netdev - Re: [PATCH] pkt_sched: QFQ Plus: fair-queueing service at DRR cost

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Date:	Wed, 21 Nov 2012 10:45:14 +0100
From:	Paolo Valente <paolo.valente@...more.it>
To:	David Miller <davem@...emloft.net>
CC:	jhs@...atatu.com, shemminger@...tta.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	rizzo@....unipi.it, fchecconi@...il.com
Subject: Re: [PATCH] pkt_sched: QFQ Plus: fair-queueing service at DRR cost

Il 20/11/2012 19:54, David Miller ha scritto:
> From: Paolo Valente <paolo.valente@...more.it>
> Date: Tue, 20 Nov 2012 18:45:13 +0100
>
>> -	struct sk_buff *skb;
>> +	struct sk_buff *skb = NULL;
>
> This is not really an improvement,
Sorry for trying this silly short cut
  now the compiler can think
> that NULL is passed eventually into qdisc_bstats_update().
>
> Please make the logic easier for the compiler to digest.
>
> For example, restructure the top-level logic into something like:
>
> 	skb = NULL;
> 	if (!list_empty(&in_serv_agg->active))
> 		skb = qfq_peek_skb(in_serv_agg, &cl, &len);
> 	else
> 		len = 0; /* no more active classes in the in-service agg */
>
> 	if (len == 0 || in_serv_agg->budget < len) {
>   ...
> 		/*
> 		 * If we get here, there are other aggregates queued:
> 		 * choose the new aggregate to serve.
> 		 */
> 		in_serv_agg = q->in_serv_agg = qfq_choose_next_agg(q);
> 		skb = qfq_peek_skb(in_serv_agg, &cl, &len);
> 	}
> 	if (!skb)
> 		return NULL;
>
> That way it is clearer, to both humans and the compiler, what is
> going on here.
>
Got it. Actually, if the first qfq_peek_skb returns NULL, then the 
example version that you are proposing apparently may behave in a 
different way than the original one: in your proposal the scheduler 
tries to switch to a new aggregate and may return a non-NULL value, 
whereas the original version would immediately return NULL. I guess that 
this slightly different behavior is fine as well, and I am preparing a 
new patch that integrates these changes.
> Thanks.
>


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