netdev - Re: [PATCH v2 net] fq_codel: fix NET_XMIT

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Message-ID: <575AAC6F.7090301@mojatatu.com>
Date:	Fri, 10 Jun 2016 08:02:55 -0400
From:	Jamal Hadi Salim <jhs@...atatu.com>
To:	Florian Westphal <fw@...len.de>,
	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Stas Nichiporovich <stasn77@...il.com>
Subject: Re: [PATCH v2 net] fq_codel: fix NET_XMIT_CN behavior

On 16-06-06 12:18 PM, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Mon, 2016-06-06 at 13:42 +0200, Florian Westphal wrote:
>>> Jamal Hadi Salim <jhs@...atatu.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