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

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Message-ID: <20100521103848.12f07c23@nehalam>
Date:	Fri, 21 May 2010 10:38:48 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.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

On Fri, 21 May 2010 18:44:35 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> 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.

Your solution is okay, but it is a special case performance hack
and we seem to be getting more and more of these lately.
This is a general problem of any producer/consumer case;
other code will have same problem (like disk requests) 
and can't use the same solution.

Maybe the RT and scheduler folks have some input because it does
seem like the same problem has occurred in other contexts before.

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