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

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Date:	Fri, 21 May 2010 08:43:49 -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 Tue, 23 Mar 2010 23:13:00 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Le mardi 23 mars 2010 à 14:45 -0700, David Miller a écrit :
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Tue, 23 Mar 2010 21:54:27 +0100
> > 
> > > Quite frankly, the real problem in this case is not the reduced
> > > throughput, but fact that one cpu can stay a long time doing the xmits
> > > to device, of skb queued by other cpus. This can hurt latencies a lot,
> > > for real time threads for example...
> > > 
> > > I wonder if ticket spinlocks are not the problem. Maybe we want a
> > > variant of spinlocks, so that cpu doing transmits can get the lock
> > > before other cpus...
> > 
> > I want to note that things operate the way they do now
> > intentionally.
> > 
> > Herbert Xu and Jamal Hadi Salim were active in this area
> > about 4 years ago.
> 
> Yes, but ticket spinlocks were added after their work (in 2008 - 2.6.25
> if I remember well) and change things.
> 
> We want cpu owning __QDISC_STATE_RUNNING being able to re-get the lock
> as fast as possible. Alexander results can show the possible speedup.

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



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