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

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Message-ID: <1274463643.2439.473.camel@edumazet-laptop>
Date:	Fri, 21 May 2010 19:40:43 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>, 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 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)



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