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Date: Wed, 20 Nov 2013 10:14:43 +0800 From: Yang Yingliang <yangyingliang@...wei.com> To: Jesper Dangaard Brouer <brouer@...hat.com> CC: <davem@...emloft.net>, <netdev@...r.kernel.org>, <eric.dumazet@...il.com>, <jpirko@...hat.com> Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size On 2013/11/19 17:38, Jesper Dangaard Brouer wrote: > On Tue, 19 Nov 2013 15:25:38 +0800 > Yang Yingliang <yangyingliang@...wei.com> wrote: > >> commit b757c9336d63f94c6b57532(tbf: improved accuracy at high rates) >> introduce a regression. >> >> With the follow command: >> tc qdisc add dev eth1 root handle 1: tbf latency 50ms burst 10KB rate 30gbit mtu 64k >> >> Without this patch, the max_size value is 10751(bytes). >> But, in fact, the real max_size value should be smaller than 7440(bytes). >> Or a packet whose length is bigger than 7440 will cause network congestion. >> Because the packet is so big that can't get enough tokens. Even all the tokens >> in the buffer is given to the packet. > > Sorry, but I don't like the commit message. The real problem is the > value in q->buffer, and that the userspace rate table cannot handle > these high rates, which you don't mention. > > I would write something like: > > The kernel no longer uses the userspace provided rate table. Thus, it > is wrong to calculate max_size based on this rate table. At high rates > this rate table gets very inaccurate, which can lead wrong calculation > of max_size. > > Consequence of max_size being too large is severe, and cause packets > being stalled in tbf_dequeue() because it cannot get enough tokens. > The max_size guards against enqueuing packet sizes above q->buffer > "time" in tbf_enqueue(). > > This patch fixes the calculation of max_size. By ... add desc ... > perhaps also mention how it is connected to p->mtu (with is also a > "time" value). > > > The rest of the patch looks okay now, one point below though. OK, change it in v4. Thanks! > > >> diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c >> index 68f9859..c194129 100644 >> --- a/net/sched/sch_tbf.c >> +++ b/net/sched/sch_tbf.c > [...] >> @@ -339,30 +326,46 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt) > [...] >> + for (n = 0; n < 65536; n++) >> + if (psched_l2t_ns(&q->rate, n) > q->buffer) >> + break; >> + max_size = min_t(u32, n, (256ULL << qopt->rate.cell_log) - 1); > > I'm a little uncertain about, if using the 65536 constant is okay, or > considered "bad style". I'll use a MACRO to instead of this constant in v4. Thanks! > > I'm still a little confused/uncertain why we need the "qopt->rate.cell_log". > Because we need max_size be smaller than mtu(user input in bytes). E.g. if user inputs like this "... burst 100KB rate 100mbit mtu 4095", without this patch, max_size is 4095. But with this patch, if don't use cell_log, max_size is 102400. I think it's not correct, so I used cell_log here. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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