netdev - Re: pkt_sched: Fix qdisc len in qdisc_peek_dequeued() [61c9eaf9008]

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Date:	Sun, 16 Nov 2014 10:15:25 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Soltys <soltys@....info>
Cc:	Jarek Poplawski <jarkao2@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: pkt_sched: Fix qdisc len in qdisc_peek_dequeued() [61c9eaf9008]
 - question

On Sun, 2014-11-16 at 14:04 +0100, Michal Soltys wrote:
> Hi,
> 
> I was wondering (probably missing some subtleties) about that particular 
> patch, namely:
> 
> 61c9eaf90081cbe6dc4f389e0056bff76eca19ec
> 
> Why would that qlen++ change be necessary ? As far as peeked qdisc sees 
> things, the packet is already deqeued and gone - so not really part of 
> the queue anymore in this context (not ever requeued either). More 
> advanced qdiscs such as say fq_codel - if for example they decide to 
> drop head during further enqueue operation - obviously won't even 
> consider the peeked packet.

peeked packet has not to be considered by qdisc : Their dequeue()
methods is called only when there is no more peeked packet.

> 
> Increasing qlen from what I can see artificially shortens queue by 1. 
> For some classful schedulers (say like hfsc that instantly peeks next 
> packet length after dequeuing), child qdiscs will be almost all the time 
> formally operating at queue size decreased by 1.

Well, the peeked packet is part of the queue, as it is not yet
transmitted.


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