[Cake] cake review comments cake_drop

[Sebastian Moeller]

Hi Kevin,


On Oct 3, 2015, at 21:42 , Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:

> Hello All,
> 
> On 18th Aug (before I'd subscribed to the list), Dave posted a message
> about reviewing cake
> https://lists.bufferbloat.net/pipermail/cake/2015-August/000364.html
> 
> In it he mentioned a change to cake_drop:
> 
> "
> 
> I would like cake_drop to be evaluated with some less exaustive
> search. It also looks wrong
> 
>      for(j=0; j < CAKE_MAX_CLASSES; j++) {
>                fqcd = &q->classes[j];
> 
> CAKE_MAX_CLASSES should actually be q->class_cnt here; I think. I am
> unsure if other references to it are correct in face of changes, on
> cleanup"
> 
> 
> After making the change and nearly submitting a pull request I've
> thought about this some more and to be blunt it makes me nervous.  My
> concern is what happens if cake is changed from say a diffserv8 config
> to a single class(bin) config and for whatever reason 'cake_drop' is
> called.  with diffserv8 the fat flow is likely to be in a class(bin)
> higher than the first....cake_drop would now only check the first class
> for the fat flow and possibly not find anything to drop....  I fear what
> happens next!


I thought the following takes care of that (called from cake_reconfigure() ):

/* Discard leftover packets from a class no longer in use. */
static void cake_clear_class(struct Qdisc *sch, u16 class)
{       
        struct cake_sched_data *q = qdisc_priv(sch);
        struct cake_fqcd_sched_data *fqcd = &q->classes[class];
        
        q->cur_class = class;
        for(q->cur_flow = 0; q->cur_flow < fqcd->flows_cnt; q->cur_flow++)
                while(custom_dequeue(NULL, sch))
                        ;
}

I probably do no understand things fully enough, but to me this seems to take care of your concern. Especially since cake_reconfigure will only touch classes from q->class_cnt up to CAKE_MAX_CLASSES. Since I am not very fluent in C I might have missed things… That means that exchanging CAKE_MAX_CLASSES with q->class_cnt in cake_drop() should be safe. The question left is then how costly is it to search empty classes? I guess, cake_drop is the slow path and hence could afford to search them all, then again cake_drop probably tests called most under heavy traffic conditions, so even the slow path performance might be noticeable to the user…

Best Regards
	Sebastian


> 
> Kevin
> 
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