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Date: Tue, 19 Nov 2013 10:38:28 +0100 From: Jesper Dangaard Brouer <brouer@...hat.com> To: Yang Yingliang <yangyingliang@...wei.com> Cc: <davem@...emloft.net>, <netdev@...r.kernel.org>, <eric.dumazet@...il.com>, <brouer@...hat.com>, <jpirko@...hat.com> Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size 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. > 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'm still a little confused/uncertain why we need the "qopt->rate.cell_log". -- Best regards, Jesper Dangaard Brouer MSc.CS, Sr. Network Kernel Developer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer -- 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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