netdev - Re: [PATCH net-next] fq_codel: add memory limitation per queue

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Message-ID: <CAM_iQpUusrr7W9diudUad+iDg3s5-4We6Z7tVzjLfMSVVJ+MMg@mail.gmail.com>
Date:	Mon, 9 May 2016 21:57:33 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Dave Täht <dave.taht@...il.com>,
	netdev <netdev@...r.kernel.org>, moeller0 <moeller0@....de>
Subject: Re: [PATCH net-next] fq_codel: add memory limitation per queue

On Mon, May 9, 2016 at 9:45 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2016-05-09 at 21:34 -0700, Cong Wang wrote:
>> On Mon, May 9, 2016 at 7:26 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Sun, 2016-05-08 at 22:07 -0700, Cong Wang wrote:
>> >> On Sun, May 8, 2016 at 9:31 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> >> > On Sun, 2016-05-08 at 21:14 -0700, Cong Wang wrote:
>> >> >
>> >> >> So when the packet is dropped due to memory over limit, should
>> >> >> we return failure for this case? Or I miss anything?
>> >> >
>> >> > Same behavior than before.
>> >> >
>> >> > If we dropped some packets of this flow, we return NET_XMIT_CN
>> >>
>> >> I think for the limited memory case, the upper layer is supposed
>> >> to stop sending more packets when hitting the limit.
>> >
>> > They doe. NET_XMIT_CN for example aborts IP fragmentation.
>> >
>> > TCP flows will also instantly react.
>>
>> But not for the NET_XMIT_SUCCESS case:
>>
>>         return ret == idx ? NET_XMIT_CN : NET_XMIT_SUCCESS;
>
>
> I believe you missed whole point of FQ (SFQ, FQ_CODEL, FQ, ...)
>
> If we dropped a packet of another flow because this other flow is an
> elephant, why should we notify the mouse that we shot an elephant ?
>
> We return NET_XMIT_SUCCESS because we properly queued this packet for
> this flow. This is absolutely right.
>

Sure, but we are talking about memory constraint case, aren't we?

If the whole system are suffering from memory pressure, the whole
qdisc should be halted?