From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size Date: Tue, 19 Nov 2013 10:38:28 +0100 Message-ID: <20131119103828.5697d5c9@redhat.com> References: <1384845939-8424-1-git-send-email-yangyingliang@huawei.com> <1384845939-8424-2-git-send-email-yangyingliang@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: , , , , To: Yang Yingliang Return-path: Received: from mx1.redhat.com ([209.132.183.28]:20233 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751380Ab3KSJik (ORCPT ); Tue, 19 Nov 2013 04:38:40 -0500 In-Reply-To: <1384845939-8424-2-git-send-email-yangyingliang@huawei.com> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, 19 Nov 2013 15:25:38 +0800 Yang Yingliang 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