From: Jamal Hadi Salim <jhs@mojatatu•com>
To: Florian Westphal <fw@strlen•de>, Eric Dumazet <eric.dumazet@gmail•com>
Cc: Cong Wang <xiyou.wangcong@gmail•com>,
David Miller <davem@davemloft•net>,
netdev <netdev@vger•kernel.org>,
Stas Nichiporovich <stasn77@gmail•com>
Subject: Re: [PATCH v2 net] fq_codel: fix NET_XMIT_CN behavior
Date: Fri, 10 Jun 2016 08:02:55 -0400 [thread overview]
Message-ID: <575AAC6F.7090301@mojatatu.com> (raw)
In-Reply-To: <20160606161843.GD7827@breakpoint.cc>
On 16-06-06 12:18 PM, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@gmail•com> wrote:
>> On Mon, 2016-06-06 at 13:42 +0200, Florian Westphal wrote:
>>> Jamal Hadi Salim <jhs@mojatatu•com> wrote:
>>>> BTW, returning NET_XMIT_CN could be confusing to tcp;
>>>> it does not mean that the packet that we are getting return
>>>> code for was dropped; it could mean _another_ packet in
>>>> the queue was dropped.
>>>
>>> Yes, but we currently conceal NET_XMIT_CN from upper layer (tcp)
>>> via the net_xmit_* macros:
>>>
>>> #define net_xmit_eval(e) ((e) == NET_XMIT_CN ? 0 : (e))
>>> #define net_xmit_errno(e) ((e) != NET_XMIT_CN ? -ENOBUFS : 0)
>>>
>>> Might be worth changing this so tcp reduces cwnd in _CN case too.
>>
>> It always had been the case.
>>
>> tcp_transmit_skb() calls tcp_enter_cwr(), unless someone broke this.
>
> No, you're right. We hide error in ip_send_skb but that function is
> not used in tcp case.
>
> So all is good here.
>
Just making sure because that little gem is not obvious;->
Eric, on the NET_XMIT_CN digression:
depends on the qdisc scheduler.
NET_XMIT_CN could mean one of two things:
"There is congestion. The packet you sent was dropped locally" and
"There is congestion. The packet you sent was not dropped locally"
Yes, they both react with tcp_enter_cwr() and from a macroscopic
view it probably doesnt matter as much because tcp state machine will
kick in at some point. I think better use could be made of such
knowledge.
cheers,
jamal
next prev parent reply other threads:[~2016-06-10 12:03 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-04 17:43 [PATCH net] fq_codel: fix NET_XMIT_CN behavior Eric Dumazet
2016-06-04 19:03 ` [net] " Florian Westphal
2016-06-04 19:40 ` Eric Dumazet
2016-06-04 19:55 ` [PATCH v2 net] " Eric Dumazet
2016-06-05 13:03 ` Jamal Hadi Salim
2016-06-05 20:30 ` Cong Wang
2016-06-05 20:54 ` Eric Dumazet
2016-06-05 20:55 ` Eric Dumazet
2016-06-05 23:30 ` Cong Wang
2016-06-06 10:49 ` Jamal Hadi Salim
2016-06-06 11:42 ` Florian Westphal
2016-06-06 14:37 ` Eric Dumazet
2016-06-06 16:18 ` Florian Westphal
2016-06-10 12:02 ` Jamal Hadi Salim [this message]
2016-06-06 14:29 ` Eric Dumazet
2016-06-07 22:39 ` David Miller
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