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

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Date:   Fri, 04 May 2018 21:10:17 +0200
From:   Toke Høiland-Jørgensen <toke@...e.dk>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cake List <cake@...ts.bufferbloat.net>
Subject: Re: [PATCH net-next v8 1/7] sched: Add Common Applications Kept Enhanced (cake) qdisc

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.

- 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.

>> +static u16 quantum_div[CAKE_QUEUES + 1] = {0};
>> +
>> +#define REC_INV_SQRT_CACHE (16)
>> +static u32 cobalt_rec_inv_sqrt_cache[REC_INV_SQRT_CACHE] = {0};
>> +
>> +/* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots
>> + * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
>> + *
>> + * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
>> + */
>> +
>> +static void cobalt_newton_step(struct cobalt_vars *vars)
>> +{
>> +       u32 invsqrt = vars->rec_inv_sqrt;
>> +       u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
>> +       u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);
>> +
>> +       val >>= 2; /* avoid overflow in following multiply */
>> +       val = (val * invsqrt) >> (32 - 2 + 1);
>> +
>> +       vars->rec_inv_sqrt = val;
>> +}
>> +
>> +static void cobalt_invsqrt(struct cobalt_vars *vars)
>> +{
>> +       if (vars->count < REC_INV_SQRT_CACHE)
>> +               vars->rec_inv_sqrt = cobalt_rec_inv_sqrt_cache[vars->count];
>> +       else
>> +               cobalt_newton_step(vars);
>> +}
>
> Looks pretty much duplicated with codel...

Cobalt is derived from CoDel, and so naturally shares some features with
it. However, it is quite different in other respects, so we can't just
use the existing CoDel code for the parts that are similar. We don't
feel quite confident enough in Cobalt (yet) to propose it replace CoDel
everywhere else in the kernel; so we have elected to keep it internal to
CAKE instead.

>> [...]
>>
>> +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?

-Toke