From: Jesper Dangaard Brouer <brouer@redhat•com>
To: Yang Yingliang <yangyingliang@huawei•com>
Cc: <davem@davemloft•net>, <netdev@vger•kernel.org>,
<eric.dumazet@gmail•com>, <brouer@redhat•com>,
<jpirko@redhat•com>
Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size
Date: Tue, 19 Nov 2013 10:38:28 +0100 [thread overview]
Message-ID: <20131119103828.5697d5c9@redhat.com> (raw)
In-Reply-To: <1384845939-8424-2-git-send-email-yangyingliang@huawei.com>
On Tue, 19 Nov 2013 15:25:38 +0800
Yang Yingliang <yangyingliang@huawei•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
next prev parent reply other threads:[~2013-11-19 9:38 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-19 7:25 [PATCH net v3 0/2] net: sched: fix some issues Yang Yingliang
2013-11-19 7:25 ` [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size Yang Yingliang
2013-11-19 9:38 ` Jesper Dangaard Brouer [this message]
2013-11-20 2:14 ` Yang Yingliang
2013-11-20 10:04 ` Jesper Dangaard Brouer
2013-11-20 12:50 ` Yang Yingliang
2013-11-23 19:06 ` Eric Dumazet
2013-11-24 7:28 ` Yang Yingliang
2013-11-24 18:40 ` Eric Dumazet
2013-11-25 3:43 ` Yang Yingliang
2013-11-25 12:04 ` [PATCH] " Yang Yingliang
2013-11-25 12:22 ` David Laight
2013-11-26 1:28 ` Yang Yingliang
2013-11-26 2:35 ` Yang Yingliang
2013-12-02 1:11 ` David Miller
2013-12-02 10:29 ` David Laight
2013-12-02 16:45 ` David Miller
2013-12-03 0:59 ` Yang Yingliang
2013-11-19 7:25 ` [PATCH net v3 2/2] net: sched: htb: fix calculation of quantum Yang Yingliang
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