netdev - Re: [RFC PATCH] net: add additional lock to qdisc to increase enqueue/dequeue fairness

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Date:	Wed, 02 Jun 2010 11:48:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>
Cc:	alexander.h.duyck@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH] net: add additional lock to qdisc to increase
 enqueue/dequeue fairness

Le vendredi 21 mai 2010 à 19:40 +0200, Eric Dumazet a écrit :
> Le vendredi 21 mai 2010 à 18:44 +0200, Eric Dumazet a écrit :
> 
> > We could use cmpxchg() and manipulate several bits at once in fast path.
> > ( __QDISC_STATE_RUNNING, __QDISC_STATE_LOCKED ... ) but making the crowd
> > of cpus spin on the same bits/cacheline than dequeue worker would
> > definitely slowdown the worker.
> > 
> > 
> 
> Maybe I am missing something, but __QDISC_STATE_RUNNING is always
> manipulated with the lock held...
> 
> We might avoid two atomic ops when changing this state (if moved to a
> separate container) in fast path (when a cpu sends only one packet and
> returns)
> 
> 

Here are two patches to implement this idea.

First patch to abstract QDISC_STATE_RUNNING access.

Second patch to add a __qstate container and remove the atomic ops.



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