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Date: Tue, 29 May 2018 10:53:02 +0300 From: Nogah Frankel <nogah.frankel@...il.com> To: Jakub Kicinski <jakub.kicinski@...ronome.com> Cc: davem@...emloft.net, jiri@...nulli.us, xiyou.wangcong@...il.com, john.fastabend@...il.com, netdev@...r.kernel.org, oss-drivers@...ronome.com, alexei.starovoitov@...il.com, nogahf@...lanox.com, yuvalm@...lanox.com, gerlitz.or@...il.com Subject: Re: [PATCH net-next 05/14] nfp: abm: add simple RED offload 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 :) >
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