[Cake] cake review comments cake_drop

Kevin Darbyshire-Bryant kevin at darbyshire-bryant.me.uk
Sun Oct 4 13:23:49 EDT 2015

On 04/10/15 11:50, Sebastian Moeller wrote:
> 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…
Good spot Sebastian!  I agree that looking through the code would
indicate that a reconfigure from say 'diffserv8' to 'besteffort' would
actually cause cake to drop all the packets in the now redundant
classes, therefore my worry is solved.  I've re-submitted the pull
request.  I may even merge it as it has been running on my router for a
couple of days without actually blowing up :-)

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