLE;
>>>> +
>>>> +       q->rate_bps = 0; /* unlimited by default */
>>>> +
>>>> +       q->interval = 100000; /* 100ms default */
>>>> +       q->target   =   5000; /* 5ms: codel RFC argues
>>>> +                              * for 5 to 10% of interval
>>>> +                              */
>>>> +
>>>> +       q->cur_tin = 0;
>>>> +       q->cur_flow  = 0;
>>>> +
>>>> +       if (opt) {
>>>> +               int err = cake_change(sch, opt, extack);
>>>> +
>>>> +               if (err)
>>>> +                       return err;
>>>
>>>
>>> Not sure if you really want to reallocate q->tines below for this
>>> case.
>>
>> I'm not sure what you mean here? If there's an error we return it and
>> the qdisc is not created. If there's not, we allocate and on subsequent
>> changes cake_change() will be called directly, or? Can the init function
>> ever be called again during the lifetime of the qdisc?
>>
>
> In non-error case, you call cake_change() first and then allocate
> ->tins with kvzalloc() below. For me it looks like you don't need to
> allocate it again when ->tins!=NULL.

No, we definitely don't. It's just not clear to me how cake_init() could
ever be called with q->tins already allocated?

I can add a check in any case, though, I see that there is one in
fq_codel as well...

-Toke