linux-kernel - Re: Linux 2.6.25-rc2

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Message-ID: <20080219130253.GA29804@Krystal>
Date:	Tue, 19 Feb 2008 08:02:53 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: Linux 2.6.25-rc2

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Pekka Enberg <penberg@...helsinki.fi> wrote:
> 
> > Mathieu, Christoph is on vacation and I'm not at all that familiar 
> > with this cmpxchg_local() optimization, so if you could take a peek at 
> > this bug report to see if you can spot something obviously wrong with 
> > it, I would much appreciate that.
> 
> hm, it's bad for at least one other reason as well (which is probably 
> unrelated to this crash):
> 
>  /*
>   * Currently fastpath is not supported if preemption is enabled.
>   */
>  #if defined(CONFIG_FAST_CMPXCHG_LOCAL) && !defined(CONFIG_PREEMPT)
>  #define SLUB_FASTPATH
>  #endif
> 
> such !PREEMPT exceptions tend to show "i didnt want to think too hard 
> about the preemptible case so just turn it off" thinking.
> 

Ingo, a comment in slub.c explains it :

/*
 * The SLUB_FASTPATH path is provisional and is currently disabled if the
 * kernel is compiled with preemption or if the arch does not support
 * fast cmpxchg operations. There are a couple of coming changes that will
 * simplify matters and allow preemption. Ultimately we may end up making
 * SLUB_FASTPATH the default.
 *
 * 1. The introduction of the per cpu allocator will avoid array lookups
 *    through get_cpu_slab(). A special register can be used instead.
 *
 * 2. The introduction of per cpu atomic operations (cpu_ops) means that
 *    we can realize the logic here entirely with per cpu atomics. The
 *    per cpu atomic ops will take care of the preemption issues.
 */

So there is more coming in the preemption area.

> Also, why isnt this "SLUB_FASTPATH" flag done in the Kconfig space?
> 

Eventually, I think only CONFIG_FAST_CMPXCHG_LOCAL will be needed (when
the code will support preemption). Therefore, this SLUB_FASTPATH define
seems to be only here temporarily.

I'm looking at the code right now.. more to come.

Mathieu

> 	Ingo

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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