netdev - Re: [BUG] net_sched: pfifo_head

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Date:	Wed, 5 Jan 2011 09:15:47 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
	Patrick McHardy <kaber@...sh.net>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	Jarek Poplawski <jarkao2@...il.com>
Subject: Re: [BUG] net_sched: pfifo_head_drop problem

On Wed, 05 Jan 2011 18:00:50 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> While reviewing CHOKe stuff, I found following problem :
> 
> commit 57dbb2d83d100ea (sched: add head drop fifo queue)
> introduced pfifo_head_drop, and broke the invariant that
> sch->bstats.bytes and sch->bstats.packets are COUNTER (increasing
> counters only)
> 
> This can break estimators because est_timer() handle unsigned deltas
> only. A decreasing counter can then give a huge unsigned delta.
> 
> My suggestion would be to change things so that sch->bstats.bytes and
> sch->bstats.packets are incremented in dequeue() only, not at enqueue()
> time.
> 
> It would be more sensible anyway for very low speeds, and big bursts.
> Right now, if we drop packets, they still are accounted in estimators.
> 
> Or maybe my understanding of estimators is wrong, and only apply to
> enqueue rate, not dequeue rate ?
> 
> If so, we should remove the 
> 
> sch->bstats.bytes -= qdisc_pkt_len(skb_head);
> sch->bstats.packets--;
> 
> done in pfifo_tail_enqueue() in case we drop the head skb.
> 
> 
> My preference would be to add dropped pack/byte rates to estimators...
> It might be good for tuning.

Agreed counters should reflect dequeued packets not enqueued packets.


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