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

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Date:	Fri, 21 May 2010 18:44:35 +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 08:43 -0700, Stephen Hemminger a écrit :

> What about having a special function (spin_lock_greedy?) that just ignores
> the ticket mechanism and always assumes it has right to next ticket.
> 

I am not sure we want to do this, for a single use case...
Adding a new lock primitive to linux should be really really motivated.

x86 ticket spinlocks are nice because we are sure no cpu is going to
stay forever in this code. For other arches, I dont know how this is
coped.

I thought about this before re-working on this patch, but found it was
easier to slightly change Alexander code, ie adding a regular spinlock
for the slowpath.

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.

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