Re: [PATCH net-next 05/14] nfp: abm: add simple RED offload

[Nogah Frankel <nogah.frankel@gmail.com>]

On 29-May-18 2:05 AM, Jakub Kicinski wrote:

Hi Jakub,

> Hi Nogah!
> 
> On Mon, 28 May 2018 18:49:51 +0300, Nogah Frankel wrote:
>>> +static int
>>> +nfp_abm_red_replace(struct net_device *netdev, struct nfp_abm_link *alink,
>>> +		    struct tc_red_qopt_offload *opt)
>>> +{
>>> +	struct nfp_port *port = nfp_port_from_netdev(netdev);
>>> +	int err;
>>> +
>>> +	if (opt->set.min != opt->set.max || !opt->set.is_ecn) {
>>
>> I am a bit worried about the min == max.
>> sch_red doesn't really support it. It will calculate incorrect delta
>> value. (And that only if tc_red_eval_P in iproute2 won't reject it).
>> You might maybe use max = min+1,  because in real life it will probably
>> act the same but without this problem.
> 
> I remember having a long think about this when I wrote the code.
> My conclusion was that the two would operate almost the same, and
> setting min == max may be most obvious to the user.

I agree.

> 
> If min + 1 == max sch_red would act probabilistically for qavg == min,
> which is not what the card would do.
> 
> Userspace now does this:
> 
> tc_red_eval_P() {
> 	int i = qmax - qmin;
>   
> 	if (!i)
> 		return 0;
> 	if (i < 0)
> 		return -1;
> 	...
> }
> 
> And you've fixed delta to be treated as 1 to avoid division by 0 in
> commit 5c472203421a ("net_sched: red: Avoid devision by zero"):
> 
> red_set_parms() {
> 	int delta = qth_max - qth_min;
> 	u32 max_p_delta;
> 
> 	p->qth_min	= qth_min << Wlog;
> 	p->qth_max	= qth_max << Wlog;
> 	p->Wlog		= Wlog;
> 	p->Plog		= Plog;
> 	if (delta <= 0)
> 		delta = 1;
> 	p->qth_delta	= delta;
> 	...
> }

I changes it to avoid division by 0, but I wasn't sure that the delta 
value of 1 will be good, just that it is better then 0.

> 
> So we should be safe.  Targets will match.  Probability adjustment for
> adaptive should work correctly.  Which doesn't matter anyway, since we
> will never use the probabilistic action...

That makes sense, and it is a better way to set this setup (DCTCP, I 
guess?) than before.

Thanks

Nogah


> 
>> Nogah Frankel
>> (from a new mail address)
> 
> Noted :)
>