netdev - Re: High contention on the sk_buff

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Date:	Thu, 19 Mar 2009 06:50:44 -0600
From:	"Peter W. Morreale" <pmorreale@...ell.com>
To:	David Miller <davem@...emloft.net>
Cc:	ghaskins@...ell.com, vernux@...ibm.com, andi@...stfloor.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org, pmullaney@...ell.com
Subject: Re: High contention on the sk_buff_head.lock

On Wed, 2009-03-18 at 18:03 -0700, David Miller wrote:
> From: Gregory Haskins <ghaskins@...ell.com>
> Date: Wed, 18 Mar 2009 17:54:04 -0400
> 
> > Note that -rt doesnt typically context-switch under contention anymore
> > since we introduced adaptive-locks.  Also note that the contention
> > against the lock is still contention, regardless of whether you have -rt
> > or not.  Its just that the slow-path to handle the contended case for
> > -rt is more expensive than mainline.  However, once you have the
> > contention as stated, you have already lost.
> 
> First, contention is not implicitly a bad thing.
> 
> Second, if the -rt kernel is doing adaptive spinning I see no
> reason why that adaptive spinning is not kicking in here to
> make this problem just go away.

The basic 'problem' with comparing RT adaptive spinning to non-rt
spinlocks is that if the lock owner is !oncpu, all spinners must break
and go to sleep, otherwise we (potentially) deadlock.  This does not
exist for non-rt spinners. 

Best,
-PWM


> 
> This lock is held for mere cycles, just to unlink an SKB from
> the networking qdisc, and then it is immediately released.
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