From: Yang Yingliang <yangyingliang@huawei•com>
To: Jesper Dangaard Brouer <brouer@redhat•com>
Cc: <davem@davemloft•net>, <netdev@vger•kernel.org>,
<eric.dumazet@gmail•com>, <jpirko@redhat•com>
Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size
Date: Wed, 20 Nov 2013 10:14:43 +0800 [thread overview]
Message-ID: <528C1B13.5090701@huawei.com> (raw)
In-Reply-To: <20131119103828.5697d5c9@redhat.com>
On 2013/11/19 17:38, Jesper Dangaard Brouer wrote:
> 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.
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.
next prev parent reply other threads:[~2013-11-20 2:15 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
2013-11-20 2:14 ` Yang Yingliang [this message]
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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